DS_Manual Segurança de Barragem

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STATE OF COLORADODAM SAFETY MANUAL

AUTHORSJohn Schurer

Eric WilkinsonJames NorfleetJohn Van SciverClint Huntington

Chin LeeAllan Rogers

MEMBERS OF THE COLORADO DIVISION OF WATER RESOURCESTHIRD PRINTING REVISED JANUARY 2002

THIS PUBLICATION CAN NOT BEREPRODUCED UNLESS PERMISSION

IS GRANTED BY THESTATE ENGINEER



CHAPTER 1 Introduction

2 Physical Principles

3 Fundamentals Of A Dam

4 Visual Inspection

5 Seepage

6 Upstream Slope

7 Crest

8 Downstream Slope

9 Outlet System

10 Spillways

11 Concrete Dams

12 Monitoring and Instrumentation

13 Maintenance

14 Standard Operating Procedure

15 Emergency Plan

CONTENTSOUTLINE



CHAPTER 1 — INTRODUCTION1.1 Intent of This Manual1.2 Definition of a Dam1.3 Minimum Expected Effort1.4 Critical Conditions1.5 Life Expectancy of a Dam1.6 Need for Stored Water1.7 Role of the Dam Owner1.8 Dam Safety Role of the

State Engineer1.9 Role of the Consulting

Engineer1.10 Summary

CHAPTER 2 — PHYSICAL PRINCIPLES2.1 Introduction2.2 Soils for Construction2.3 Particle Size2.4 Soil Structure2.5 Compaction2.6 Moisture Control

2.7 Comparison of PathwaysAvailable for the Flow of Water Through a GivenMaterial

2.8 Water Pressure2.9 Flow Path2.10 Lengthening the Flow Path2.11 Summary

CHAPTER 3 — FUNDAMENTALS OF A DAM3.1 Introduction3.2 Functions of a Dam3.3 Review3.4 Earth Dams3.5 Rockfill Dams3.6 Concrete Dams3.7 Spillways3.8 Outlet Systems3.9 Summary

CHAPTER 4 — VISUAL INSPECTION4.1 What and Why so

Important4.2 Inspection Equipment and

Its Use4.3 Inspection Observations

That Must Be Recorded4.4 Sighting Technique4.5 Some Typical Problems4.6 Inspection Procedure4.7 Record Keeping4.8 Evaluation of Inspection

Observations4.9 Suggested Planning4.10 Crucial Inspection Times4.11 Additional Considerations4.12 Summary4.13 Inspection Report Form

CHAPTER 5 — SEEPAGE5.1 Introduction5.2 Items of Particular Concern5.3 Special Inspection

Techniques5.4 Field Conditions

Encountered5.5 Summary

CHAPTER 6 — UPSTREAM SLOPE6.1 Introduction6.2 Items of Particular Concern6.3 Special Inspection

Techniques6.4 Typical Materials6.5 Field Conditions

Encountered6.6 Summary

CHAPTER 7 — CREST7.1 Introduction7.2 Typical Materials7.3 Items of Particular Concern7.4 Special Inspection

Techniques7.5 Problem Conditions Found

On the Crest7.6 Summary

CHAPTER 8 — DOWNSTREAM SLOPE8.1 Introduction8.2 Items of Particular Concern8.3 Special Inspection

Techniques8.4 Typical Materials8.5 Problems Found in the Field

8.6 SummaryCHAPTER 9 — OUTLET SYSTEM

9.1 Introduction9.2 Typical Outlet

Configurations9.3 Important Principles9.4 Items of Particular Concern9.5 Special Inspection

Techniques andRequirements

9.6 Typical Materials9.7 Types of Outlet Valves9.8 Field Conditions to Look For

9.9 SummaryCHAPTER 10— SPILLWAYS

10.1 Introduction10.2 Important Problems10.3 Commonly Used Materials10.4 Procedure for Inspection10.5 Commonly Encountered

Problems and ActionsRequired

10.6 Summary

OUTLINE



CHAPTER 11— CONCRETE DAMS11.1 Introduction11.2 Items of Particular Concern11.3 Special Inspection

Techniques andRequirements

11.4 Other Conditions to Look For11.5 Summary

CHAPTER 12— MONITORING ANDINSTRUMENTATION12.1 Introduction12.2 Factors That Cause Change

In Condition12.3 Careful Monitoring Can

Prevent Costly Problems12.4 Monitoring Leakage12.5 Monitoring Displacements12.6 Specialized Instrumentation12.7 Summary

CHAPTER 13— MAINTENANCE13.1 Tree, Brush, and Weed Control13.2 Earthwork 13.3 Rodent Control13.4 Concrete Structures13.5 Steel Structures and Metal

Components13.6 Outlet Gates—Mechanical

Maintenance13.7 Electrical13.8 Hydraulic System

CHAPTER 14— STANDARD OPERATINGPROCEDURE14.1 Introduction14.2 Putting the Procedure

Together14.3 Assigning Responsibility14.4 Summary

CHAPTER 15— EMERGENCY PLAN15.1 Introduction15.2 General Information15.3 Reporting Incidents15.4 Potential Problems and

Immediate DefensiveActions

15.5 Helpful Suggestions15.6 Additional Considerations15.7 Model Plan



1.1 INTENT OF THIS MANUAL

1.1-1 GOALIn the State of Colorado there are more than1,800 water storage dams that are subject tothe state’s dam safety program. In the interestof public safety and of extending the useful life

of these structures, this manual provides specificguidance that will enable the dam owner tocarry out his responsibility to maintain a safedam, avoid costly repairs, and prolong the lifeof the dam.

1.1-2 METHODInformation is provided on the basic workingsof a dam, inspection and monitoring of a dam’sperformance, and general guidelines for carry-ing out routine maintenance. Additionally, sug-gested operating procedures and a model emer-gency preparedness plan are included. Thisinformation is presented in a manner designedto help an inexperienced person becomeacquainted with the activities required to main-tain a safe dam.

1.1-3 IMPLEMENTATIONThe goals mentioned above can be achievedthrough cooperation of the dam owners and theState Dam Safety Engineers.OWNERSDam owners are potentially liable for all dam-ages associated with their dams should theyfail or break due to negligence. To minimizethis liability, owners should carry out the fol-lowing:

a. Periodic visual inspection of the dam.b. Prompt reporting and correction of any

adverse conditions found during theinspection.

c. Monitoring of questionable conditions thatmay affect the safety of the dam.

d. Performing regular periodic maintenancewhen required.

e. Retaining an experienced engineer to pro-vide the investigations, analyses, reports,plans, and specifications required for theconstruction of new dams and for theimprovement or safe operation of exist-

ing dams.f. Making repairs where and to the extent

required.g. Complying with the established laws, rules,

and regulations pertaining to dam construc-tion and operation.

OFFICE OF THE STATE ENGINEERThe State Engineer is responsible for:

a. Evaluation of each dam and related struc-ture to determine annually the safe storagelevel as required by CRS 1973 37-87-107as amended.

b. Assisting the owners by investigating ques-tionable conditions that are found and advis-ing them on prudent remedial action. Theseactions may include having the conditionassessed by a consulting engineer.

c. When unsafe conditions are found, order-ing required repairs and reduced storage lev-els until the repairs are completed.

d. Reviewing plans and specifications for theconstruction of new dams and major repairs,alterations, and enlargements to existingdams. These are thoroughly evaluated forcompliance with current dam safety stan-dards, design criteria, and state requirements.

e. Periodic on-site inspection of constructionprocedures and quality control techniquesto verify conformance with approved plansand specifications.

1.1-4 BENEFITSBy using proper inspection and maintenanceprocedures, dam owners can achieve an increaseddegree of safety for the structure. Timely inspec-tion and maintenance will also reduce the pos-sibility of loss of use of the structure and theneed for costly repairs.

1.2 DEFINITION OF A DAMDams addressed in this manual are man-made barri-ers, constructed on natural terrain in order to controlor store water. In most cases the reservoir basin issited entirely in unaltered natural terrain. Natural lakeswhich have been developed by constructing facilitiesfor releasing the lakes’ contents are also included.

1.3 MINIMUM EXPECTED EFFORT BY THEOWNERUnfortunately there is a possibility, however remote,that any given dam might experience partial or totalfailure, causing extensive damage in the downstreamflood plain.Minimum expected effort is the amount and intensi-ty of effort the owner must make in fulfilling his obli-gation to persons and properties downstream from hisdam to assure the dam does not experience partial ortotal failure. The required actions include thoroughvisual inspection, accurate monitoring when required,recording and interpreting information gained frominspection and monitoring, regularly scheduled rou-tine maintenance, making required repairs in a time-ly manner, and operating the dam in a way that willgive the greatest assurance of safety. The requiredintensity of effort will vary in relation to the loss thatwould be experienced in terms of loss of life, thedownstream development, and the value of the struc-ture itself. The suggested ranges of effort required forthe following types of dams are:HIGH HAZARD DAMS—Failure of the dam wouldcause the loss of human life.Daily—Surveillance by the owner or caretaker.

CHAPTER 1

INTRODUCTION



Weekly—Monitoring of seepage.Monthly—Thorough visual inspection. Gathering,immediately plotting, and interpreting observationwell and piezometer data.Annually—Reading horizontal and vertical controlmonuments (more frequently if necessary).

Test operation of outlet and spillway mechanical com-ponents.Routine maintenance as required.SIGNIFICANT HAZARD DAMS—Failure of thedam would cause extensive property damage but isnot expected to cause loss of human life.Weekly—Surveillance by the owner or caretaker.Monitoring of seepage.Monthly—Thorough visual inspection when storage isin excess of one-half the maximum gage rod reading.Gathering, plotting and interpreting observation welland piezometer data when storage is in excess of one-half the maximum gage rod reading.Annually—Test operation of outlet and spillwaymechanical components.Reading of horizontal and vertical control monumentsafter a satisfactory performance record has been estab-lished for the dam.Routine maintenance as required.LOW HAZARD DAMS—Failure would cause littledamage beyond the loss of the dam structure itself.Monthly—Surveillance by the owner or caretaker.Monitoring of seepage when storage is in excess of one-half the maximum gate rod reading.Gathering, plotting, and interpreting observation well

and piezometer data when storage is in excess of one-half the maximum gage rod reading.Annually—Thorough visual inspection. (If the reser-voir is full all year, visual inspection should be donequarterly.) Testing operation of the outlet works.Every Five Years—Reading of horizontal and verti-cal control monuments after a satisfactory perform-ance record has been established.Routine maintenance as required.

1.4 CRITICAL CONDITIONSFloodsWhen floods caused by severe rain storms, or exces-

sive snowmelt runoff are predicted, the spillways of alldams should be inspected to detect any areas that willneed special protection while passing the flood flows.During the flood and after flows subside, the spillwayshould be inspected to identify any damage needingrepair.WindstormHigh winds can cause damaging wave action. Theupstream slope must be monitored during severe wind-storms to allow prompt repair of damaged areas.Thorough inspection when the storm subsides willallow required maintenance to be identified andprompt repairs initiated.

EarthquakeIf the effects of an earthquake are identified in thearea of the dam site or if tremors have been reported,the dam should be thoroughly inspected immediatelyand at weekly intervals for four to six weeks after thequake. Conditions triggered by an earthquake canoften take several weeks to appear.

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PHOTO 1.4-1RUNOFF DAMAGE

FLOWS ERODING UNPROTECTED SPILLWAY CHANNEL. Sand-bags and plastic sheet placed on right side to divert flow. Note full out-let discharge to minimize flow through spillway.

PHOTO 1.4-2WINDSTORM DAMAGELarge cavern eroded into embankment after wave action destroyed con-crete protective coating on the upstream face.



1.5 LIFE EXPECTANCY OF A DAMOne of the most compelling economic concerns of adam owner is how to prolong the useful life of hisdam. The oldest dam in the United States as shownon the National Inventory of Dams is known as PoduncPond, located on the Podunc River near the town of Pleasant Valley, Connecticut. This 17-foot-high dam,impounding 110 acre feet, was constructed in 1730and has been in service more than 250 years.In Colorado, extensive dam building was initiated inthe early 1900s, utilizing the engineering know-howand construction techniques available at that time.These dams, which were located in sparsely populat-ed rural areas, occasionally experienced partial fail-ure which caused little damage in the downstreamflood plain. Historically, the required repairs weremade as quickly as economic conditions permitted.Although these older dams were built without some of the safety-oriented features included in recently con-structed dams, their useful lives can be extended great-ly if the owners exercise the appropriate effort to assurethe dams remain in good condition. Providing themoney necessary to carry out periodic maintenanceand required repairs is essential to prolong the life of the dam.The recently published costs of proposed or newlyconstructed water storage projects clearly indicate thatmaking all possible effort to keep an older structure inservice is preferable to attempting to build a new dam.When an owner constructs a new dam he shouldensure, at the end of construction, that all portionsof the structure are at the line and grade specified onthe approved plans. This will allow inspection andmonitoring to be carried out with a reliability levelthat will help assure the safety and longevity of thenew dam.

1.6 NEED FOR STORED WATERColorado is a semi-arid region with average annualprecipitation being less than 16 inches per year. Theearly settlers quickly found that natural precipitationwas not adequate to support their farming efforts.Runoff from snowpack in the high mountain areasusually is finished by early July and natural precipi-tation is usually not sufficient to bring irrigated cropsto maturity. Control of the runoff for future usebecomes vital, since most streams receiving snow meltproduce two-thirds of their annual volume of runoff inMay, June and July.The critical need for stored water which can be madeavailable on demand is present in all areas of the state.Owners of these dams play a vital role in society bymaking water available for the various required uses.

Although the use of the stored water directly benefitsthe owner, a number of the secondary benefits areenjoyed by people in the community.

1.7 ROLE OF THE DAM OWNERInitially the dam owner identifies the need for theproject, finds a method to finance and reimburse proj-ect costs, then contracts for the design and construc-tion work. Even before construction is complete, theowner assumes liability for any type of damage thatmight occur because of the uncontrolled release of stored water. Although a great number of people enjoythe benefits provided by the dam and reservoir, theowner has total responsibility for the dam’s safety,

and must provide the funds and effort necessary toobtain the maximum useful life from the structure.

1.8 DAM SAFETY ROLE OF THE STATE ENGINEERColorado statutes charge the State Engineer withresponsibility for public safety in relation to dams.This responsibility is addressed by three primaryareas of activity:

a. Physical inspection of dams. Based on conditionsidentified during these inspections, the dam own-ers are directed to take action to correct any defi-ciencies and required storage restrictions are orderedto assure the safe operation of the dam. Theseinspections are conducted by experienced registered

PHOTO 1.4-3EARTHQUAKE DAMAGE

Settlement of 2.5 feet on dam crest first noted about 2 weeks after anearthquake in the area.

PHOTO 1.5EARLY CONSTRUCTION

METHODS TYPICAL OF AN EARLIER ERA.



professional engineers in the Dam Safety Branch.Follow up of the inspection often includes review-ing questionable conditions on site with the damowner, explaining the problems and suggesting thebest and/or most economical way to proceed inassuring the dam’s safety. Frequently, further stud-ies by a consulting engineer are recommended.

b. Review of plans and specifications for new damprojects and major repair work, enlargement, orrehabilitation of existing dams for conformancewith design standards and effective constructiontechniques. Experienced registered professionalengineers in the Dam Safety Branch conduct thesereviews and work closely with the design engi-neers in order to provide a safe and workabledesign and construction plan.

c. Periodic inspection of construction work beingdone on new or existing dams. These inspectionsare performed in order to verify that the work con-forms to the approved plans and specifications.Any problem that is noted is discussed with theproject engineer and corrective action is identi-fied and implemented.

1.9 USE OF AVAILABLE STAFFIn order to provide maximum utilization of availablemanpower, the following dam safety priorities havebeen established:

1. ENSURE PUBLIC SAFETY—Both the fieldinspection and design review activities are specif-ically directed toward public safety. All possibleeffort is made to assure that each dam remains ina safe condition. If problems are found during an

inspection which threaten the safety of the dam,temporary reductions in storage behind the damare ordered until required repairs are made.When plans are submitted for repairs to existingdams, both these plans and original constructionplans as well as known site conditions are scruti-nized to assure that all potential problem areashave been properly addressed and the completedrepairs will make the dam safe.

2. MAXIMIZE THE SAFE STORAGE CAPACITYAVAILABLE WITHIN THE STATE—This usu-ally involves cooperation among the dam owner,his consulting engineer, and the State Engineer.

Efforts are focused on resolving problems thatcause the reservoir to be operated at a restrictedlevel in the interest of public safety.

3. PROVIDE DIRECTION AND ASSISTANCE TOOWNERS OF DAMS THAT ARE OPERATINGAT FULL STORAGE CAPACITY—Requiredmonitoring, operational schedules, and mainte-nance that will help assure the safety of the damand contribute to its longevity are communicatedto and discussed with dam owners for theirimplementation.

1.10 ROLE OF THE CONSULTING ENGINEERThe consulting engineering community is deeplyinvolved in the total process of planning, designing,constructing, evaluating, and maintaining water stor-age dams. Their research and expertise can often pro-vide the critical balance that determines whether aproposed project can be justified economically. Thisengineering community has the responsibility to designand construct water storage facilities according to themost recently proven technology in design and effi-cient construction practice.Dams are constructed largely of locally available nat-ural materials and are founded on natural materials.Natural materials vary widely in their characteristicsand distribution from site to site and within a givensite. Engineering services for dam design and con-struction monitoring are often extensive and time con-suming because of the unique conditions to be evalu-ated at each dam site. There is no such thing as astandard dam design. Engineering investigations andstudies are required to develop a reasonably econom-ical and serviceable dam project.

SUMMARYWater must be available on demand to meet the needs of society. Dams are constructed to provide water storage reser-

voirs to meet these needs, and with that construction thedam owners becomes solely responsible for the safety, safeoperation, and long life of his dam. In addition, the StateEngineer inspects the dams and regulates dam constructionin order to assure public safety. By a cooperative effortbetween the dam owner, the State Engineer, and the con-sulting engineering community, dams can be constructedand maintained in accordance with the highest safety stan-dards. This manual provides information that will assist thedam owner in effectively carrying out his responsibility topublic safety and extending the useful life of his dam.

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2.1 INTRODUCTIONThis chapter presents some of the physical principlesthat make it possible for a dam to hold water. Knowl-edge of what allows the dam to function will help theowner to understand why various inspection and main-tenance items must be performed.

2.2 SOILS FOR CONSTRUCTIONSoil is the material on the earth’s surface producedby the breakdown of rock due to weathering and ero-sion.Soils used in construction usually serve the follow-ing functions:

1. Provide weight and/or stability.

2. Provide resistance to the flow of water.

3. Provide for the controlled flow of water.Soils normally encountered in earth embankments con-tain a mixture of sands, silts, clays, and rock frag-ments. The individual particles range in size frommicroscopic to approximately an 8-inch maximumdimension. Silt types are identified based on the pre-dominant particle size.Silt and clay have different degrees of plastic behav-ior. A clay can be molded into a shape, while silt willbreak up when attempts are made to mold it. Somecharacteristics of sand, silt, and clay for use in a damare examined below.

2.3 PARTICLE SIZEParticle size is especially important because it influ-ences the flow of water through a soil mass. A com-parison of average sizes of various soil components is

shown below. The dimension of a clay particle will beassigned one unit to allow comparisons among sand,silt, clay, and water.

2.4 SOIL STRUCTUREAny natural deposit of soil will contain various sizeparticles and spaces called voids which are filled withair or water. A useful material for demonstrating soilstructure is sand of the type found on the beach or inchildren’s sand boxes.In illustrations of soil structure, these symbols willbe used:

Water— A droplet of water.

This droplet is normally made up of many molecules.

Air— A small bubble of air.

Sand— A sand grain.

Silt— Will be shown larger than actual sizeindicated in Figure 2.3-1.

Clay— Will be shown very much larger thanindicated in Figure 2.3-1 for ease of illustration.

a. SANDA sample is prepared by placing a few shovelsfull of dry sand into a mound. Water is now sprin-

kled on the mound until some water begins toflow away from the base. The sand is allowed todrain for a few minutes. A small sample is takenand pressed or molded in the hand.

CHAPTER 2

PHYSICAL PRINCIPLES

FIGURE 2.4-1 SAMPLE PREPARATION

SIZE RANGE ENGLISHITEM METRIC UNITS UNITS

Rock 76.2MM 2M 2.9 In. 6.56 Ft.

Gravel 4.76MM 76.2MM 9.19 In. 2.9 In.

Sand 0.074MM 2.0MM 0.003 In. 0.079 In.

Silt 0.005MM 0.074MM 0.0002 In. 0.003 In.

Clay Smaller than .005MM Smaller than 0.0002 In.

Water

Molecule 0.00000003MM 0.00000001 In.

If the clay particle and a water molecule are each magnified65,000 times, the water molecule can barely be seen.

FIGURE 2.3-1 PARTICLE SIZE



Imagine that this sample is carefully sliced in half.Placed under a powerful magnifier, the sliced sur-face would look like that shown in Figure 2.4-2.

Sand particles occupy the largest portion of thearea viewed. Air and water occupy the remainderof the area.

b. SILTA silt material is prepared in a similar manner tothe sand. The structure will contain silt particles,water and air. The sliced surface of the silt sam-ple under a powerful magnifier would appearmuch like that shown below.

Again we see that the silt particles make up themajority of the area but are interlaced with air andwater.

c. CLAYA similarly prepared sample of clay must beviewed under a microscope. We see that the clayparticles are interlaced with air and water.

d. NATURAL DEPOSITA sample of a natural deposit of soil can containa combination of rock, gravel, sand, silt, and clay.It can also include some organic material and of course will contain air and water.

These examples show that any soil deposit con-tains water and air in addition to the solid soilparticles. The voids that are filled by the air andwater provide pathways for water to flow throughthe soil. By reducing the number of voids, the soilwill contain fewer pathways for flow.

2.5 COMPACTIONThe sand sample contains large amounts of water andair. The only change from its natural condition afteradding water is the application of pressure by hand.What can be done to reduce the amount of air and

water in the sample to allow fewer spaces for flow?Squeezing on it harder seems to be a logical answer;that is, applying more pressure to the sample.Applying greater pressure to the sample will reduce theamount of space available for air and water in twoways:

1. The air between grains can be squeezed out, reduc-ing the void areas.

2. Fragments are broken off from some particles, orparticles are deformed, allowing the void area tobe reduced further.

6

MAGNIFIED VIEWFIGURE 2.4-2 SAND STRUCTURE

MAGNIFIED VIEWFIGURE 2.4-3 SILT STRUCTURE

MAGNIFIED VIEWFIGURE 2.4-4 CLAY STRUCTURE

Very small droplets of water and air bubbles take up a por-tion of the area viewed.

MAGNIFIED VIEW

FIGURE 2.4-5 STRUCTURE OF NATURAL DEPOSIT

FIGURE 2.5-1 REDUCING VOIDS



FIGURE 2.5-2 BREAKAGE CAUSES ADDITIONALREDUCTION OF VOIDS.

Also note that the sand grain surfaces in contact witheach other have increased after the application of addi-tional pressure.Compaction is usually obtained by applying an impactforce to the surface of the soil as shown in the exam-ple below. The soil used will contain equal parts of sand, silt, and clay.

FIGURE 2.5-3 THE COMPACTION PROCESS

FIGURE 2.5-4 COMPACTION IN LAYERS

In Figure 2.5-3c, a tamping tool is used to pound a 3-inch layer of soil with enough force to create animprint about 1 ⁄ 2 inch deep. The pounding is continueduntil the entire surface of the sample has been com-pacted. (See Figure 2.5-3 d).Now the soil layer that was 3 inches thick is only 2 1 ⁄ 2inches thick, although it contains the same amount of soil particles. Most of the air has been forced out of the soil sample.In order to fill the container with a soil of uniformdensity, successive 3-inch layers are placed, thencompacted to a 2 1 ⁄ 2-inch thickness, as shown in Fig-ure 2.5-4.

Compaction is required to attain a number of positivechanges in a soil:The area available for water to flow through is reduced.The strength of the soil is increased because of theincreased particle-to-particle contact.The properties of the soil become more predictable.

The soil has increased resistance to erosion.The soil provides more weight per volume (more solidparticles).

2.5-1 PLACEMENT IN LAYERS —was used iorder to obtain a uniformly dense material. Sinceplacing the materials in layers requires a lot of work, is it necessary? Let us take a look at a com-paction procedure similar to the one above; how-ever, this time the container will not be filled inlayers, but will be filled close to the top before thematerial is pounded with the tamping tool.

FIGURE 2.5-6 RESULT OF COMPACTION WITHOUT

CONTROLLED LAYERS

Figure 2.5-5d shows that only the upper por-

tion of the material has become compacted, themiddle portion shows only a small change fromthe loose state, and the bottom portion of thesample is unchanged from the loose state (Fig-ure 2.5-5a). The primary reason for this is theway the impact from the tamping tool is trans-mitted through the soil.

FIGURE 2.5-5COMPACTION WITHOUT LAYERS



FIGURE 2.5-6 TRANSMISSION OF COMPACTION FORCE

The pressure directly under the foot of the tool is fair-ly uniform; however as the depth increases, the impactis spread out over a greater area and has less effecton the soil. Also, in the lower parts of the sample, theimpact is not great enough to cause the grains to break or move against each other.

The examples shown in Figures 2.5-3 through 2.5-6illustrate why it is necessary to place a soil in con-trolled layers in order to obtain a uniformly densematerial. The thickness of the layer that can be effec-tively compacted depends on the type and weight of the equipment being used. Figures 2.5-7 through 2.5-11 show various types of compaction equipment.

2.5-2 VIBRATION AIDS IN COMPACTIONSoil materials having contact surfaces that resist par-ticle-to-particle movement can be compacted moreeffectively when vibration is applied. Usually rocks,cobble, gravel, and sands require vibration in addi-tion to impact force in order to obtain the desiredcompaction. Figure 2.5-11 shows a vibrating rollerwhich is equipped with a mechanism that causes theroller drums to vibrate as it passes over the materialbeing compacted.

2.5-3 COMPACTION EQUIPMENTSome of the more common compaction equipmentare shown here.

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FIGURE 2.5-7—POGO STICK, SIMILAR TO TAMPING TOOL.—Isused in tight spots not accessible to larger equipment.

FIGURE 2.5-10—SELF-PROPELLED ROLLER.—Used in open areas.

FIGURE 2.5-9—SHEEPSFOOT ROLLER.—Used in open areas.

FIGURE 2.5-8—WACKER.—Is used in areas larger equipment cannotreach.



As can be seen, the pogo stick and wacker aremuch smaller and lighter than the sheepsfootand self-propelled roller. Soils compacted withthe lighter equipment must be placed in thinlayers not exceeding 4 inches, while soils com-pacted by the heavier equipment can be placedin layers up to 12 inches.

2.6 MOISTURE CONTROLThe amount of moisture (water) contained in a soilprior to placement determines several important char-acteristics of the material during and after compaction:

How easy the material is to work with (e.g.,not mud or dust).How much effort will be required to obtain thegreatest practical density.How dense a soil can be made by a givenamount of compactive effort.

The moisture content of the soil under considerationfalls into three categories:

Optimum (just right) —When the moisturecontent is at the Optimum Level, there is justenough moisture to allow most grains or parti-cles of soil to move or slide into a more com-pact structure while the air filling a significantportion of voids in the soil is forced out.

AT OPTIMUM MOISTURE CONTENT, A GIVENCOMPACTIVE EFFORT WILL PRODUCE THEGREATEST DENSITY.

Too Dry —When not enough moisture is pres-

ent, the grains or particles resist change ormovement between surfaces and the materialdoes not reach the desire density after appli-cation of a specified compactive effort.Too Wet —When too much water is present, itallows water to fill a large portion of the voidsin the sample. When compactive effort is applied,the pressure is transferred to the water and air,resulting in little change in the soil structure.

2.7 COMPARISON OF PATHWAYS AVAILABLEFOR THE FLOW OF WATER THROUGH AGIVEN MATERIALWe have seen how the structure of sand, silt, and clayprovide very different patterns or pathways for theflow of water through the soil. In order to give a com-parison of the area available for water to flow throughvarious materials, the following illustration is pre-sented. These commonly used materials will be shown:

Sand—properly compactedSilt—properly compactedClay—properly compactedConcrete—properly compactedAsphalt—properly compactedOne-fourth inch steel plate—in new condition

To establish a basis for quick comparison, a shadedsquare will be used to represent the portion of solidparticles and the portion of voids (normally filled bywater or air) of a given area of a material we are view-ing. The area representing the solid portion is shadedand the void area is clear.

FIGURE 2.7-1—COMPARISON OF AREA AVAILABLE FOR WATERTO FLOW THROUGH. (NOT TO SCALE)

FIGURE 2.5-11—VIBRATORY ROLLER.Drums vibrate as roller moves.



are waterproof. Both tanks are filled with sand. Eachtank filled with water; then the valve is opened. Byregulating the inflow, the water level is kept at PointA for both tanks. The flow path is much longer in thetank with baffles.

2.11 SUMMARY

1. Soil is an essential construction material.

2. Particle size and type of soil determine many of itsconstruction properties.

3. Soil composition and compactness influences how

water can flow through that soil.4. compaction of a soil reduces the area available

for water to flow through and increases soilstrength.

5. Controlled layer placement is required for prop-er compaction.

6. Moisture control is vital to producing the mostdense compacted soil material.

7. Water pressure at a point increases directly as theheight of water above the point increases.

8. Water pressure at a point will cause flow in any

direction if a flow path is available.9. Lengthening the flow path through a given mate-

rial reduces the amount of water that will flowthrough the material.

FIGURE 2.10-2 ANOTHER WAY TO INCREASE THE FLOW PATHAlthough the height of water above Point B is equal for both tanks the flowfrom the tank having the longer flow path will be much less.



1. Upstream slope—This is the part of the dam that is in contact withthe reservoir water. On earthen dams the upstream slope must beprotected from the erosive action of waves by rock riprap, concreteor some other material.

2. Crest—The top part of the dam. Usually a roadway is establishedacross the crest to provide access to the dam for operation, inspectionand maintenance.

3. Downstream slope—This is the slope or face of the dam away fromthe reservoir water. This area requires some form of protection, suchas grass from the erosive effects of rain and surface flows.

4. Principal spillway—Allows discharge of water from the reservoirwhen the water level exceeds the top of the principal spillway. Prin-cipal spillways are used to allow small inflows to be released fromthe reservoir.

5. Emergency spillway—Allows the inflow from large storms to bereleased from the reservoir before the water level rises high enoughto overtop the dam.

6. Outlet discharge structure—This structure protects the downstreamend of the outlet pipe from erosion and is often designed to slow downthe velocity of released water to prevent erosion of the stream chan-nel.

7. Outlet Control House—Protects the operating mechanism for theoutlet control valve from weather and from vandalism.

8. Energy Dissipator—This structure slows the fast moving spillwayflows in order to prevent erosion of the stream channel.

9. Stilling Basin—An area where spillway flows become equal to streamflow velocities.

10. Outflow Channel—A natural stream channel that transports reser-voir releases.

11. Reservoir—A man made facility for the storage and controlled releaseof water.

12. Log and Safety Boom—A net like device installed to prevent logs,debris or boaters from entering a water discharge facility.

13. Gage Rod—A measuring device that shows the water level in thereservoir.

14. Horizontal and Vertical Control Points—These are small monumentsthat are securely embedded in the surface of the dam. Any move-ment of the monument indicates a movement in the dam itself. Move-ments are detected by using accurate survey procedures.

15. Permanent Monument—Fixed monuments placed away from thedam which allow movements in the control points to be observed byusing accurate survey procedures.

16. Piezometer (Open Well)—Allows the level of saturation within thedam to be measured.

17. Toe Drain and Outfall—Carry seepage water away from the sam andcan allow seepage quantities to be measured.

18. Abutment Seepage—Reservoir water that moves through seams orpores in the natural abutment material and exits as seepage.

19. V-Notch Weir—A device for measuring the flow of water.20. Seepage Outflow Ditch—A shallow ditch that is excavated to prevent

seepage water from collecting in ponds.21. Access Road—A roadway that provides access to the dam.22. Access Gate—A sturdy gate to prevent unauthorized access and dis-

courage vandalism.23. Turn Around—Provided for the convenience of maintenance vehicles.

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3.2 FUNCTIONS OF A DAMThe primary requirements of a dam in order to holdback water safely are:

1. Contain the water and resist leakage.

2. Maintain shape.

3. Resist movement in a downstream direction.The greater the depth of stored water behind the dam,the greater the pressures are and the greater the resist-ance must be to leakage and movement. Resistanceto leakage is important since the purpose of the damis to store water. Resistance to movement is impor-tant too, because the pressure of the stored water tendsto push the dam downstream. For example, the sketchbelow shows a concrete dam that would move down-stream if it were not designed to resist the pressureof the water.

FIGURE 3.2-1CONCRETE DAM WITH LOAD FROM RESERVOIR

The function of maintaining shape is more related todams constructed of earth material or rock. The shaperefers to the outline of the dam or the profile. The final

shape of the dam is a result of design which determinesthe amount of material necessary to resist leakage andmovement. If the shape of the dam changes, it may nolonger be able to perform its required functions. Thesketches below show that a slide has occurred and thedam can no longer hold back the water.

FIGURE 3.2-2EARTH DAM—FAILS TO HOLD BACK WATER

The dam surroundings must also function as a unit. Thesite on which the dam is constructed becomes a partof the dam. The dam site has three important parts:

1. The foundation

2. The abutments.

3. The reservoir basin.The site is an integral part of the dam and must houp the structure as well as help hold back the water

3.2-1 THE FOUNDATIONThe part of the dam site which must supportthe dam is the foundation. Although other fac-tors are involved, the first task of the foundationis to provide firm support for the entire dam. Asoft foundation, for example, would not sup-port the weight of the dam.

FIGURE 3.2-3FOUNDATION CONDITIONS

Because the purpose of the dam is storage of water, the foundation must also resist the flowof water under the structure. A clay materialor unfractured hard rock, for example, wouldresist the flow of water under the structuremuch more effectively than sand or gravel.

3.2.2 THE ABUTMENTS AND RESERVOIRBASINThe other parts of the site which must be asstrong as the foundation and resist the flow of

water are the abutments. The abutments are theareas where the ends of the dam meet with thesurrounding terrain.

FIGURE 3.2-4THE DAM SITE

The abutments must offer support to the struc-ture in the length-wise, upstream-downstream,and vertical directions.The basin behind the dam or the area coveredby the reservoir is just as important as the damitself. Its size and shape determine the volume of the reservoir. Like the dam, the foundation, andthe abutments, the basin must contain the water.

3.2-3 THE DAM SITEUnfortunately, nature does not always providethe most ideal site for a dam. Therefore, specialdesigns and treatments of the site are necessary

14



in some cases to produce a useful storage facil-ity. Many factors are involved in selecting thedam site.The dam site must be able to:1. Hold up the dam.2. Hold the dam in place.

3. Hold back water.Many times the type of dam selected is a resultof the site conditions. Even though the bestefforts are employed in site selection, design,and construction, all dams will experience someseepage.Three types of dams are discussed in this chap-ter: earth, rockfill, and concrete. The discussionincludes typical sections identifying the parts of each, the role of each part, and how each type of dam functions to achieve the primary purposeof holding back water safely.

3.3 REVIEW

The following list is provided for easy review and quick reference when looking at the details of the three typesof dams:Required Functions of a DamAll dams must:1. Contain the water and resist leakage.2. Maintain shape.3. Resist downstream movement.

The foundation, abutments, and reservoir basin must:

1. Hold up the dam.2. Hold the dam in place.3. Hold back the water.

Important Physical Principles

1. The greater the depth of water is above any point,the greater the pressure is at that point.

2. Water travels in streams, drops, and tiny droplets.Because the water molecule is small, special pre-cautions must be taken to prevent excessive flowof water through earthen materials.

3. The amount of water that will move through a soilis determined by:

a. The height of water above the point being con-sidered.

b. The type of material.c. The amount of space available for the water to

flow through the soil after compaction of thatmaterial.

d. The length of the flow path.4. Proper moisture control allows an earthen materi-

al to be compacted efficiently. Proper compactionresults in the fewest paths for seepage and the great-est strength.

5. Steel, concrete, and asphalt provide almost totalresistance to the flow of water.

3.4 EARTH DAMSUnderstanding the physical principle of how waterbehaves under pressure is primary to understanding thebehavior of an earth dam. Let us look at a single dropof water at some depth in the reservoir. The pressuretrying to force the drop of water through the dam isproportional to the height of water above the drop. Thesketch below shows a droplet of water at Point A in thereservoir. The pressure forces the droplet through thedam to Point B.

As the drop of water is pushed horizontally throughthe soil of the dam, gravity is also pulling the dropdownward. The path is similar to that shown in theabove sketch. Now, if we follow the paths of severaldrops of water at different depths in the reservoir, thepattern traced by the droplets is similar to that shownin the following sketch.

The sketch above illustrates the concept of seepagethrough an earth dam. If the area available for flow ina given soil structure is changed, the paths will change.The items that will influence the area available for floware material grain size and amount of compaction. If the material selected has larger grain size, or if it is notwell compacted, the result is an unacceptable amountof seepage as shown in the following sketch.

FIGURE 3.4-1SEEPAGE PATH

FIGURE 3.4-2FLOW PATH



The first two sketches represent undesirable condi-tions because uncontrolled seepage is exiting on thedownstream slope of the dam. The situation developsbecause the structure does not provide sufficient resist-ance to the flow of water. Gravel and sand, because of their larger grain size, have more area available forflow than finer-grained material such as clay. Com-pacting clay soils further reduces the area availablefor flow. As the water moves through the soil structure,some of the pressure which is pushing it through thedam is dissipated. The path of the water must be madelong enough to use up all the pressure pushing on thedrop at point A by the time the drop reaches point B.

We have seen how the grain size of the material andthe amount of compaction influences the area availablefor flow. For a given material, the length of the flowpath will determine the amount of water that flowsthrough as seepage. In the next section we will seehow various configurations are used to control seep-age, allow the dam to maintain its shape, and preventdownstream movement.

3.4-1 TYPICAL CONFIGURATIONSThe selection of shape and materials allows aneffective dam to be constructed of locally avail-able material. The following cross-sections rep-resent three most common earth embankments.

The three different cross sections are designedfrom knowledge of the principles used to con-trol the path of the water through the dam:

1. Selection of material by grain size.2. Compaction3. Elongation of the path by increasing the base

width.The homogeneous dam, for example, controlsthe flow of water by lengthening the path. Thepath is lengthened by increasing the base width;however, without drains seepage can exit thedam above the toe. The zoned dam providescontrol of the flow of water with a core hav-ing a very high resistance to flow.The sketches below show the difference in thepaths of water between a homogenerous damand a zoned dam.

The middle section (core) of the zoned dam hamore resistance to flow and dissipates the pres-sure caused by the reservoir in a short distance.In the case of the homogenerous dam, theheight of visible seepage water near the toe canexit as high as 1/3 the height of the reservoirwater surface. The appearance of water on thedownstream slope can endanger the safety of the structure. Therefore, every effort is madein design of a dam to minimize the chance forwater to exit on the downstream face.In the case of the zoned dam with draincoarse-grained material is installed which haslittle resistance to the flow of water.

16

FIGURE 3.4-3MATERIALS INFLUENCE FLOW PATH

FIGURE 3.4-4RESISTANCE TO FLOW

FIGURE 3.4-5TYPICAL CROSS SECTIONS

Type: Homogenerous Zoned Zoned w/DrainSystem

Characteristics Constructed of Constructed of Drain system of one material. two or more granular material

mater ia ls . Centra l a llows the con-core or midsec- trolled exit of tion has greater seepage.resistance to flow.

FIGURE 3.4-6TYPE OF CROSS SECTION INFLUENCES FLOW PATH



3.5 ROCKFILL DAMSMost rockfill dams are similar in shape to earth dams.The difference is that rock fragments make up the pri-mary material used for construction. The choice of constructing a rockfill dam versus constructing anearth dam is usually based on availability of materials.The sketch below illustrates one configuration of arockfill dam.

FIGURE 3.5-1TYPICAL ROCKFILL CROSS SECTION

Because rock fragments alone would leave large open-ings for seepage flow, a central core, like that in thezoned earth dam, is required. Also, note that the coreextends into the foundation to help control the flowof water under the dam. A transition zone is usuallynecessary to protect the core from erosion. The tran-sition zone is designed to keep the fine-grained corematerials from being washed into or through the rock-fill.

3.6 CONCRETE DAMSConcrete dams are the least common types in Col-orado. Concrete is probably the most durable materi-al for building dams and has a very high resistance toseepage.A concrete dam is unique in that it directly transferssome of the pressures created by the stored water to thefoundation and abutments. A concrete dam, therefore,is very dependent upon the ability of the foundation

and abutments to hold the dam in place.There are two basic designs for concrete dams: grav-ity and arch. The sketches below show how the two aretied into the foundation and abutments.

3.6-1 CONCRETE GRAVITY DAM

FIGURE 3.6-1CONCRETE GRAVITY DAM

Without a proper key or tie into the abutmentsand foundation, and without enough weight, agravity dam could tip over or slide downstreamas shown below.

FIGURE 3.6-2GRAVITY DAM MUST RESIST RESERVOIR LOADS

The choice of a concrete gravity dam over othertypes is based on the available material and thesite conditions.

3.6-2 CONCRETE ARCH DAM

FIGURE 3.6-3—CONCRETE ARCH DAM

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4.1 WHAT AND WHY SO IMPORTANTVisual inspection performed on a regular basis is themost economical aid a dam owner can use to assure thesafety and long life of the structure. Visual inspectionis a straightforward procedure that allows any properlytrained person to make an accurate assessment of adam’s condition. The inspection involves careful exam-ination of the surface on all parts of the structure. Theequipment required is not expensive and the inspectionusually can be completed in less than one day.The essentials of the inspection will be discussed aftera look at useful inspection equipment and inspectionfindings that must be recorded.

4.2 INSPECTION EQUIPMENT AND ITS USE

figure 4.2-1 inspection equipment—Notebook, pencil, hand-level, cam-

era, tape recorder and tape, binoculars, probe, hard hat, pocket tape,flashlight, bonker, shovel, rock hammer, bucket and stopwatch, bug spray,flagging tape and stakes, waterproof boots.

INSPECTION CHECK LIST—Serves as a reminderto inspect for all important conditions. An example ispresented a the end of this chapter.NOTEBOOK AND PENCIL—It is very important towrite down observations at the time they are made.This reduces mistakes and the need to return to thearea to refresh the inspector’s memory.TAPE RECORDER—A small portable tape recordercan also be used effectively to make a record of fieldobservations.

CAMERA—Photographs provide a reliable record of observed field conditions. They can be valuable incomparing past and present configurations. An inex-pensive model usually takes pictures good enough forinspection records.HAND LEVEL—This is needed to locate accuratelyareas of interest and to determine embankment heightsand slope.PROBE—A probe can provide information on con-ditions below the surface, such as the depth and soft-ness of a saturated area. Also, by observing moisturebrought up on the probe’s surface, the inspector candecide whether an area is saturated or simply moist.

An effective and inexpensive probe can be made byremoving the head from a golf club.HARD HAT—A hard hat is recommended for inspect-ing large outlets or working in construction areas.POCKET TAPE—Many descriptions are not accurateenough when estimated or paced. The pocket tape pro-

vides accurate measurements which allow meaningfulcomparisons to be made.FLASHLIGHT—The interior of an outlet in a smalldam can often be inspected adequately without crawl-ing through by using a good flashlight or fluorescentlantern.SHOVEL—A long-handled shovel is useful in clear-ing drain outfalls, removing debris, locating monitor-ing points, and killing snakes and rodents.ROCK HAMMER—Questionable-looking riprap orconcrete can be checked for soundness with a rock hammer. Care must be taken not to break through thinspots or cause unnecessary damage.

BONKER—The condition of support material behindconcrete or asphalt faced dams cannot be determinedby observing the surface of facing. By firmly tappingthe surface or the facing material, conditions belowcan be determined by the sound produced when thematerial is tapped. Facing material fully supported byfill material produces a “click” or “bink” sound, whilefacing material that is over a void or hole in the fac-ing produces a “clonk” or “bonk” sound. The bonkercan be made of 1 1 ⁄ 4-inch hard wood dowel with a metaltip firmly affixed to the tapping end. A rubber shoelike those on some furniture legs is recommended forthe other end to allow the bonker to be used a a walk-ing aid on steep, slippery slopes.

BINOCULARS—These are useful for inspecting lim-ited access areas especially on concrete dams.GALLON CONTAINER AND TIMER—These areused to make accurate measurements of leakage flows.Establishing the time it takes the seepage flow to fillthe gallon bucket enables the inspector to calculatethe number of gallons per minute. Various containersizes may be required, depending on the flow rates.STAKES AND FLAGGING TAPE—These are used tomark areas requiring future attention and to stake thelimits of existing conditions, such as cracks and wetareas, to allow future comparison.WATERTIGHT BOOTS—These are often requiredwhen inspecting various areas of the dam site wherestanding water is present.BUG REPELLENT—Biting bugs can gravely reducethe efficiency and effectiveness of the inspector andsour his disposition.SNAKE BITE KIT—In areas where rattlesnakes orother poisonous snakes might be present, it is recom-mended that a snake bite kit be kept handy.

CHAPTER 4

VISUAL INSPECTION



4.3 INSPECTION OBSERVATIONS THAT MUST BERECORDEDThe visual inspection is performed to allow the damowner to be knowledgeable about the condition of hisdam and to allow any problems to be identified whenthey begin to develop. An accurate and detaileddescription of the condition of the dam observed dur-ing each inspection will make possible meaningfulcomparisons of conditions.All measurements and descriptive details that arerequired to portray an accurate picture of the dam’scurrent condition must be recorded. This informationfalls into three categories:LOCATION—The location of any questionable areaor condition must be accurately described to allowthat area or condition to be properly evaluated. Thelocation along the length of the dam, as well as heightabove the toe or distance down from the dam’s crest,must be established and recorded. The same applies toconditions associated with the outlet or spillway.EXTENT OF AREA—The length, width, and depth orheight of any area where a suspected problem is found.DESCRIPTIVE DETAIL—A brief yet detaileddescription of a condition or observation must begiven. Some description items are:QUANTITY OF DRAIN OUTFLOWSQUANTITY OF SEEPAGE FROM POINT ANDAREA SOURCESCOLOR OR QUANTITY OF SEDIMENT INWATERDEPTH OF DETERIORATION IN CONCRETELENGTH, DISPLACEMENT, AND DEPTH OFCRACKSIS AREA MOIST, WET, OR SATURATED?IS PROTECTIVE COVER ADEQUATE?IS SURFACE DRAINAGE ADEQUATE?DO SLOPES LOOK TOO STEEP?DOES DETERIORATION APPEAR TO BERAPID OR SLOW?HAVE CONDITIONS CHANGED?The above listing of inspection findings that must berecorded is not meant to be a complete list but is toserve as a guide. Keep in mind that if the inspectorthinks a condition has changed since the last inspec-tion he must make note of it. He should also take aphoto and put it into his file, carefully noting the dateand writing a description of the scene shown onthe photo.

4.4 SIGHTING TECHNIQUEA sighting technique similar to that used when select-ing straight pieces of lumber can be useful in identi-fying misalignments as well as high or low areas alonga surface. The technique is illustrated.

FIGURE 4.4-1 SIGHTING TECHNIQUE

When checking alignment on parts of the dam centereyes along the line being viewed. Sighting along theline, move from side to side a little to view the linefrom several angles.For example, sighting along the edge of the crest:

FIGURE 4.4-2 SIGHTING ALONG CREST

Looking through a pair of binoculars will help to makechanges more obvious.

4.5 SOME TYPICAL PROBLEMSVarious conditions are seen during the inspection thatmay indicate a developing problem. A few of thesewill be discussed here to demonstrate what the inspec-tor needs to do and how the equipment should behelpful.

22



24

Drain outfalls should be examined tomake sure there are no obstructions inthe pipe. The flashlight and probe areuseful here. Flow quantities requireaccurate measurement. Bucket meas-urements or a “V” notch weir are usu-ally used for these measurements. Out-flows should be examined for soilparticles.

The entire abutment area downstreamfrom the dam should be carefullyinspected for leakage. Special atten-tion must be given to the contactbetween the man-made embankmentand the natural terrain. Leakage at thecontact can cause erosion of embank-ment material.

4.5-6 SLOPE PROTECTIONRiprap is designed to protect the earth-en embankment material against waveaction. When deteriorated areas arediscovered, the extent and threat to theembankment materials need to bedetermined. Photos should be taken.

When the rock used for riprap is break-ing down, the extent of the damagemust be assessed to determine if theembankment is threatened. The rock hammer can be used to see how soundand durable the rock seems to be.

Holes or cracks in the upstream fac-ing can allow underlying embankmentmaterial to be pumped out by waveaction. The entire facing must beexamined for small openings, espe-cially along construction joints.

Voids under upstream facings canallow the facing over the void to col-lapse, jeopardizing the safety of thedam. To check for these undetectedvoids, the bonker is used. Bonkie-sounding areas should be flagged forfurther investigation.

Deteriorated or spilled areas must beexplored with the rock hammer todetermine if immediate repairs arerequired. Even if the remaining mate-rial seems sound, concrete must berepaired if the reinforcing is exposed.

Channel protection displaced by highvelocity flows can lead to rapid dete-rioration in the unprotected area.Extent of unprotected area and rate of deterioration should be determined.The probe and flagging stakes can beuseful here.

4.5-7 CONCRETE STRUCTURESAll deteriorated areas must be care-fully examined and checked with arock hammer. The objective is to deter-mine if the deterioration affects thefunction of the structure. Location andextent of deterioration should be notedand photos taken.

Displacements can be found by sight-ing. All apparent displacements shouldbe recorded to allow a worsening con-dition to be verified during futureinspections. Cracks must be exploredto see if reinforcing is exposed, sincerapid deterioration of reinforcing canlead to failure of the structure. Photosshould be taken.

4.5-8 STEEL—METALLocation and extent of cracks, holes,or deteriorated areas must be record-ed. These areas may require scrapingclean to allow an assessment of theextent of the deterioration. Photosshould be taken.

4.6 INSPECTION PROCEDURETo obtain the best results and allow for consistent record-ing of inspection findings, it is best to follow a specif-ic sequence when making the inspection. Before dis-cussing the sequence a few ideas on technique will bediscussed.



3. Equipment should be assembled.

4. Entry and meeting arrangements for personsaccompanying the inspector should be made.

5. Provisions for outlet shut off and dewatering, if required, should be made.

6. A good weather day, if possible, should be chosen.4.10 CRUCIAL INSPECTION TIMES

There are a few special times when an inspection isrecommended:

1. Prior to a predicted major rainstorm or heavy snowmelt: check spillway, outlet channel, and riprap.

2. During or after severe rainstorm: check spillway,spillway and outlet channel, and riprap.

3. Severe windstorm: check riprap performance dur-ing a storm and after it has subsided.

4. Earthquake in area: make complete inspection

right after the event and weekly inspections forthe next six weeks to detect any delayed effects.

5. First filling: a regular schedule of frequent com-plete inspections must be scheduled during theperiod the dam is being filled for the first time.This activity is to assure that the design and siteconditions are serving as predicted. The inspec-tion and filling schedule are prescribed by thedesign engineer and approved by the State Engi-neer.

4.11 ADDITIONAL CONSIDERATIONSEND OF CONSTRUCTION—The end of construc-tion condition is very important. Surfaces that areshown as plane or uniform surfaces on the drawingsmust be as close as possible to a plane surface at theend of construction. An uneven surface prevents accu-rate observation of change in that surface.KEEP THE DAM CLEAR OF OVERGROWTH—Overgrown areas of the dam are difficult to inspectand do not get adequately inspected. All portions of thedam and an area extending a minimum of 20 feetbeyond the downstream toe should be kept clear of obscuring growth. Protective grass cover should beencouraged.

4.12 SUMMARYThis chapter briefly outlined the general procedurefor visual inspection of a dam. Visual inspection isthe most economical aid the dam owner can use toassure the safety and long life of his dam.

In the chapters that follow, specific problems occurringon various portions of the dam are discussed.

4.13 INSPECTION REPORT AND FORMSTwo different inspection report forms are included.These forms can be reproduced and used or may serveas an aid for the owner in developing his own form.

26



DAM INSPECTION REPORT

Name of Dam __________________________________ C: _______ Date: ______________ Division ______ Dam ID ____________

Type of Dam (circle): EARTHFILL, ROCKFILL, CONCRETE, OTHER________________________________________________________

Estimate Actual Capacity: ____________________________________ Estimate Surface Area: ___________________________________

Estimate Height: ____________________________________________ Gage Rod Reading: ______________________________________

Waterlevel—Feet Below Spillway: _____________________________ Estimate Spillway Width: _________________________________

Estimate Freeboard (Spillway to Top of Dam): ____________________________________________________________________________

Use: IRRIGATION, MUNICIPAL, OTHER ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

DIRECTIONS: Mark an “X” in the Yes or No column and circle the word or phrase which applies.

1. Are the roads to the dam adequate to allow ACCESS BY EMERGENCY EQUIPMENT and TRAVEL ACROSS THE DAM(i.e., TRUCKS, AMBULANCES)?

2. Is there DEBRIS, TREES, or BRUSH on the upstream slope that prevent seeing the entire surface of the slope?

3. Are there TREES or BRUSH on the CREST, or DOWNSTREAM SLOPE that prevent seeing the entire surface of the slope?

4. Are there CRACKS, SLIDES, SLUMPS, BOILS, SETTLEMENT or OTHER on the UPSTREAM SLOPE, CREST, orDOWNSTREAM SLOPE?

5. Are there RODENT HOLES or ERODED GULLIES on the UPSTREAM or DOWNSTREAM SLOPE?

6. Is the upstream slope eroded from wave action?

7. Is there FLOWING WATER or LARGE BOGGY SPOTS at the toe of the dam?

8. Are there FLOWS OF WATER or WET SPOTS above the toe of the dam?

9. Is the riprap DISPLACED or BROKEN DOWN or MISSING?

10. Are there toe drains?

11. Is the water from the TOE DRAINS or LEAKS found to be MUDDY or SANDY?

12. Are any of the concrete portions excessively CRACKED or SPALLED?

13. Is the OUTLET CONTROL or GATE found to be STUCK, BROKEN, or EXCESSIVELY CORRODED?

14. Is the outlet control easy to get to?

15. Is released water UNDERCUTTING THE OUTLET or ERODING THE EMBANKMENT?

16. Does the spillway channel show significant EROSION, BACKCUTTING or DETERIORATION?

17. Is the spillway obstructed with FLASHBOARDS, TREES, DEBRIS, BRUSH or OTHER?

18. Is there evidence that the dam has been overtopped?

19. Are spillway WALLS, FLOOR, CONTROL SECTION, and ENERGY DISSIPATOR in POOR condition?

20. Is the outlet pipe BLOCKED, BROKEN, or EXCESSIVELY CORRODED or OTHER?

21. Is the reservoir usually full YEAR ROUND, OVER 1/2 OF YEAR, or LESS THAN 1/2 OF YEAR?

22. Should this dam be promptly inspected by a field engineer from the State Engineer’s offices?

23. Additional Comments: ________________________________________________________________________________________________

______________________________________________________________________________________________________________________

______________________________________________________________________________________________________________________

______________________________________________________________________________________________________________________

Inspected By: __________________________________________

Yes No



DAM INSPECTION REPORT FORM

NAME OF DAM ___________________________________________ DATE: ________________________________________________

DAM HEIGHT: ____________________________________________ (FT) MAX. RES. CAPACITY: ________________________(A.F.)

MAXIMUM GAGE ROD: ________________________________(FT) TODAY’S GAGE HEIGHT: ___________________________(FT)

NOTE:

a) Enter 1 below if: No problems found in this area, the whole area looks all right.

b) Circle items of particular concern.

UPSTREAM SLOPE: ________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

CREST ____________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

DOWNSTREAM SLOPE______________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

SEEPAGE AREAS __________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

OUTLET __________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

SPILLWAY ________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

REQUIRED MAINTENANCE OR ACTION ______________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________

INSPECTOR’S SIGNATURE _________________________________________________________________________________

Page 1 of



UPSTREAM FACE

CRESTDOWNSTREAM FACE

PROTECTION

UNIFORMITYDISPLACEMENTSCRACKINGEROSION

RODENT ACTIVITY

OBSCURING GROWTHWETNESSCHANGES IN CONDITION

AREAS OF DAM ITEMS TO ADDRESS

SEEPAGE LOCATIONCHARACTERISTICS OF AREA(i.e. SOFT, BOGGY, FIRM)QUANTITY

TRANSPORTED ORDEPOSITED MATERIAL

EFFLUENT QUANTITY ANDCOLOR

EXTENT OF AREACONCENTRATED FLOWSBOILSCOLOR

TOE DRAIN

OUTLET DETERIORATIONACCESSIBILITYCONDUIT LEAKABE

AROUND CONDUIT

OPERABILITYCONDITIONGATE LEAKABE

UNDERCUTTING

SPILLWAY DETERIORATIONCONDITION OF CONTROL

SECTION

CHANNEL PROTECTION

CHANNEL OBSTRUCTIONSEROSION OR BACK

CUTTING IN CHANNEL

DAM INSPECTION REPORT FORM CHECK LIST

Page 2 or



5.1 INTRODUCTIONAn earth dam is judged to be effective or ineffectivebased on its ability to form a water barrier. In Chap-ter 2, we learned how different soils can help form abarrier. In Chapter 3, various configurations of anearth embankment were shown along with the pathswater will take through the embankment. Techniquesand procedures for inspecting the dam were present-ed in Chapter 4.Seepage is discussed before any other portion of thedam because detailed observation of the location andextent of seepage water appearing on the ground sur-face can provide accurate information about a dam’sperformance and condition. By correctly interpretingseepage patterns, the owner can identify and correctmany potential problems before they threaten the safe-ty of the dam. Every area that is producing or has pro-duced seepage must be identified and continually mon-itored. A record of the extent of flow areas and flowquantities must be kept.

5.2 ITEMS OF PARTICULAR CONCERNExcessive seepage can jeopardize the dam in two ways:HIGH VELOCITY FLOWS THROUGH THE DAMCAN SET UP PROGRESSIVE EROSION OF THEEMBANKMENT MATERIALS, LEADING TOFAILURE OF THE DAM.SATURATED AREAS OF THE EMBANKMENT ORABUTMENT CAN MOVE IN MASSIVE SLIDESAND LEAD TO FAILURE OF THE DAM.Seepage in any area on or near the dam can be dan-gerous. All seepage should be treated as a potentialproblem.

5. SPECIAL INSPECTION TECHNIQUESSEARCH—Since seepage can be present but not seen,an intensive search must be made of all downstreamareas where seepage water might emerge. Even inshort grass cover, the seepage may not be seen andmust be walked over to be found.

PROBING—Probing can help identify the limits of saturated areas, the degree of wetness or saturation,and the depth of softened surface materials.DIFFERENCE IN VEGETATION—Vegetative growthin moist or saturated areas is often of a different typethan surrounding areas or has a deeper green color ormore lush nature than drier neighboring areas. Whena difference is noted the greener area must be care-fully examined for seepage.TRACKS—The depth of footprints the inspectorleaves in wet or saturated material shows how softsurface materials have become.

5.4 FIELD CONDITIONS ENCOUNTERED

The following table presents most of the harmful con-ditions caused by seepage that will be found at anactual dam site. For each condition the cause, harmdone, and required corrective actions are given. Anyof the conditions that require assessment by an engi-neer should be reported to the State’s Dam SafetyEngineer at the time it is discovered.

5.5 SUMMARYCareful inspection for and accurate assessment of seepage can help the dam owner identify potentialproblems before they threaten the safety of the dam.Regular inspection of the dam can identify any changesin previously noted conditions that may indicate asafety problem.Quick reaction to conditions requiring attention willpromote the safety and long life of the dam and pos-sibly prevent costly repairs.

CHAPTER 5

SEEPAGE



PROBLEM

5.4-1

EXCESSIVE MUDDY WATEREXITING FROM A POINTSOURCE

5.4-2

EXCESSIVE AMOUNT OFWATER EXITING FROM APOINT SOURCE

5.4-3

WATER EXITING FROM APOINT SOURCE HIGH ON THEEMBANKMENT

CAUSES & HARM DONE

Cause :

1. Water has created an open path-way, channel, or pipe through thedam. The water is eroding and car-rying embankment material.

2 Large amounts of water haveaccumulated in the downstreamslope. Water and embankment mate-rials are exiting at one point. Sur-face agitation may be causing themuddy water.

Harm :

Continued flows can further erodeembankment materials. This can leadto failure of the dam.

Cause:

Water has created an open pathwayor pipe through the dam.

Harm:

Continued flows can further erodeembankment materials. This can leadto failure of the dam.

Cause:

1. Rodents, frost action, or poor con-struction have allowed water to cre-ate an open pathway or pipe throughthe embankment.

Harm:

1. Continued flows can saturate por-tions of the embankment and lead toslides in the area.

2 Continued flows can further erodeembankment materials and lead tofailure of the dam.

ACTION REQUIRED

Action:

1. Begin measuring outflow quanti-ty and establishing whether water isgetting muddier, staying the same,or clearing up.

2. If quantity of flow is increasing,the water level in the reservoir shouldbe lowered until the flow stabilizesor stops.

3. A qualified engineer shouldinspect the condition and recommendfurther actions to be taken.

ENGINEER REQUIRED

Action:

1. Begin measuring outflow quanti-ty.

2. If quantity of flow is increasing,the water level in the reservoir mayneed to be lowered until the flow sta-bilizes or stops.


ENGINEER REQUIRED

Action:

1. Begin measuring outflow quanti-ty.

2. If quantity of flow is increasing,the water level in the reservoir needsto be lowered until the leak stops.

3. Search for opening on upstreamside and plug it if possible.

4. A qualified engineer shouldimmediately inspect the conditionand recommend further action to betaken.

ENGINEER REQUIRED

32



PROBLEM

5.4-4

WATER EXITING FROMRODENT HOLES

5.4-5

STREAM OF WATER EXITINGTHROUGH CRACKS NEARTHE CREST

5.4-6

SEEPAGE WATER EXITING ASA BOIL IN THE FOUNDATION

CAUSES & HARM DONE

Cause :

Diggings by the rodent have short-ened the flow path.

Harm :

Continued flows can further erodeembankment material and lead tofailure of the dam.

Cause:

1. Severe drying has caused shrink-age of embankment material.

2. Settlement in the embankment orfoundation is causing the transversecracks.

Harm:

Flow through the crack can causefailure of the dam.

Cause:

Some portion of the foundationmaterial is providing a flow path.this could be caused by a sand orgravel layer in the foundation.

Harm:

Increased flows can lead to erosionof the foundation and failure of the

dam.

ACTION REQUIRED

Action:

1. Locate any entrance points on theupstream slope and plug them.

2. If the quantity of flow is increas-

ing, the water level in the reservoirneeds to be lowered until the leak stops.

3. Bring a halt to the rodent activi-ty (See Chapter 13).

4. A qualified engineer shouldinspect the condition and recommendfurther actions to be taken

ENGINEER REQUIRED

Action:

1. Plug the upstream side of thcrack to stop the flow.

2. The water level in the reservoirshould be lowered until it is belowthe level of the cracks.


ENGINEER REQUIRED

Action:

1. Examine the boil for transporta-tion of foundation materials.

2. If soil particles are moving down-stream, sandbags or earth should beused to create a dike around the boil.The pressure created by the waterlevel within the dike may controlflow velocities and temporarily pre-vent further erosion.

3. If erosion is becoming greater,the reservoir level should be low-ered.


ENGINEER REQUIRED



PROBLEM

5.4-7

SEEPAGE WATER EXITINGFROM A POINT ADJACENT TOTHE OUTLET

5.4-8

LEAKAGE IN OR AROUNDSPILLWAY

5.4-9

SEEPAGE FROM A CON-STRUCTION JOINT OR CRACKIN CONCRETE STRUCTURE

CAUSES & HARM DONE

Cause :

1. A break in the outlet pipe.

2. A path for flow has developedalong the outside of the outlet pipe.

Harm :

Continued flows can lead to rapiderosion of embankment materialsand failure of the dam.

Cause:

1. Cracks and joints in geologic for-mation at spillway are permittingseepage.

2. Gravel or sand layers at spillwayare permitting seepage.

Harm:

1. Could lead to excessive loss of stored water.

2. Could lead to a progressive fail-ure if velocities are high enough tocause erosion of natural materials.

Cause:

Water is collecting behind structurebecause of insufficient drainage orclogged weep holes.

Harm:

1. Can cause walls to tip in and over.Flows through concrete can lead torapid deterioration from weathering.

2. If the spillway is located withinthe embankment, rapid erosion canlead to failure of the dam.

ACTION REQUIRED

Action:

1. Thoroughly investigate the areaby probing and/or shoveling to seeif the cause can be determined.

2. Determine if leakage water is car-rying soil particles.

3. Determine quantity of flow.

4. If flow increases or is carryingembankment materials, reservoirlevel should be lowered until leak-age stops.


ENGINEER REQUIRED

Action:

1. Examine exit area to see if typeof material can explain leakage.

2. Measure flow quantity and check for erosion of natural materials.

3. If flow rate or amount of erodedmaterials increases rapidly, reservoirlevel should be lowered until flowstabilizes or stops.

4. A qualified engineer shouldinspect the condition and recommend

further actions to be taken.ENGINEER REQUIRED

Action:

1. Check area behind wall for pud-dling of surface water.

2. Check and clean as required; drainoutfalls, flush lines, and weep holes.

3. If condition persists a qualifiedengineer should inspect the condi-tion and recommend further actionsto be taken.

ENGINEER REQUIRED

34



PROBLEM

5.4-16

BULGE IN LARGE WET AREA

5.4-17

TRAMPOLINE EFFECT INLARGE SOGGY AREA

5.4-18

LEAKAGE FROM ABUTMENTSBEYOND THE DAM

5.4-19

LARGE INCREASE IN FLOW ORSEDIMENT IN DRAIN OUTFALL

CAUSES & HARM DONE

Cause :

Downstream embankment materialshave begun to move.

Harm :

Failure of the embankment due tomassive sliding can follow these ini-tial movements.

Cause:

Water moving rapidly through theembankment or foundation is being

controlled or contained by a well-established turf root system.

Harm:

Condition indicates excessive seep-age in the area. If control layer of turf is destroyed, rapid erosion of foundation materials could result infailure of the dam.

Cause:

Water moving through cracks andfissures in the abutment materials.

Harm:

1. Can lead to rapid erosion of abut-ment and evacuation of the reservoir.

2. Can lead to massive slides nearor downstream from the dam.

Cause:

A shortened seepage path orincreased storage levels.

Harm:

1. Higher velocity flows can causeerosion of drain then embankmentmaterials.

2. Can lead to piping failure.

ACTION REQUIRED

Action:

1. Compare embankment cross-sec-tion to the end of construction con-dition to see if observed conditionmay reflect end of construction.

2. Stake out affected area and accu-rately measure outflow.


ENGINEER REQUIRED

Action:

1. Carefully inspect the area for out-flow quantity and any transportedmaterials.


ENGINEER REQUIRED

Action:

1. Carefully inspect the area todetermine quantity of flow andamount of transported material.

2. A qualified engineer or geologistshould inspect the condition and rec-ommend further actions to be taken.

ENGINEER REQUIRED

Action:

1. Accurately measure outflowquantity and determine amount of increase over previous flow.

2. Collect jar samples to compareturbidity.

3. If either quantity or turbidity hasincreased by 25% a qualified engi-neer should evaluate the conditionand recommend further actions.

ENGINEER REQUIRED



6.1 INTRODUCTIONNow our attention will be concentrated on problemsthat can be found on the upstream slope, in order toallow the owners to identify conditions that threatenthe safety and long life of a dam. Although most of these items can be corrected by normal maintenance,more serious conditions will require inspections bythe State Engineer’s office and may require furtherinvestigation by an experienced consulting engineer.The items requiring the attention of an engineer willbe noted as “ENGINEER REQUIRED” on the FieldConditions Chart—Upstream Slope which is provid-ed at the end of this chapter.

PHOTO 6.1—EXCELLENT UPSTREAM FACE. Note uniform surfaceor riprap protection.

6.2 ITEMS OF PARTICULAR CONCERN

As the owner criss-crosses the upstream slope duringan inspection, he should look carefully for these threeitems:CracksSlidesLack of protection of the upstream faceThe first two of these conditions may indicate seri-ous problems within the embankment.Looking for and spotting cracks is difficult. The slopemust be transversed in such a manner that the inspec-tor is likely to walk over the cracks. Cracks may beonly an inch or two wide but 2 or 3 feet deep. Usual-ly a depth of 3 feet shows that the crack is not a harm-

less drying type crack. A 20-foot-long line of recent-ly dislodged riprap along the upstream slope couldindicate a crack underneath the riprap. Cracks indi-cate possible foundation movement, embankment fail-ure, or a surface slide.Slides are almost as difficult to spot as cracks. Theirappearance is subtle, since there may be only about2 feet of settlement or bulging out from the normalslope in a distance of perhaps 100 feet. Also, whenthe dam was finished, it may not have been uniform-ly graded by the bulldozer or grader operator. A goodfamiliarity with how the slope looked at the end of construction helps identify any new slides.

The lack of protection against wave action on theupstream slope leads to erosion, and the decrease of the embankment width and/or elevation which couldallow water to overflow the crest.

PHOTO 6.2—EROSION OF UPSTREAM FACE DUE TO LACK OFPROTECTION AGAINST WAVE ACTION.

6.3 SPECIAL INSPECTION TECHNIQUESWhen walking on riprap, caution should be used toavoid losing one’s footing. Most important, a criss-cross path should be used for inspecting the slope sothat cracks and slides can be more easily seen. Manytimes the water-line alignment will indicate a changein the uniformity of the slope. The inspector shouldstand at one end of the dam and sight along the waterline. Also, if a crack is seen, the crest and downstreamslope in that area should be carefully inspected to noteany other changes in that area of the embankment thatcould be associated with the upstream crack.

FIGURE 6.3-1—USING CRISSCROSS INSPECTION PATH

6.4 TYPICAL MATERIALSThis information is presented to describe the array of materials that can be used on the upstream slope.

1. UNPROTECTED EMBANKMENT is composeof soils only and is exposed to wave action andsusceptible to erosion. Soil alone is adequate foronly small, gently sloped embankments where noscraping is occurring. When scraping does occur,protection must be improved to protect the dam.

CHAPTER 6

UPSTREAM SLOPE



b. ROCK RIPRAP WITH BEDDING is composedof rock graded from 3 ⁄ 16 inch to 3 1 ⁄ 2 inches, whichprevents soil from being washed out through thevoids in the larger rock rirrap. Riprap is largerrock that is designed for thicknesses that preventwave action from eroding the embankment. Usingvarious sizes of rock is important so that rockslodge between other rocks forming a dense mat.The slope should be gentle so that rocks do nottumble to the bottom of the slope due to over-steepness.

c. CONCRETE is comprised of concrete slabs thatprovide a facing covering the upstream slope.Joints must be watertight to prevent erosion of soil that is behind the concrete.

d. SOIL CEMENT is a mixture of water, pulver-ized soil, and portland cement. When properlyconstructed, it provides low cost, durable protec-tion against wave action.

e. ASPHALT PAVING is asphalt concrete laidagainst a designed gravel bedding.

f. STEEL is composed of steel plate welded togeth-er with a good design that includes expansion-contraction joints.

6.5 FIELD CONDITIONS ENCOUNTERED

6.6 00The three major problems encountered for an upstreamslope are:CracksSlidesLack of protection from wave actionThe upstream slope needs a thorough inspection, sincethe slope protection and water stored can hide prob-lems. When the reservoir is emptied, the slope shouldbe thoroughly inspected for settlement areas, rodentactivity, sinkholes, or slides. Also, the reservoir basin(bottom of the reservoir) should be inspected for sink holes or settlement.The dam’s owner, by applying the maximum prudenteffort, can identify any changes in previously notedconditions that indicate a safety problem. A consci-entious annual maintenance program will address andcontrol most of the conditions identified above. When

a questionable condition is found, the state’s damsafety engineers should be notified immediately.Quick corrective action to conditions requiring attentionwill promote the safety and extend the useful life of the dam while possibly preventing costly future repairs.

40

PROBLEM

6.5-1

SCARPS, BENCHES,OVERSTEEP AREAS

6.5-2

SINKHOLE

CAUSES & HARM DONE

Cause :

Wave action, local settlement, or iceaction cause soil and rock to erodeand slide to the lower part of the

slope forming a bench.Harm :

This eroded area lessens the widthand possible height of the embank-ment and could lead to increasedseepage or overtopping of the dam.

Cause:

The piping of embankment materi-al or foundation material causes asink hole. The cave-in of an erodedcavern can result in a sink hole. Asmall hole in the wall of an outletpipe can develop a sink hole.

Harm:

This condition can empty a reservoirthrough a small hole in the wall of an outlet pipe or can lead to failureof a dam as soil pipes through thefoundation or a pervious portion of the dam.

ACTION REQUIRED

Action:

Determine exact cause of scarps. Donecessary earthwork, restore em-bankment to original slope, provide

adequate protection (bedding andriprap). See Chapter 13.

Action:

Inspect other portions of the dam forseepage or additional sink holes.Identify exact cause of sink holes.Check seepage and leakage outflowsfor dirty water.

A qualified engineer should inspectthe conditions and recommend fur-ther actions to be taken.

ENGINEER REQUIRED



PROBLEM

6.5-3

SLIDE, SLUMP, OR SLIP

6.5-4

BROKEN DOWN, MISSINGRIPRAP

6.5-5

EROSION BEHIND POORLYGRADED RIPRAP

6.5-6LARGE CRACKS

CAUSES & HARM DONE

Cause :

Earth or rocks move down the slopealong a slippage surface because theywere on too steep a slope, or thefoundation moves. Also, look forslides in reservoir basin.

Harm :

A series of slides can lead to obstruc-tion of the outlet or failure of thedam.

Cause:

Poor quality riprap has deteriorated.Wave action or ice action has dis-

placed riprap. Round and similar-sized rocks have rolled downhill.

Harm:

Wave action against these unpro-tected areas decreases embankmentwidth.

Cause:

Similar-sized rocks allow waves topass between them and erode smallgravel particles and soil.

Harm:

Soil is eroded away from behind theriprap. This allows riprap to settle,providing less protection anddecreased embankment width.

Cause:A portion of the embankment hasmoved due to loss of strength, or thefoundation may have moved, caus-ing embankment movement.

Harm:

Can lead to failure of the dam.

ACTION REQUIRED

Action:

Evaluate extent of the slide. Moni-tor slide. (See Chapter 12.)

Draw the reservoir level down if

safety of dam is threatened.A qualified engineer should inspectthe conditions and recommend fur-ther actions to be taken.

ENGINEER REQUIRED

Action:

Re-establish normal slope. Placebedding and competent riprap. (See

Chapter 13).

Action:

Re-establish effective slope protec-tion. Place bedding material ENGI-NEER REQUIRED for design of gradation and size of rock for bed-ding and riprap.


ENGINEER REQUIRED

Action:Depending on the amount of embankment involved, draw reser-voir level down.


ENGINEER REQUIRED



PROBLEM

6.5-7

CRACKS DUE TO DRYING

6.5-8

BEAVER OR MUSKRAT ACTIV-ITY

6.5-9

CRACKED DETERIORATEDCONCRETE FACE

CAUSES & HARM DONE

Cause :

The soil loses its moisture andshrinks, causing cracks.

Note:

Usually seen on crest and down-stream slope mostly

Harm :

Heavy rains can fill up cracks andcause small portions of embankmentto move along internal slip surface.

Cause:

Holes, tunnels, and caverns are

caused by animal burrowings. Cer-tain habitats like cattail type plantsand trees close to the reservoirencourage these animals.

Harm:

If a tunnel exists through most of thedam, it can lead to failure of the dam.

Cause:

Concrete deteriorated due to weath-ering. Joint filler deteriorated or dis-placed.

Harm:

Soil is eroded behind the face andcaverns can be formed. Unsupportedsections of concrete crack. Ice actionmay displace concrete.

ACTION REQUIRED

Action:

1. Monitor cracks for increases inwidth, depth, or length.

2. A qualified engineer should

inspect the condition and recommendfurther actions to be taken.

ENGINEER REQUIRED

Action:

Remove rodents. Determine exactlocation of digging and extent of tun-neling. Remove habitat. Repair dam-ages. (See Earthwork, Chapter 13.2.)

Action:

1. Determine cause. Either patchwith grout or contact engineer forpermanent repair method.

2. If damage is extensive a qualifiedengineer should inspect the condi-tions and recommend further actionsto be taken.

ENGINEER REQUIRED

42



PHOTO 6.5-1—UNIFORM POORLY SIZED RIPRAP LEADS TOEROSION OF SOIL BEHIND RIPRAP.

PHOTO 6.5-4—MUSKRAT DAMAGE

PHOTO 6.5-2—CRACK IN UPSTREAM SLOPE PHOTO 6.5-5—BADLY DETERIORATED CONCRETE FACE

PHOTO 6.5-3—CRACK IN UPSTREAM SLOPE PHOTO 6.5-6—SLIDE AT UPSTREAM SLOPE.

Note water line alignment



44

PHOTO 6.5-7—EROSION AT UPSTREAM FACE

PHOTO 6.5-8—21'X21' SINK HOLE IN RESERVOIR BASIN



7.1 INTRODUCTIONNow our attention will be concentrated on the crestof the dam in order to allow owners to identify con-ditions that threaten the safety and long life of theirdam. Although some of these conditions can be cor-rected by normal maintenance, more serious defi-ciencies will require inspection by the State Engi-neer’s Office and may require further investigation byan experienced consulting engineer. The conditionsrequiring the attention of an engineer will be desig-nated by the notation “ENGINEER REQUIRED’ onthe table presented in Section 7.5 of this chapter.

7.2 TYPICAL MATERIALSThis information is presented to describe to the read-er the array of materials that can be found on the crestof dams. The type of materials that are used or foundon the crest are often dictated by dam design or antic-ipated usage of the crest, including access and road-way requirements. Common materials encounteredinclude native earth, gravel, rock, concrete, or asphalt.When access across the dam is needed only for main-tenance operations that can be scheduled during favor-able weather conditions, no special crest surfacing isrequired. In these cases, the crest surfacing is usual-ly composed of native earthen materials placed duringoriginal embankment construction. If access acrossthe dam is imperative under all weather conditionsfor the safe and routine operation of the dam, the crestshould be surfaced with a minimum of 4 inches of gravel or road base material.If the dam is a rockfilled structure, the crest will alsobe rock. Again, if access is required, the top of therockfill is often smoothly finished or gravel is placed

on the crest to provide a smooth roadway.If the crest conveys a road or highway, it should besurfaced with a properly engineered roadway of road-base material, asphalt, or reinforced concrete. Thecrest surfacing must be capable of withstanding theexpected traffic loading and preventing damage to theunderlying dam structure.If the dam contains an impervious core, adequate pro-tective material should be provided on the crest toprotect the core from damage by frost heave and fromthe formation of drying cracks at the top of the imper-vious core. In all cases, it is preferable that the mate-rial used to cover the crest be a material that will not

shrink or crack when dried out. This will prevent theformation of drying cracks in the embankment andthe possible infiltration of surface runoff into the dam’scross-section through the surface cracking.

7.3 ITEMS OF PARTICULAR CONCERNOn the crest, some of the more threatening conditionsthat may be identified during inspection are:7.3-1 LONGITUDINAL CRACKING can indicate

localized instability, differential settlement,and/or movement between adjacent sections of the embankment. Longitudinal cracking is char-acterized by a single crack or a close, parallelsystem of cracks along the crest in a direction

more or less parallel to the length of the dam.

(See Photo 7.3-1) These cracks, which are con-tinuous over their length and are usually greaterthan 1 foot deep, can be differentiated fromdrying cracks which are usually intermittent,erratic in pattern, shallow, very narrow, andnumerous.Longitudinal cracking may precede vertical dis-placement as the dam attempts to move to a posi-tion of greater stability. (See Photo 7.3-2.) Ver-tical displacements on the crest are usuallyaccompanied by displacements on the upstreamor downstream face of the dam. An example of the development of a longitudinal crack into ver-tical displacement can be seen in Figure 7.3-1.

CHAPTER 7

CREST

PHOTO 7.3-1—SEVERE LONGITUDINAL CRACKING. Cracking pat-tern indicates horizontal movement between adjacent sections of the dam.Note the close parallel cracks.



7.3-2 TRANSVERSE CRACKING can indicate dif-ferential settlement or movement between adja-cent segments of the dam. Transverse cracking isusually a single crack or a close, parallel system of cracks which extend across the crest in a directionmore or less perpendicular to the length of the dam.This type of cracking is usually greater than 1 footin depth and can easily be distinguished from dry-ing cracks.

Transverse cracking poses a definite threat to thesafety and integrity of the dam. If the crack should progress to a point below the reservoirwater surface elevation, seepage could progressalong the crack and through the embankmentcross-section. This could evolve into a piping sit-uation, and if not corrected, lead to the failureof the dam. An example of the progression of the transverse crack can be seen in Figure 7.3-2.

46

FIGURE 7.3-1LONGITUDINAL CRACK PRECEDES EMBANKMENT FAILURE

FIGURE 7.3-2TRANSVERSE CRACKING

Because of difference in settlement between high and low sections of theembankment, a transverse crack can form through embankment section.

FIGURE 7.3-2

VERTICAL DISPLACEMENT ON THE CREST OF A DAM. In its ini-tial stages, this condition was detected as a longitudinal crack.



FIGURE 7.3-3TRANSVERSE CRACKING LEADS TO FAILURE

7.3-2 MISALIGNMENT can indicate relative move-ment between adjacent portions of the dam in direc-tions perpendicular to the axis of the dam (SeePhoto 7.3-4).

PHOTO 7.3-3—Well-graded straight crest of a newly completed dam.

Photo 7.3-4—Crest in poor condition showing evidence of misalignment.

If these conditions are identified or suspected, the StateEngineer’s Office should be notified immediately.

7.4 SPECIAL INSPECTION TECHNIQUESThe items of particular concern noted above are usu-ally visibly detectable during a thorough and com-plete inspection of the dam, as described in Chapter 4.

While detection of longitudinal cracking, vertical dis-placement, or transverse cracking depends on a care-ful, thorough, and methodical visual inspection of thecrest surface, misalignment may only be detectableby a general overview of the dam from either abut-ment. (See Figure 4.4-2.)If the crest is straight for the length of the structure,alignment is best checked by standing away from thedam on either abutment and sighting along theupstream and downstream edges of the crest. Oncurved dams, alignment amy best be checked by stand-ing at either end of a straight segment of the dam andsighting along the crest’s upstream and downstreamedges, noting any curvature or misalignment in thatsection.

7.5 PROBLEM CONDITIONS FOUND ON THECRESTFollowing are sketches of conditions that may be foundon the crest of the dam during an inspection. Whilemost of the conditions on the following tables can becorrected by routine and periodic maintenance con-ducted by the owner, some of the conditions noted areof a nature that threaten the safety and integrity of thedam and require the attention of an experienced pro-fessional engineer. Conditions requiring the attentionof an engineer are identified by the notation “ENGI-NEER REQUIRED” under the “ACTIONREQUIRED” column of the tables.

7.6 SUMMARYThe dam’s crest usually provides the primary accessfor inspection and maintenance. Since surface waterwill pond on the crest unless that surface is well main-tained, this part of the dam usually requires periodicregrading. Problems found on the crest should not begraded over. When a questionable condition is found,the state’s dam safety engineers should be notifiedimmediately.Quick corrective action applied to conditions requir-ing attention will promote the safety and extend theuseful life of the dam while possibly preventing cost-ly future repairs.



CONDITION FOUND

7.5-1

LONGITUDINAL CRACK

7.5-2

VERTICAL DISPLACEMENT

CAUSES & HARM DONE

Cause :

1. Uneven settlement between adja-cent sections or zones within theembankment.

2. Foundation failure causing loss of support to embankment.

Harm :

1. Creates local area of low strengthwithin embankment. Could be thepoint of initiation of future structur-al movement, deformation, or fail-ure.

2 Provides entrance point for sur-face run-off into embankment, allow-ing saturation of adjacent embank-ment area. and possible lubrication

which could lead to localized fail-ure.

Cause:

1. Vertical movement between adja-cent sections of the embankment.

2. Structural deformation or failurecaused by structural stress or insta-bility, or by failure of the founda-tion.

Harm:

1. Provides local area of lowstrength within embankment whichcould cause future movement.

2. Leads to structural instability orfailure.

3. Provides entrance point for sur-face water that could further lubri-cate failure plane.

4. Reduces available embankmentcross section.

ACTION REQUIRED

Action:

1. Inspect crack and carefully recordlocation, length, depth, width, align-ment, and other pertinent physicalfeatures. Immediately stake out lim-its of cracking. Monitor frequently.(See Section 12.5-2.)

2. Engineer should determine causeof cracking and supervise steps nec-essary to reduce danger of dam andcorrect condition.

3. Effectively seal the cracks at thecrest’s surface to prevent infiltrationby surface water. (See Section 13.2.)

4. Continue to routinely monitorcrest for evidence of further crack-ing.

ENGINEER REQUIRED

Action:

1. Carefully inspect displacementand record its location, vertical andhorizontal displacement, length, andother physical features. Immediate-ly stake out limits of cracking. (SeeSection 12.5.)

2. Engineer should determine causeof displacement and supervise allsteps necessary to reduce danger todam and correct condition.

3. Excavate area to the bottom of the displacement. Backfill excava-tion, using competent material andcorrect construction techniques,under supervision of engineer.

4. Continue to monitor areas rou-tinely for evidence of future crackingor movement. (See Section 12.5.)

ENGINEER REQUIRED

48



CONDITION FOUND

7.5-3

TRANSVERSE CRACKING

7.5-4

CREST MISALIGNMENT

CAUSES & HARM DONE

Cause :

1. Uneven movement between adja-cent segments of the embankment.

2. Deformation caused by structur-

al stress or instability.Harm :

1. Can provide a path for seepagethrough the embankment cross sec-tion.

2. Provides local area of lowstrength within embankment. Futurestructural movement, deformation,or failure could begin at this point.

3. Provides entrance point for sur-face run-off to enter embankment.

Cause:

1. Movement between adjacent por-tions of the structure.

2. Uneven deflection of dam underloading by reservoir.

3. Structural deformation or failurenear area of misalignment.

Harm:

1. Area of misalignment is usuallyaccompanied by low area in crestwhich reduces free board.

2. Can produce local areas of lowembankment strength which maylead to failure.

ACTION REQUIRED

Action:

1. Inspect crack and carefully recordcrack location, length, depth, width,and other pertinent physical features.Stake out limits of cracking. (SeeSection 12.5-2.)

2. Engineer should determine causeof cracking and supervise all stepsnecessary to reduce danger to damand correct condition.

3. Excavate crest along crack to apoint below the bottom of the crack.Then backfill excavation using com-petent material and correct con-struction techniques. This will sealthe crack against seepage and sur-face run-off. (See Section 13.2.) This

should be supervised by engineer.4. Continue to monitor crest rou-tinely for evidence of future crack-ing.) (See Section 12.5.)

ENGINEER REQUIRED

Action:

1. Establish monuments across crestto determine exact amount, location,and extent of misalignment. (SeeSection 12.5-1.)

2. Engineer should determine causeof misalignment and supervise allsteps necessary to reduce threat todam and correct condition.

3. Monitor crest monuments on aschedule basis following remedialaction to detect possible future move-ment. (See Section 12.5-1.)

ENGINEER REQUIRED



CONDITION FOUND

7.5-5

LOW AREA IN CREST OF DAM

7.5-6

SINKHOLE IN CREST

CAUSES & HARM DONE

Cause :

1. Excessive settlement in theembankment or foundation directlybeneath the low area in the crest.

2. Internal erosion of embankmentmaterial.

3. Foundation spreading towardupstream and/or downstream direc-tion.

4. Prolonged wind erosion of crestarea.

5. Improper final grading followingconstruction

Harm:

Reduces freeboard available to pass

flood flows safely through spillway.

Cause:

1. Rodent activity.

2. Hole in outlet conduit is causingerosion of embankment material.

3. Internal erosion or piping of embankment material by seepage.

4. Breakdown of dispersive clayswithin embankment by seepagewaters.

Harm:

1. Void within dam could causelocalized caving, sloughing, insta-bility, or reduced embankment crosssection.

2. Entrance point for surface water.

ACTION REQUIRED

Action:

1. Establish monuments along lengthof crest to determine exact amount,location, and extent of settlement increst. (See Section 12.5-1.)

2. Engineer should determine causeof low area and supervise all stepsnecessary to reduce possible threatto the dam and correct condition.

3. Re-establish uniform crest eleva-tion over crest length by placing fillin low area using proper construc-tion techniques. (See Section 13.2.)This should be supervised by engi-neer.

4. Re-establish monuments acrosscrest of dam and monitor monu-ments on a routine basis to detectpossible future settlement. (See Sec-tion 12.5-1.)

ENGINEER REQUIRED

Action:

1. Carefully inspect and record loca-tion and physical characteristics(depth, width, length) of sinkhole.

2. Engineer should determine causeof sinkhole and supervise all stepsnecessary to reduce threat to damand correct condition.

3. Excavate sinkhole, slope sides of excavation, and backfill hole withcompetent material using proper con-struction techniques. (See Section13.2.) This should be supervised byengineer.

ENGINEER REQUIRED

50



CONDITION FOUND

7.5-10

RUTS ALONG CREST

7.5-11

PUDDLING ON CREST—POOR DRAINAGE

7.5-12

DRYING CRACKS

CAUSES & HARM DONE

Cause :

Heavy vehicle traffic without ade-quate or proper maintenance or prop-er crest surfacing.

Harm :1. Inhibits easy access to all partsof crest.

2. Allows continued development of rutting.

3. Allows standing water to collectand saturate crest of dam.

4. Operating and maintenance vehi-cles can get stuck.

Cause:

1. Poor grading and improperdrainage of crest.

2. Localized consolidation or set-tlement on crest allows puddles todevelop.

Harm:

1. Causes localized saturation of thecrest.

2. Inhibits access to all portions of the dam and crest.

3. Becomes progressively worse if not corrected.

Cause:

Material on the crest of dam expandsand contracts with alternate wettingand drying of weather cycles. Dryingcracks are usually short, shallow, nar-row, and numerous.

Harm:

Provides point of entrance for sur-face run-off and surface moisture,causing saturation of adjacentembankment areas. This saturationand subsequent drying of the damcould cause further cracking.

ACTION REQUIRED

Action:

1. Drain standing water from ruts.

2. Regrade and recompact crest torestore integrity and provide proper

drainage toward upstream slope. (SeeSection 13.2.)

3. Provide gravel or roadbase mate-rial to accommodate traffic.

4. Perform periodic maintenance andregrading to prevent reformation of ruts.

Action:

1. Drain standing water from pud-dles.

2. Regrade and recompact crest torestore integrity and provide properdrainage toward upstream slope. (SeeSection 13.2.)

3. Provide gravel or roadbase mate-rial to accommodate traffic.

4. Perform periodic maintenance andregrading to prevent reformation of

ruts.

Action:

1. Seal surface of cracks with a tight,impervious material. (See Section13.2.)

OR

2. Routinely grade crest to provideproper drainage and fill cracks.

OR

3. Cover crest with non-plastic (notclay) material to prevent large mois-ture content variations with respect totime.

52



CONDITION FOUND

7.5-13

CREST CAMBER

CAUSES & HARM DONE

Cause :

Results from construction. Propor-tionally more fill is placed on crest inhigher segments of the embankmentduring construction to compensatefor anticipated settlement within thedam and foundation.

Harm :

None.

ACTION REQUIRED

Action:

None.

PHOTO 7.5-2—LOW AREA IN THE CREST OF A DAM, caused by afoundation failure.

PHOTO 7.5-1.A—VERTICAL DISPLACEMENT ON CREST.

PHOTO 7.5-3—SINKHOLES IN THE CREST OF A DAM, caused byconsolidation of low density embankment material.

PHOTO 7.5-1.B—VERTICAL DISPLACEMENT ON THE CREST OF ADAM. This vertical movement was associated with a slip on the down-stream face.

SMALLSINKHOLE

LARGESINKHOLE



54

PHOTO 7.5-4—THE CREST OF A DAM PARTIALLY OVERGROWN

AND OBSCURED DUE TO EXCESSIVE BRUSH GROWTH.

PHOTO 7.5-7—RUTS IN THE CREST OF A DAM

PHOTO 7.5-8—LONGITUDINAL DRYING CRACKS ON THECREST OF A DAM. These cracks are caused by drying of the crest sur-face and are only 6-8 inches deep.

PHOTO 7.5-9—CAMBER INCORPORATED INTO CREST DURINGCONSTRUCTION. Camber is greatest over highest section of the dam

PHOTO 7.5-5—RODENT HOLE FOUND ON THE CREST OF ADAM

PHOTO 7.5-6—RUNOFF GULLIES FORMING ON THE DOWN-STREAM SHOULDER OF THE CREST



8.1 INTRODUCTIONNow our attention will be concentrated on the down-stream slope in order to allow the owners to identifyconditions that threaten the safety and long life of thedam. Although most of these items can be corrected bynormal maintenance, more serious conditions willrequire inspection by the State Engineer’s Office andmay require further investigation by an experiencedconsulting engineer. The items requiring the attentionof an engineer will be noted “ENGINEERREQUIRED.”

8.2 ITEMS OF PARTICULAR CONCERNOn the downstream slope some of the more threaten-ing conditions that may be identified during inspectionsare:CRACKSSLIDESSEEPAGECracks can indicate settlement, drying and shrinkage,

or a slide developing in the embankment. Whateverthe cause, cracks should be monitored and changes inlength and width noted. A suggested method for mon-itoring cracks is presented in Section 12.5-2.Drying cracks may appear and disappear seasonallyand normally will not show vertical displacement likethat associated with settlement cracks or slide cracks.(See Figure 8.2-1.)

FIGURE 8.2-1—DISPLACEMENT AT CRACKSlides are easily spotted and require immediate eval-uation by the State Engineer’s Office. There are, how-ever, early warning signs of a slide. A bulge in theembankment or vertical displacement at a crack in theembankment may indicate sliding.Seepage occurs at all dams in varying degrees. Themost potentially dangerous condition is the appear-ance of seepage on the downstream face above thetoe of the dam. Seepage on the downstream slope canlead to a slide or failure of the dam by piping. Referto Chapter 5 for details concerning seepage.If these three conditions are identified or suspected, theState Engineer’s Office should be notified immediately.

8.3 SPECIAL INSPECTION TECHNIQUESIf the downstream slope is covered with heavy brush orvegetation, a more concentrated search must be madeto identify cracks or seepage. A concentration of thick

green grass on the slope usually indicates a seepagearea. A crack such as the one shown in the photo belowis difficult to locate. To help distinguish drying cracksfrom other types, the ground surface adjacent to the damshould be examined for similar cracking patterns.

PHOTO 8.3-1—Three-fourth-inch-wide crack, well hidden in grass cover.

Constant vigilance by the owner is necessary in orderto identify potentially dangerous situations which threat-en the safety and long life of the dam.

8.4 TYPICAL MATERIALSThis information is presented to describe the materialsthat can be used on the downstream slope.The two most common materials are unprotectedembankment (compacted fill) and rock.Embankment or compacted fill is usually covered withgrasses to prevent erosion. An example is shown in thephoto below.

Photo 8.4-1—Early stage of growth for grass slope protection.

The fill material is usually from a local source and addi-tional material for making repairs is usually available.A thick stand of vegetation such as that shown belowis not recommended because it obscures visual inspec-tion of the surface.

CHAPTER 8

DOWNSTREAM SLOPE



8.5 PROBLEMS FOUND IN THE FIELDThe following charts are presented to help the ownerreadily identify problems that appear on the down-stream slope. The charts also point out the HAZ-ARDOUS problems where evaluation by an ENGI-NEER IS REQUIRED.

8.1 SUMMARYThe downstream slope is especially important duringinspection because it is the area where evidence of developing problems appears most frequently. Thearea requires especially detailed inspection. In order toassure the safety of the dam, it is important to keepthis area free from obscuring growth.When cracks, slides or seepage are noted in this area,the State Engineer’s Office should be notifiedimmediately.

56

PHOTO 8.4-2—UNDESIRABLE AMOUNT OF VEGETATION

Rock is placed on the compacted fill for erosion pro-tection and is the preferred method of protecting theslope from erosion.

PHOTO 8.4-3—Example of well-placed riprap on the downstream slope.



PROBLEM

8.5-1

EROSION

8.5-2

TRANSVERSE CRACKING

8.5-3

LONGITUDINAL CRACKING

CAUSES & HARM DONE

Cause :

Water from intense rainstorms orsnow-melt carries surface materialdown the slope, resulting in contin-uous troughs.

Harm :

If allowed to continue, erosion canlead to eventual deterioration of thedownstream slope which can short-en the seepage path.

Cause:

1. Drying and shrinkage of surface

material is most common.2. Differential settlement of theembankment also leads to transversecracking (e.g., center settles morethan abutments).

Harm:

1. Shrinkage cracks allow water toenter the embankment. This pro-motes saturation and increases freezethaw action.

2. Settlement cracks can lead toseepage of reservoir water throughthe dam.

Cause:

1. Drying and shrinkage of surfacematerial.

2. Downstream movement or settle-ment of embankment.

Harm:

1. Can be an early warning of apotential slide.

2. Shrinkage cracks allow water toenter the embankment and freezingwill further crack the embankment.

3. Settlement or slide indicating lossof strength in embankment can leadto failure.

ACTION REQUIRED

Action:

1. The preferred method to protecteroded areas is rock or riprap. (SeeSection 6.4 for information onriprap.)

2≠ Re-establishing protective grass-es can be adequate if the problem isdetected early.

Action:

1. If necessary plug upstream end

of crack to prevent flows from thereservoir.

2. A qualified engineer shouldinspect the conditions and recom-mend further actions to be taken.

ENGINEER REQUIRED

Action:

1. If cracks are from drying, dressarea with well-compacted materialto keep surface water out and natu-ral moisture in.

2. If cracks are extensive, a quali-fied engineer should inspect the con-ditions and recommend furtheractions to be taken.

ENGINEER REQUIRED



PROBLEM

8.5-4

SLIDE/SLOUGH

8.5-5

SLUMP(LOCALIZED CONDITION)

8.5-6

SINK HOLE/COLLAPSE

CAUSES & HARM DONE

Cause :

1. Lack of or loss of strength of embankment material.

2. Loss of strength can be attributed

to infiltration of water into theembankment or loss of support bythe foundation.

Harm :

Can lead to failure of the dam.

Cause:

Preceded by erosion undercutting aportion of the slope. Can also befound on relatively steep slopes.

Harm:

Can expose impervious zone toerosion.

Cause:

Lack of adequate compaction; rodenthole below; piping through embank-ment or foundation.

Harm:

Shortens seepage path, can lead towash out of embankment.

ACTION REQUIRED

Action:

HAZARDOUS

1. Measure extent and displacementof slide.

2. If continued movement is seen,begin lowering water level untilmovement stops.

3. Have a qualified engineer inspectthe condition and recommend fur-ther action.

ENGINEER REQUIRED

Action:

1. Inspect area for seepage.2. Monitor for progressive failure.


ENGINEER REQUIRED

Action:

1. Inspect for and immediatelyrepair rodent holes. Control rodentsto prevent future damage.


ENGINEER REQUIRED

58



PROBLEM

8.5-7

TREES/OBSCURING BRUSH

8.5-8

RODENT ACTIVITY

8.5-9

LIVESTOCK/CATTLETRAFFIC

CAUSES & HARM DONE

Cause :

Natural vegetation in area.

Harm :

Large tree roots can create seepagepaths. Brushes can obscure visualinspection and harbor rodents.

Cause:

Over-abundance of rodents.

Harm:

reduces length of seepage path. Canlead to piping failure.

Cause:

Excessive travel by livestock espe-cially harmful to slope when wet.

Harm:

Creates areas bare of erosion pro-tection and causes erosion channels.Allows water to stand. Area suscep-tible to drying cracks.

ACTION REQUIRED

Action:

1. Remove all large, deep-rootedtrees and shrubs on or near theembankment. Properly backfill void.See Chapter 13.

2. Control all other vegetation onthe embankment that obscures visu-al inspection. See Chapter 13.

Action:

1. Control rodents to prevent addi-

tional damage. (See Chapter 13.)2. Backfill existing rodent holes. (SeeChapter 13.)

Action:

1. Fence livestock outside embank-ment area.

2. Repair erosion protection, i.e.,riprap, grass.



Cavitation is reduced by introducing air through a ventpipe at a point downstream of the control valve, wherea pressure drop is expected. (See Figure 9.2-1). The ventpipe establishes atmospheric pressure so that a partialvacuum is not created, and cavitation is avoided.

9.3-2 Location of Outlet Valve: Figures 9.2-1 and

9.2-2 show valves at several possible locationsalong the conduit. The preferred location is at theupstream end. In this configuration the pipe down-stream of the valve is not pressurized. Also, theoutlet conduit may be dewatered for inspectionor repair, and in an emergency involving failureof the conduit. (See Section 9.4.) None of theseadvantages is present when the valve is locatedat the downstream end of the conduit, and thepipe is continually pressurized under full reservoirhead. If a downstream valve is used for flow reg-ulation, the system should also have a guard gatenear its upstream end. (See Figure 9.2-2.)

9.4 ITEMS OF PARTICULAR CONCERNFigures 9.4-1, 9.4-2, and 9.4-3 show progressive fail-ure of an outlet system by three typical modes. Thesetypes of failures are associated with a hole whichdevelops in the conduit, followed by erosion of thesurrounding embankment. These conditions requirethe assistance of an engineer.

FIGURE 9.4-1—Failure caused by leakage along the outside of the out-let pipe.

The outlet conduit is pressurized by the downstreamvalve which has no upstream guard gate.

a. Hole develops due to rusting, cavitation, settlement,etc. of conduit. Seepage waters flow out intoembankment, and along conduit, to toe of dam.

b. Hole in conduit enlarges and seepage increases.A piping failure starts and progresses upstream.

c. The piping failure is complete when it reaches thehole in the outlet conduit. The reservoir evacu-ates, washing away part of the dam and the down-stream portion of conduit.

FIGURE 9.4-2—Reservoir emptied through the outlet because of a hole

in the upstream end of the pipe.Upstream valve. Outlet conduit is not pressurized.

a. Hole develops due to rusting, etc. Water seepsfrom reservoir into conduit.

b. Hole in conduit enlarges, seepage increases, anda piping failure starts and progresses toward thereservoir. A sinkhole develops on upstream slope.

c. Sinkhole enlarges and reservoir evacuates throughoutlet conduit.

Figure 9.4-3—Turbulence and pressure created by debris in the pipecauses sinkhole in the downstream slope.

Upstream valve. Outlet conduit becomes pressurizedbecause of a buildup of debris in the pipe.

a. Hole develops due to rusting, etc. Debris partial-ly blocks conduit, causing water to seep intoembankment.

b. Hole is enlarged by additional release of waterand a cavity develops above pipe. Pumping actionof released water slowly erodes embankment.

64



c. Cavity collapses, leaving a sinkhole on the down-stream slope. Reservoir does not evacuate sinceupstream control valve is undamaged. See Photo9.4-1.

PHOTO 9.4-1—SINKHOLE ON DOWNSTREAM SLOPE ABOVE OUT-LET CONDUIT. Failure occurred as described in Figure 9.4-3.

9.5 SPECIAL INSPECTION TECHNIQUES ANDREQUIREMENTSInspection of the outlet system is necessary to con-firm that the system is functioning properly, as well asto detect problems which could lead to failure.

9.5-1 Testing the Outlet Systema. All valves should be fully opened and

closed at least once a year. This limits cor-rosion buildup on control stems and gateguides and provides an opportunity tocheck for smooth operation of the system.Jerky or erratic operation could indicateproblems, requiring more detailed inspec-tion.

b. The system should be checked through thefull range of gate settings. Slowly open thevalve, checking for noise and vibration.

Certain valve settings may result in greaterturbulence. Check for noise which soundslike gravel being rapidly transportedthrough the system. This sound indicatesthat cavitation is occurring. These gate set-tings should be avoided. (See Section 9.l3.)

c. Check the operation of all mechanical andelectrical systems associated with the out-let. Backup electric motors, power gener-ators, and power and lighting wiring shouldfunction as intended and be in a safe con-dition. (See Chapter 13, Sections 13.6, 13.7,and 13.8.)

9.5-2 Inspection of Outlet System: Accessible portions of the outlet, such as the outfall structureand control, can be easily and regularly inspect-ed. However, sever problems (Section 9.4) arecommonly associated with deterioration or failureof portions of the system which are either buriedin the dam or normally under water.a. Outlet pipes 30 inches or greater in diam-

eter can be inspected internally, providedthe system has an upstream valve, allow-ing the pipe to be dewatered. Refer to Table9.8 for conditions to look for. Tapping theconduit interior with a hammer will helplocate voids which may exist behind thepipe. This type of inspection should be per-formed at least once a year.

b. Small diameter outlet pipes can be inspect-ed by remote TV camera. The camera ismoved through the conduit and transmitsa picture to an equipment truck, where it

can be viewed by a technician. This typeinspection is expensive and usually requiresthe services of an engineer. However, if noother method of inspection is possible, theuse of TV equipment is recommended atleast once every five years.

c. Outlet intake structures, wetwells, and out-let pipes with only downstream valves, arethe most difficult to inspect because theyare usually under water. These should bescheduled for inspection when the reser-voir is drawn down or at five year inter-vals. If a definite problem is suspected, orif the reservoir remains full over extendedperiods, divers should be hired to performan underwater inspection.

9.6 TYPICAL MATERIALSTable 9.6-1 describes materials commonly used foroutlet conduits. The information is general and intend-ed to aid the dam owner by identifying potential weak-nesses of each material type. For more specific infor-mation, refer to footnote references.



TYPEDRAWING ORDESCRIPTION ADVANTAGES DISADVANTAGES

4. CAST-INPLACECONCRETECONDUIT

High strength.Any size or shape can be con-structed.

Special forming required;added cost.

TABLE 9.6-1 (CONTINUED)MATERIALS USED IN OUTLET PIPES

MISCELLANEOUSMATERIALS

1. VCP

Vitrified Clay Pipe

Bell and Spigot, MortaredJoint

Will not corrode.Smooth interior; less resistanceto flow.

Brittle; pipe and joints easilydamaged by impact or differ-ential settlement. Not nor-mally used as pressure pipe;

joints may leak.

2. POLYETHY-LENE

(REF.5)

Heat Fused Butt Joint Flexible and very tough; willnot develop stress cracks.Lightweight and will not cor-rode.Very smooth; little resistanceto flow. Watertight.

Limited in-place testing. Maycollapse or creep under highloading; concrete encasementmay be required.

3. PVC Glued Collar Joint Lightweight and inexpensive.Easy assembly; no specialtools.Smooth and will not corrode.

Limited in-place testing. May“sunburn” and become brittleif exposed to sunlight forextended periods.

4. WOOD STAVE Steel Bands Will not rust.Smooth and not damaged byfreezing. Lightweight.

Relatively short life unlesskept saturated. High cost of maintenance. Seldom used incurrent design. High leakage;low strength.

REFERENCES: 1. Corrugated Metal Pipe, Catalog C-1B, Thompson Pipe & Steel Co., Denver, CO.2. Handbook of Cast Iron Pipe, 3rd Ed., Cast Iron Pipe Research Association.3. Engineering Manual, Lock Joint Concrete Pressure Pipe, Interpace Corporation.4. Sewer Pipe Manual, Johns-Manville.5. Driscopipe 7600, Phillips Products Co., Inc.6. Water Supply and Sewage, by Steel, pp. 114-115.

9.7 TYPES OF OUTLET VALVESTable 9.7-1 shows several specific types of valves,with the intent of aiding the dam owner in inspectionof his outlet. Valves can be generally classified basedupon their intended function as follows:Guard Gate: A valve located at or near the upstreamend of the pipe and maintained in the fully open posi-tion. The valve may be closed to dewater the conduitfor inspection, maintenance, or in an emergencyinvolving failure of the outlet system. (See Section9.4.) The guard gate is designed to be either fullyopen or fully closed. It should not be used as a reg-ulating valve.

Regulating Valve: A valve used to adjust the rate of water release. It may be located at any point alongthe conduit, but must function well under the full rangeof valve settings. For most dams, an airvent is required

just downstream of this valve to reduce turbulenceand cavitation. (See Section 9.3.)Free Discharge Valve: A regulating valve, located atthe downstream end of the outlet pipe and designed todischarge directly into the air.



TYPE

SLIDE GATEOR

SLUICE GATE(Ref. 2)

GATE VALVE

(Ref. 1)

BUTTERFLYVALVE

(Ref. 2)

BALL VALVE,PLUG VALVE,OR CONE VALVE.

(Ref. 1)

FREE DISCHARGE VALVE

(HOWELL-BUNGER,ORHOLLOW JET)

(Ref. 3)

FLAP GATE

(Ref. 2)

SHEAR GATE

(Ref. 1)

DRAWING OR DESCRIPTION USES AND DISCUSSION

The most common type of valve. Usedas regulating or guard valve at

upstream end of outlet pipe, or in wetwell. Mechanically simple and avail-able in a variety of sizes and shapes.Suitable for dams of low to moderateheight. If used to regulate flow, an air-vent is recommended just downstreamof the valve.

Typically used as a regulating valveor guard valve at the upstream end of the conduit or in an outlet well. Sincethe valve is watertight, the outlet wellremains dry. The gate leaf is wedge-shaped and seals into a tapered seat.

If a gate valve is used for regulation, anairvent is recommended just down-stream of the valve.

Performs best as an upstream guardgate. If used for regulation, certainvalve openings cause turbulence andcavitation. Airvent required down-stream of valve, if used for regulation.

Used in low head situations or as aguard valve. Intermediate valve set-tings may result in turbulence and cav-itation (airvent recommended).

Located at downstream end of outletconduit, discharging into the air (freedischarge). Excellent regulating valve;functions well in a wide range of release rates. Used primarily on highdams where outlet pressures are great.Upstream guard gate required.

Used as guard gate at upstream end of outlet conduit. Normally in the fullyopen position. Valve is closed only todewater the outlet conduit for mainte-nance, inspection, or in an emergency.

68

TABLE 9.7-1OUTLET VALVES

REFERENCES: 1. Water Supply and Sewage, by Steel, pp. 152-153.2. Waterman Catalog.3. Handbook of Dam Engineering, p. 538.



CONDITION FOUND

9.8-3 CONTROL WORKS

9.8-4

FAILURE OF CONCRETEOUTFALL STRUCTURE

9.8-5

OUTLET RELEASES ERODINGTOE OF DAM

CAUSES & HARM DONE

1. BROKEN SUPPORTBLOCK:CauseConcrete deterioration.Excessive force exerted on con-trol stem by attempting to opengate when it was jammed.Harm:Causes control support block totilt; control stem may bind. Con-trol headworks may settle. Gatemay not open all the way. Sup-port block may fail completely,leaving outlet inoperable.

2. BENT/BROKEN CONTROLSTEM:CauseRust. Excess force used toopen or close gate. Inadequateor broken stem guides.Harm:Outlet is inoperable.

3. BROKEN/MISSING STEMGUIDES:CauseRust. Inadequate lubrication.Excess force used to open orclose gate when it was jammed.Harm:Loss of support for control

stem. Stem may buckle andbreak under even normal use,(as in this example).

Cause:

Excessive side pressures on nonre-inforced concrete structure. Poorconcrete quality.

Harm:

Loss of outfall structure exposesembankment to erosion by outletreleases.

Cause:

Outlet pipe too short. Lack of ener-gy-dissipating pool or structure atdownstream end of conduit.

Harm:

Erosion of toe oversteepens down-stream slope, causing progressivesloughing.

ACTION REQUIRED

Any of these conditions can meanthe control is either inoperable or atbest partially operable. Use of thesystem should be minimized or dis-continued. If the outlet system has a

second control valve, consider usingit to regulate releases until repairscan be made. (Refer to Chapter 13.6,operating gates, maintenance.) Engi-neering assistance is recommended.

Check for progressive failure bymonitoring typical dimension, suchas “d” shown in figure.

Repair by patching cracks and pro-viding drainage around concretestructure. Total replacement of out-fall structure may be required.

Extend pipe beyond toe (use a pipeof same size and material, and formwatertight connection to existingconduit).

Protect embankment with riprap oversuitable bedding.

70



10.1 INTRODUCTIONThe main function of a spillway is to provide a safeevacuation route for excess water that has entered thereservoir after a large storm or rapid snow-melt. If thespillway is of inadequate size the dam can overtop.Overtopping is the main cause of dam failure. Defectsin the spillway can cause the dam to fail by rapid ero-sion produced by floodwater. Figure 10.1 shows thesketch of a typical spillway.

FIGURE 10.1PROFILE OF A TYPICAL SPILLWAY

Required spillway capacity is determined by severalfactors, such as drainage area, magnitude or intensityof the storm, storage capacity of the reservoir, the speedwith which rain water would flow into the reservoir, andhow rapidly water would build up in the reservoir. Aspillway which is too small will cause the water surfacein the reservoir to rise above the crest of the dam. Whenthe dam site is hit by a large storm, overtopping of thedam will usually create severe damage to the dam orthe dam foundation, especially in the case of an earthdam. If the over-flow or overtopping persists for sometime, the dam can erode away and fail.

This potential danger is not widely understood becauselarge storms tend to have a low frequency of occur-rence. Sometimes a dam owner will say, “It neverrained like that in this area.” But an owner never knowswhen that big storm will happen at his dam site.Nobody anticipated the Big Thompson flood of July31, 1976. But suddenly it became a tragic fact. Teninches of rain fell in a matter of five hours in someareas. The resulting flood killed 139 persons with 5persons still missing.The main purpose of this chapter is to identify com-mon problems associated with spillway structures,outline procedures of spillway inspection, and sug-gest remedial actions for problems identified during theinspection.

10.2 IMPORTANT PROBLEMSThere are four major types of problems that can pre-vent a spillway from functioning properly. As soon asany of these problems is identified, remedial stepsmust be taken to correct the defect.

10.2-1 ObstructionThe spillway channel may be obstructed byexcessive growth of grass and weeds, thick brush, trees, debris, or landslide deposits. Anobstructed spillway will have a substantiallyreduced discharge capacity. This reduced capac-

ity can create serious problems, include

Grass is usually not considered as an obstruc-tion. But tall weeds and brush should be peri-odically cleared and trees removed as soon asthey are noticed. Brush and debris can be entan-gled with trees to form an effective obstruction.When this happens, an even and smooth flowpattern cannot be maintained. Consequently,flow capacity of the spillway will be reduced.Any substantial amount of dirt deposited in thespillway channel from sloughing, landslideabove the channel, or sediment transport intothe area must be immediately removed. Time-ly removal of large rocks is especially impor-tant. Presence of rocks in the channel wouldobstruct flow and encourage erosion. Their sud-den plunge to the stilling basin also results inabrasion of the channel lining and damage tothe stilling basin.

10.2-2 Lack of Ability to Resist ErosionWhen a large storm occurs, the spillway isexpected to carry a large amount of water formany hours. Severe erosion damage or com-plete wash-out could result if the spillway lacksthe ability to resist erosion. If the spillway isexcavated in a hard rock formation or linedwith concrete, erosion is usually not a prob-lem. But if the spillway is excavated in sandy,deteriorated granite, clay, or silt deposits, ero-sion protection is very important. Generally,resistance to erosion can be increased if thespillway channel has a mild slope, or if it iscovered with a layer of grass or riprap withbedding material.

10.2-3 DeteriorationA spillway cannot be expected to perform prop-erly if it is deteriorated. Deterioration includescollapse of side slopes, weathering of materi-al, disintegration of riprap, breakdown of con-crete lining, erosion of approach section,sloughing of chute channel, excessive siltationof stilling basin or discharge channel, and lossof protective grass cover. These can lead toflows under and around the protective materi-al which can cause severe erosion. Remedialactions must be taken as soon as any sign of deterioration has been detected, even duringstorm flows.

10.2-4 CracksDrying cracks in an earth spillway channel areusually not regarded as a functional problem.Missing rocks in a riprap lining can be con-sidered as a “crack” in the protective cover,and this must be repaired at once.Cracks in concrete lining are commonlyencountered in the spillway channel. The cracksmay be caused by uneven foundation settle-ment, slab displacement, or excessive earth orwater pressure. Large cracks will allow water towash out fine materials below or behind theconcrete slab, causing erosion and leading to

more cracks. An extensive crack can cause theconcrete slab to be severely displaced. Conse-

CHAPTER 10

SPILLWAYS



quently, the slab may be dislodged and washedaway by the flow. A severely cracked concretespillway should be examined by and repairedunder the supervision of an engineer.

10.3 COMMONLY USED MATERIALSEarth —An emergency spillway may be dug in natu-

ral earth and left unlined if the flow velocity is notexpected to be higher than 5 feet per second. Cohesivetype soil or tough clay is preferred because it affordsgreater resistance to erosion. Measures should be takento encourage an even growth of grass over the spillwaybecause it provides more protection against erosion.Side slopes should be made as mild as possible tominimize any chance of slope failure.Earth with Control Sill —It is sometimes desirable toconstruct a control sill across the spillway channel.The sill is essentially a concrete wall buried deep in thechannel. The wall is usually perpendicular to the direc-tion of flow, and its crest or the top surface is sup-posed to be horizontal (or level) and flush with thechannel surface. The sill serves as a control sectionat which discharge may be measured. In the case of aflood, the sill will evenly distribute the flow acrossthe channel, thus minimizing erosion of the down-stream portion. The sill will also halt backcutting toassure that the upstream portion of the channel willnot be damaged by erosion. The area downstream of the sill may be protected with riprap.Riprap with Bedding Material —Riprap is a layerof randomly sized rocks carefully placed to protectunderlying soils. Rirrap protection is one of the mosteconomical ways to protect an earth spillway againsterosion, provided that good quality rock is readily

available. Adequate depth of riprap protection isrequired at the entrance section, chute bottom, andbanks, and the stilling basin. In order to prevent earthparticles under the riprap from being washed away, alayer of bedding material should be evenly spreadover the channel bottom or side slope before the riprapis placed. See Chapter 13 for more details about theconstruction and maintenance of bedding material andriprap.Concrete —Concrete, especially (steel bar) reinforcedconcrete, is one of the most commonly used materialsfor spillways. A concrete spillway can be expected tolast many decades. During construction, sound aggre-gates must be used, and the concrete properly mixedand carefully cured. Addition of air-entraining agentsto the mixing of concrete will render the concretemore resistant to the harmful effect of freeze-thawcycles. Construction joints and expansion joints areusually present at suitable intervals to prevent frac-turing or shrinkage cracks. The joint must be filledwith sealants. Adjacent concrete panels or slabs areconstructed flush relative to each other at the joint toavoid uplift pressure or erosion caused by flowingwater. Adequate reinforcing steel bars must be usedto prevent large cracks and failure. Weep holes andunderdrainage systems are installed to avoid build-upwater pressure.

A concrete spillway should be designed by and con-structed under the supervision of a professional engineer.Wood —In a relatively small structure, wood pilesmay be used to provide protection to toe areas andfoundation at the spillway. Occasionally, woodplanks are used to construct a large flume servingas a spillway.Since wood material decays easily, it is not recom-mended for permanent purposes above the water sur-face. Wood piles submerged below water all the time,however, can be expected to last a long time. Spe-cially treated wood will also last longer than untreat-ed material.

10.4 PROCEDURE FOR INSPECTIONSpillway inspection is an important part of a damsafety program. The basic objective of spillwayinspection is to detect any sign of obstruction, ero-sion, deterioration, misalignment, or cracks. Identi-fication of obstruction is relatively simple. Inspec-

tion procedures regarding erosion resistance,deterioration, and cracks are briefly outlined belowaccording to the spillway type.

10.4-1 Earth Spillway and Earth with Control SillWhen inspecting an earth spillway, find outwhether side slopes have sloughed, or whetherthere is an excessive vegetative growth in thechannel. Look for signs of erosion and rodentactivities. Use a probe to obtain a comparativefeel of the hardness and moisture content of the soil. Note location of particularly wet orsoft spots. See if the stilling basin or drop struc-ture is properly protected with rocks or riprap.Since some erosion is unavoidable duringspilling, determine whether such erosion mightendanger the embankment itself. If the spill-way is installed with a sill, determine if thereis any crack or misalignment of the sill. Alsolook for any erosion beneath or downstream of the sill.

PHOTO 10.4-1—GABION DROP STRUCTURE. A special type of rockprotection against erosion.

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10.4-2 Concrete SpillwayCommonly encountered defects and generalinspection procedures for concrete spillway areoutlined as follows:

a. Cracks—Hairline cracks are usually harm-less. Large cracks should be carefully

inspected. Note the location, width, length,and orientation of the crack. Determine if concrete around the crack has deteriorat-ed or whether reinforcing bars are exposed.

b. Spalling—Spillway surfaces exposed tofreeze-thaw cycles often suffer from sur-face spalling. Chemical action, contami-nation, and unsound aggregates can alsocause spalling. If spalling is extensive, drawa sketch of the spalled area and show thelength, width, and depth of the area. Exam-ine closely to see if the remaining concretehas deteriorated or reinforcing bars areexposed. Tap the concrete with a “bonker”or rock hammer to see if there is emptyspace below the surface. Shallow spallingshould be examined from time to time tosee if it is getting worse. Deep spallingmust be repaired by an experienced personas soon as possible.

FIGURE 10.2—WEEP HOLE

c. Drains—Walls of spillway are usuallyequipped with weep (or drain) holes. Occa-sionally, spillway chute slabs are alsoequipped with weep holes. If all holes are

dry, it is probably because the soil behindthe wall or below the slab is dry. If someholes are draining while others are dry, thenperhaps the dry holes are plugged by mudor mineral deposits. Probe the plugged holeto determine probable causes of the block-age. Plugged weep holes increase chancesfor failure of the retaining wall or chuteslab. Try to clean out dirt or deposit andrestore draining ability. If this does notwork, rehabilitation work must be per-formed under the supervision of a profes-sional engineer as soon as possible.

d. Joints—Spillway retaining walls and chuteslabs are normally constructed in sections.Between adjoining sections, gaps or jointsmust be tightly sealed with flexible mate-rials such as tar, epoxies, or other chemicalcompounds. Sometimes rubber or plasticdiaphragm or copper foil is used to seal the

joint watertight. During inspection, notethe location, length, and depth of any miss-ing sealant. Also, probe the open gap anddetermine if soil behind the wall or belowthe slab has been undermined.

c. Misalignment—Spillway retaining wallsor chute slabs may be displaced from theiroriginal position by foundation settlementor earth or water pressure. Sight carefullyat the upstream or downstream end of thespillway near the wall to determine if it hasbeen tipped inward or outward. Relativedisplacement or offset between neighboringsections can be readily identified at the

joint. Measure the horizonta l as wel l asvertical displacement as explained in Sec-tion 12.5-2.3. A fence line on top of theretaining wall is usually erected in a straightline at the time of construction. Any curveor distortion of the fence line may indicatethat the wall has deformed.

At the time of construction, the entire spillwaychute is supposed to form a smooth sur-face. Measurement of relative movementbetween neighboring chute slabs at the jointwill give a good indication of the slab dis-placement.

Misalignment or displacement of the wall orthe slab is often associated with cracks. Aclear description of crack patterns should berecorded to photos taken to help in under-standing the nature of the displacement.

10.5 COMMONLY ENCOUNTERED PROBLEMS ANDACTIONS REQUIRED —The following chart is a listof problems frequently encountered when inspecting aspillway. Probable causes of each problem are present-ed along with recommended remedial actions.

10.6 SUMMARYThe spillway is a very important part of a dam. If it isnot designed with adequate capacity or constructedand maintained properly, overtopping of the embank-ment may occur during a large storm, resulting in fail-ure of the dam and serious damage to downstreamproperties, or even death of downstream residents.A spillway should always be kept free of obstruction,have the ability to resist erosion, and be protectedfrom deterioration. It should be emphasized that adam and reservoir represent not only a potential pub-lic hazard, but also a substantial investment. The dam’sowner can identify any changes in previously notedconditions that indicate a safety problem. A consci-entious annual maintenance program will address and



76

CONDITION FOUND

10.5-1

Debris or other obstructions

10.5-2

Excessive erosion in earth-slidecauses concentrated flows

10.5-3

End of spillway chute undercut

CAUSES & HARM DONE

Cause :

Accumulation of slide materials,dead trees, excessive vegetativegrowth, etc., in spillway channel.

Harm :

Reduced discharge capacity; over-flow of spillway; overtopping of dam. Prolonged overtopping cancause failure of the dam.

Cause:

Discharge velocity too high; bottomand slope material loose or deterio-rated; channel and bank slopes toosteep; bare soil unprotected; poorconstruction; protective surfacefailed.

Harm:

Disturbed flow pattern; loss of mate-rial, increased sediment load down-

stream; collapse of banks; failure of spillway; can lead to rapid evacua-tion of the reservoir through theseverely eroded spillway.

Cause:

Poor configuration of stilling basinarea. Highly erodible materials.Absence of cutoff wall at end of chute.

Harm:

Structural damage to spillway struc-ture; collapse of slab and wall; leadsto costly repair.1. Higher velocityflows can cause erosion of drain thenembankment materials.

ACTION REQUIRED

Action:

Clean out debris periodically; con-trol vegetative growth in spillwaychannel. Install log boom in front of spillway entrance to intercept debris.

Action:

Minimize flow velocity by properdesign. Use sound material. Keepchannel and bank slopes mild.Encourage growth of grass on soilsurface. Construct smooth and well-compacted surfaces. Protect surfacewith riprap, asphalt, or concrete.Repair eroded portion using soundconstruction practices.

Action:

Dewater affected area; clean outeroded area and properly backfill.Improve stream channel below chute;provide properly sized riprap in still-ing basin area. Install cutoff wall.

control most of the conditions identified above. Whena questionable condition is found, the State Engineer’sOffice should be notified immediately. Quick correc-

tive reaction to conditions requiring attention will pro-mote the safety and long life of the dam and possiblyprevent costly future repairs.

TABLE 10.5SPILLWAY INSPECTION – SPECIFIC ITEMS TO LOOK FOR



CONDITION FOUND

10.5-7

Breakdown or loss of riprap

PHOTO 10.5-1—Failed spillway gabion wall

10.5-8

Material deterioration-spallingand disintegration of riprap, con-crete, etc.

PHOTO 10.5-1—Deterioration of right D/Swingwall of spillway.

10.5-9

Poor surface drainage

PHOTO 10.5-3—Water exiting at joint orcrack because of poor drainage.

CAUSES & HARM DONE

Cause :

Slope too steep; material poorlygraded; failure of subgrade; flowvelocity too high; improper place-ment of material; bedding materialor foundation washed away.

Harm :

Erosion of channel bottom andbanks; failure of spillway.

Cause:

Use of unsound or defective materi-als; structure subjected to freeze-thaw cycles; improper maintenancepractices; harmful chemicals.

Harm:

Structure life will be shortened; pre-mature failure.

Cause:

No weep holes; no drainage facili-ty; plugged rains.

Harm:Wet foundation has lower support-ing capacity; uplift pressure due toaccumulated seepage water maycause damage to spillway chute;accumulation of water may alsoincrease total pressure on spillwaywalls and cause damage.

ACTION REQUIRED

Action:

Design a stable slope for channelbottom and banks. Riprap materialshould be well graded (the materialshould contain small, medium, andlarge particles). Subgrade should beproperly prepared before placementof riprap. Install filter fabric if nec-essary. Control flow velocity in thespillway by proper design. Riprapshould be placed according to spec-ification. Services of an engineer arerecommended.

ENGINEER REQUIRED

Action:

Avoid using shale or sandstone forriprap. Add air-entraining agent whenmixing concrete. Use only cleangood quality aggregates in the con-crete. Stell bars should have at least1 inch of concrete cover. Concreteshould be kept wet and protectedfrom freezing during curing. Timbershould be treated before use.

Action:

Install weep holes on spillway walls.Inner end of hole should be sur-rounded and packed with graded fil-tering material. Install drain systemunder spillway near downstream end.Clean out existing weep holes. Back-flush and rehabilitate drain systemunder the supervision of an engineer.

ENGINEER REQUIRED

78



CONDITION FOUND

10.5-10

Concrete erosion, abrasion, andfracturing

PHOTO 10.5-4—FAILED SPILLWAYFracturing of spillway chute slab

CAUSES & HARM DONE

Cause :

Flow velocity too high (usuallyoccurs at lower end of chute in rel-atively high dams); rolling of grav-el and rocks down the chute; cavitybehind or below concrete slab.

Harm :

Pock marks and spalling of concretesurface may progressively becomeworse; small hole may cause under-mining of foundation, leading to fail-ure of structure.

ACTION REQUIRED

Action:

Remove rocks and gravels from spill-way chute before flood season. Raisewater level in stilling basin. Usegood quality concrete. Make sureconcrete surface is smooth.

ENGINEER REQUIRED



11.1 INTRODUCTIONNow our attention will be concentrated on concretedams in order to allow the owner to identify readilyconditions that threaten the safety and long life of hisstructure. Although some of these items can be cor-rected by normal maintenance, more serious condi-tions will require inspection by the State Engineer’sOffice and may require further investigation by anexperienced consulting engineer. The items requiringthe attention of an engineer will be noted below as“ENGINEER REQUIRED.”From a safety standpoint, the principal advantage of concrete dams is their near immunity to failure byerosion during overtopping. Embankment slides andpiping failures, typical of earth dams, are also avoid-ed. Refer to Chapter 3, Section 6, for discussion of the basic principles which describe the behavior of concrete dams.Although concrete dams comprise less than 1% of thetotal number of dams within the state, they are, onaverage, of greater height and storage capacity thanearth structures. This makes them potentially morehazardous to life and property. It is important that damowners be aware of the principal modes of failure andthat they be able to discern between conditions whichthreaten the safety of the dam and those which mere-ly indicate a need for maintenance.

11.2 ITEMS OF PARTICULAR CONCERNConcrete dams fail for reasons different than earthdams. Several of these more serious problems are dis-cussed below. It is emphasized that, should these con-ditions be discovered during inspection, the ownershould obtain engineering assistance immediately.

11.2-1 Structural CracksThese are caused by overstressing of portionsof the dam and result because of inadequatedesign, poor construction technique, or faultymaterials. Structural cracks are often irregular,meaning they run at an angle to the major axesof the dam and may exhibit abrupt changes indirection. These cracks also have noticeableradial, transverse, or vertical displacement. (SeeFigure 11.1.)

(A) VIEW LOOKING UPSTREAM AT A CONCRETE DAM. NOTE IRREG-ULAR STRUCTURAL CRACK WITH VERTICAL DISPLACEMENT.

(B) VIEW LOOKING DOWN ON CONCRETE ARCH DAM. NOTECRACK WITH RADIAL DISPLACEMENT

(C) A VIEW LOOKING DOWN ON A CONCRETE GRAVITY Y DAM.

NOTE IRREGULAR STRUCTURAL CRACK WITH TRANSVERSEDISPLACEMENT.

FIGURE 11.1Figure 11.1 shows displacement measured at a single location. The amountof displacement often varies along the length of a structural crack. Thisvariation occurs because a portion of the dam has moved in relation to theoriginal alignment. In any case, the presence of structural cracks couldbe an indication of progressive failure of the abutment, the foundation, orthe dam itself. Engineering assistance should be obtained.

11.2-2 Foundation or Abutment WeaknessAs explained in Chapter 3, Section 6, concretedams transfer substantial load to the abutmentsand foundation. Although the concrete of thedam may endure, the natural terrain may crack,crumble, or move in a massive slide. If this

occurs, support for the dam is lost, causing it tofail. Examples of this process are shown in Fig-ures 11.2 and 11.3

CHAPTER 11

CONCRETE DAMS



FIGURE 11.2OVERTURNING FAILURE OF A CONCRETE GRAVITY DAM,CAUSED BY A WEEK FOUNDATION. THE SHAPE OF THE DAMDID NOT CHANGE. It failed because it lost support of the natural ter-rain. (Drawings are section view.)

PHOTO 11.2-1ALKALI-AGGREGATE REACTION. Note typical cracking pattern of exposed concrete.

FIGURE 11.3

ABUTMENT FAILURE OF A CONCRETE ARCH DAM. (Drawings arein plan view.)Impending failure of the foundation or abut-ments is difficult to detect because initial move-ments are often very small. This problem willbe further discussed in Section 11.3

11.2-3 Deterioration Due to Alkali-Aggregate ReactionSevere deterioration can result from a chemi-cal reaction between alkali present in cementsand certain forms of silica present in someaggregates. This chemical reaction producesbyproducts in the form of silica gels whichcause expansion and loss of strength withinthe concrete.Alkali reaction is characterized by certainobservable conditions, such as: “Cracking, usu-ally of random pattern on a fairly large scale,and by excessive internal and overall expan-sion. Additional indications are a gelatinousexudation and whitish amorphous deposit onthe surface, and lifeless, chalky appearance of the freshly fractured concrete.” 1 An example oalkali-aggregate reaction is shown in Photo11.2-1.

1Concrete Manual, U.S. Bureau of Reclamation, p. 8.

82

(A) CONCRETE ARCH DAM WITH A FAULT PLANE CUTTINGTHROUGH ONE ABUTMENT. Block “A” is unstable.

(b) Foundation starts to fail, allowing dam to be moved by the force of thewater.

(c) Dam collapses and water is suddenly released.

(B) BLOCK “A” MOVES DOWNSTREAM REDUCING SUPPORT FORTHE DAM. A structural crack develops, and seepage begins at weak abutmentand through dam.



contact with the reservoir water. This resultsin cracks which extend from the crest for somedistance down each face of the dam. Thesecracks will probably be at construction joints,if these are provided.Shrinkage cracks can be a sign that certain por-tions of the dam are not carrying load. 1 Thetotal compressive load must then be carried bya smaller percentage of the structure. It maybe necessary to restore load-carrying capabil-ity be grouting affected areas. This work requires the assistance of an engineer.

11.4-3 Deterioration Due to Spalling:Almost every concrete dam in Colorado expe-riences continued minor deterioration due tothe severe nature of the climate.Spalling is the process by which concrete chipsand breaks away, as a result of freeze-thawaction. Since it usually affects only the surfaceof the structure, it is not ordinarily considereddangerous. However, if allowed to continue,spalling will cause structural damage, particu-larly if the dam is of thin cross section. Also,repair is necessary when reinforcing becomesexposed to the elements.The method of repair of spalled areas dependsupon the depth of the deterioration. Repairshould be considered temporary unless seep-age through the dam can be halted. (Refer toChapter 13, Section 4.)In severe situations, engineering assistance isrequired to determine the best method of repair.Photos 11.4-3 and 11.4-4 are examples of dam-

age due to severe spalling.

Photo 11.4-3—Structural damage. Spalling has exposed reinforcing.

1 Design of Gravity Dams , United State Bureau of Reclamation, pp. 109-111.

PHOTO 11.4-4SEVERE SPALLING HAS EXPOSED REINFORCEMENT ON THE

UPSTREAM FACE OF THIS DAM.

11.4-4 Minor Leakage:Leakage through concrete dams, althoughunsightly, is not usually dangerous, unlessaccompanied by structural cracking. The worsteffect may be to promote minor deteriorationdue to the elements through freeze-thaw action.Increases in seepage could indicate that, throughchemical action, materials are being leachedfrom the dam and carried away by the flowingwater. Decreases in seepage may also occur asmineral deposits are formed in the seepagechannel. In neither case is the condition inher-ently dangerous, and detailed study is requiredbefore it can be determined that repair is nec-essary for other than cosmetic reasons.Photo 11.4-2 shows an example of typicalminor seepage eat a construction joint. Referto Chapter 5 for a general discussion of seep-

age.11.5 SUMMARY

Concrete dams pose a special problem for the inspec-tor because of the difficulty in gaining close accessto the nearly vertical surfaces. Regular inspection witha pair of powerful binoculars can initially identifyareas where change from surrounding areas is occur-ring. When these changes are noted a detailed close upinspection should be arranged for. Any questionablecondition requires immediate evaluation by an expe-rienced engineer. Since the failure of concrete damscan occur suddenly, even a hint of a problem must be

84



12.1 INTRODUCTIONThe previous chapters have dealt with the visualinspection of a dam. Although visual inspection is infact a simple and very useful form of monitoring, thischapter deals with the more accurate physical meas-urement of changes which occur in dams. The moni-toring of concrete dams is not specifically addressedin this chapter, but many of the principles of moni-toring earth dams can be applied to concrete dams.The owner of a dam frequently hears the term instru-mentation. Instrumentation refers to the method andequipment used to make physical measurements of dams. However, instrumentation is not a substitute forinspection; it is a supplement to the visual observa-tions made during an inspection.

1.2 FACTORS THAT CAUSE CHANGE INCONDITIONAn earth dam will normally undergo several predictablechanges throughout the life of the structure. Thechanges which occur and the factors causing thechanges have been identified through the use of instru-mentation. Furthermore, instrumentation has made itpossible to distinguish between normal and abnormalchanges. Knowledge of the changes which occur hasenabled the engineer to design and build dams higherand higher with more and more confidence. Thechanges which occur are: 1) vertical displacement (set-tlement); 2) horizontal displacement (change in align-ment); and 3) internal wetting of the embankment.Knowing that changes occur is one thing, but moreimportant is understanding the cause. The first changesthe dam experiences occur during construction. Asthe height of the dam is increased, the material in the

lower portion of the dam is compressed due to theweight of the material on top. The foundation mayalso compress (settle) due to the weight of the embank-ment. The changes which occur during construction arenot limited to settlement. Instrumentation of earthdams under construction has shown that horizontaldisplacement (spreading) occurs as well. First fillingof the reservoir will create a new imbalance of forcescausing more horizontal displacement and additionalsettlement occurs as the wetting of the embankmentprogresses. Other factors which cause continualchanges are variations in: 1) the depth of stored water(gage height); 2) the length of time maximum stor-age depth is present; and 3) the speed with which the

reservoir is drawn down.12.3 CAREFUL MONITORING CAN PREVENT

COSTLY PROBLEMS (i.e., SLOPPY MEASUR-ING TECHNIQUES OR PRACTICES AND POOR-LY MAINTAINED EQUIPMENT CAN HIDE APROBLEM OR CREATE A FALSE ALARM)The solution is obvious. The owner must have a con-sistent and systematic approach to monitoring of thedam. Also, by having concise records of the meas-urements, the owner can quickly determine if a prob-lem is developing. The owner should fully understandthe purpose of each instrument in order to understandthe use of recorded measurements.

12.4 MONITORING LEAKAGEALL DAMS WILL LEAK to varying degrees, butEVERY LEAK SHOULD BE MONITORED.The potential for a dam to leak will vary accordingto the design of the embankment, the ability of thecutoff to prevent leakage under the dam, and the tight-

ness of the natural abutments.Leakage should first appear at the toe drain if the damwas constructed with a drain system. If the dam does nothave a drain system, leakage may appear on the down-stream face.

12.4-1 WET AREASIf the area is damp, the perimeter of the wet areashould be staked out and the length and widthof the area should be recorded. Also the degreeof wetness, such as boggy, surface moist but firmunderfoot, etc., should be described. An exampleof staking wet areas is shown in Figure 12.4-1.

FIGURE 12.4-1—STAKING WET AREAS

When the leak produces a measurable flow of water, the quantity should be monitored. First,confine the flow through drainage channelsaway from the embankment. Then measure thequantity flowing by creating a drop in thedrainage channel and installing a pipe, a weir,or a flume.

12.4-2 PIPE FOR TIMED BUCKET MEASURE-MENT WITH STOP WATCHThe most accurate and direct measurement canbe obtained by catching the flow from a pipe ina container of known volume and timing howlong it takes to fill the container as shown inFigure 12.4-2. The flow rate should be record-ed in gallons per minute.

FIGURE 12.4-2—BUCKET AND STOPWATCH METHOD

CHAPTER 12

MONITORING AND INSTRUMENTATION



12.4-3 WEIRA weir, on the other hand, can save time, but themeasurement is not as direct as the bucket andstop watch. The rate of flow at a weir is relat-ed to the height of water flowing over the crestof the weir. The most commonly used, V-notchand rectangular weirs, are shown in Figures12.4-3 a and b.

FIGURE 12.4-3—STANDARD WEIRS

The V-notch weir is the most accurate for flowsless than 450 gal/min (1 ft 3 /sec.). If the ownerhas to make a choice the V-notch weir is pre-ferred over the rectangular weir. For compari-son tables of values for the two weirs are shownin Figures 12.4-4 and 5. A typical installationof a V-notch weir is shown in Figure 12.4-4.For flows greater than 450 gal/min., larger V-notch weirs can be used or a flume can beinstalled.

(Tabular values may be used only with w eirs of the dimensionsgiven in the sketch below.)

FIGURE 12.4-4DISCHARGE OF 90° V-NOTCH WEIRS

(Tabular values may be used only with w eirs of the dimensionsgiven in the sketch below.)

FIGURE 12.4-5DISCHARGE OF STANDARD 1-FOOT CONTRACTED

RECTANGULAR WEIR

86



FIGURE 12.4-6—TYPICAL V-NOTCH WEIR INSTALLATION

12.4-4 FLUMES

For larger flows, the Parshall flume is preferredto larger V-notch weirs because the flume willnot restrict the flow as much as the weir.Parshall flumes like that shown in Figure 12.4-7 can be purchased through a manufacturer and

is shown here only as a guide to help the ownerunderstand the methods used to measure leak-age quantities.

FIGURE 12.4-7 PARSHALL FLUME

PARSHALL FLUME(Installation and flow measurement

according to manufacturer’s instructions)

12.4-5 TURBIDITY AND SEDIMENTJust as important as accurate measurement of leakage quantities is observation of changes inthe turbidity and amount of sediment in thewater. Turbidity is a measure of the amount of soil particles suspended in the water. A visualdescription would be the color, e.g., clear,cloudy, etc. The sediment will usually be larg-er particles which settle out in a jar sample of the water. An increase in the turbidity or sedi-ment may indicate that the water is carryingsoil with it as it travels through the dam, a verydangerous condition. Each time the quantity ismeasured, an evaluation of the turbidity andsediment should be made to observe any change.The easiest method of comparing observationsis to collect a sample of the water in a quart

jar marked with the date collected and retainthe sample. A different jar should be used untilfive or six samples have been collected. Thenthe jars can be reused, starting with the onecontaining the oldest sample. This way, eachnew sample can be compared with the previ-ous samples to observe any change in the tur-bidity or amount of sediment in the water.

12.4-6 RECORDSAlways remember to record reservoir gage rodheight along with leakage quantities. It is alsohelpful to have an area map describing the loca-tion and extent of the leakage similar to themap shown in Figure 12.4-8. All pertinent fea-tures of topography and sources which may becontributing to the leakage should be includedon the map.

FIGURE 12.4-8—TYPICAL MAP OF LEAKAGE AREAIn addition, leakage from toe drains and adescription of any visual changes in the qual-ity of the water (i.e., turbidity, amount of sed-iment) should be recorded.A photograph of the leakage or wet area is alsohelpful in describing the situation.A sample form for recording leakage meas-urements and wet area dimensions is shown onFigure 12.4-9. The format should be adjusted tofit each individual situation.



12.4-7 SEEPAGE EXITING ON THE DOWN-STREAM SLOPELeakage which poses the most hazardous threatto the safety of the dam is that which appearson the downstream slope. This leakage is oftena result of the flow of water through the dam asdescribed in Chapter 3. Figure 12.4-10 showshow the condition progresses with time.

FIGURE 12.4-10—WETTING OF EMBANKMENT

12.4-8 OBSERVATION WELLSThis progress of flow through the dam is eas-ily observed by using observation wells. Theuse of observation wells will confirm whetheror not the dam is behaving as designed to resistflow.Figure 12.4-11 shows a typical observation wellinstallation.

FIGURE 12.4-11—TYPICAL OBSERVATION WELL INSTALLATION

Water in embankment enters the standpipethrough the slotted portion and rises to the samelevel as the water in the soil around the obser-vation well.Each reading of an observation well should becompared to previous readings.

A change in the reading should be evaluated asfollows:

1. Draw a profile of the water surface on thecross section of the dam along with thereservoir water surface (similar to that shownin Figure 12.4-13).a. If the profile appears normal or what can

be expected for the current height of thereservoir water surface, continue normalmonitoring program. (See Chapter 3.)

b. If the profile appears unusual it may indi-cate a potentially dangerous situation (seeChapter 3). Contact State Engineer’s

Office and your consulting engineer.12.4-9 RECORDING FORM

A sample recording sheet is shown in Figure12.4-9. Note that space is provided for drawingcross sections. This enables the observer to eval-uate the condition immediately. Graphs shouldalso be maintained showing the entire historyof observation well measurements. The formatof the graphs should make them easy to updateafter each measurement. This will enable theobserver to see the relationship between the cur-rent reading and previous readings graphically.The plot of observation well readings can besimilar to Figure 12.4-12. Note that the reser-voir elevation is also shown because it is thepredominant variable which influences the pro-file of water level within the embankment.

88



12.5 MONITORING DISPLACEMENTSMonitoring displacements can be helpful in under-standing the normal behavior of a dam as well as beinguseful in determining if a potentially hazardous con-dition is developing. The displacements, both hori-zontal and vertical, are more commonly measured onthe surface of the embankment. Measuring displace-ments of points on the surface is usually accomplishedby conventional surveying methods such as leveling oralignment. Other methods are described in Section12.6. The movements described above are not limitedto just the embankment but can sometimes be tracedto a point below the dam in the foundation. Internaldisplacement monitoring schemes can be complex andexpensive. Therefore, the measurement of displace-ments is usually monitored on the surface, unless aproblem develops.

12.5-1 ALIGNMENT AND SETTLEMENTA simple system for monitoring displacementson the surface consists of a few permanentpoints across the crest of the dam. The pointsare usually marked with a 3-foot length of 1-inch diameter rebar set in concrete. A typicalinstallation is shown in Figure 12.5-1.

FIGURE 12.5-1—INSTALLATION OF PERMANENT POINTS

The top of the rebar is marked with a cross orcenter punched hole. All points are initially seton one line-of-sight established between aninstrument station on one abutment and a tar-get station on the other abutment as shown inFigure 12.5-2.

FIGURE 12.5-2—PLAN OF ALIGNMENT SYSTEM

The alignment system measures the change inthe point’s position relative to the line of sight.Subsequent measurements are compared withthe initial. The amount of horizontal displace-ment from the line-of-sight and the change inelevation from the initial is reported. The rareof settlement and horizontal displacement withtime or reservoir gage height can be observed.The single line-of-sight system can be expandedto include two or even three lines-of-sight to mon-itor points across the upstream and downstreamface. More often the alignment monitoring systemis used to establish behavior patterns of the damespecially during filling of the reservoir or duringthe construction of modifications to the dam.

12.5-2 CRACKS ON THE EMBANKMENTThe owner is frequently faced with special situ-ations where the temporary and immediate mon-itoring of potentially dangerous conditions is need-ed. Sometimes this calls for a little imagination onthe part of the owner and the use of commonmaterials one may find in his own backyard.Suppose a small crack is observed on theembankment. It would be important to know if the crack enlarges. An easy method of monitor-ing the crack is to drive rebar or stakes on bothsides to monitor additional separation and ver-tical displacement on one side of the crack rel-ative to the other side. Also, the end of the crack should be staked to determine if the crack islengthening. The example is shown in Figure12.5-3 a and b. This scheme can be used to mon-itor both longitudinal and transverse cracking.

FIGURE 12.5-3—MONITORING CRACKS ON EMBANKMENT



12.5-3 SLIDE ON EMBANKMENTAnother special situation which would requireimmediate attention is a slide. The methodshown is simple yet reliable and utilizes thesame principle as the alignment method. Astrong wire is stretched across the slide andtied to pins outside the slide area. At intervalsalong the wire, pins are driven into the slidemass as shown in Figure 12.5-4. If additionalmovement occurs, the amount is directly deter-mined by measuring the distance between thepins and the wire.

FIGURE 12.5-4—MONITORING A SLIDE

12.5-4 DISPLACEMENTS OF CONCRETESTRUCTURES

The owner should also concentrate on moni-toring changes in the concrete structures asso-ciated with the dam, such as the spillway andoutlet works. The owner should monitor verti-cal, horizontal, and lateral displacements; struc-tural cracking; and tilting of walls in spillwaysor the drop structure for the outlet. A few sim-ple methods are illustrated in Figure 12.5-5.

FIGURE 12.5-5—MEASURING DISPLACEMENTSThe information from routine measurement of displacements will tell the owner if movementis continuing and at what pace.

12.6 SPECIALIZED INSTRUMENTATIONThe requirements for specialized instrumentation aremore often specific to each individual dam site andrequire an evaluation by an engineer. Entire books havebeen written on the three types of specialized instru-mentation presented in this section. Therefore, this sec-tion will provide the owner of a dam only general infor-mation to allow the owners to communicate better withthe engineer.The three types of instrumentation presented are 1)piezometers, 2) inclinometers, and 3) triangulation/tri-lateration. Following the section are charts the ownercan use for quick reference.

12.6-1 PIEZOMETERSUnlike the observation well, which directly showsthe height of water in the embankment or foun-dation, a piezometer indicates the pressure exert-ed by the height of water above the tip of thepiezometer. The pressure, though, can be relatedto the height of water in the embankment orfoundation. The similarity between the piezome-ter and observation well would make thepiezometer less desirable because of addedexpense. However, the piezometer can performother functions which an observation well cannot.The advantage of the piezometer can best beshown in a case where foundation water pressureis being measured. The case in point is wherethe equivalent height of a column of water forthe pressure being measured by the piezometer

would exceed the height of the dam.

92



FIGURE 12.6-1—PIEZOMETER VS. OBSERVATION WELL

For the example above, the equivalent height of water for a piezometer reading of 30 lb/in 2 is 70feet, 20 feet above the crest of the dam. Do notbe misled by the example because it is not typi-cal. However, the situation can develop andshould be evaluated by an engineer. There areseveral types of piezometer available (i.e., opentube, hydraulic, pneumatic, electric, etc.). Selec-tion is made based on use, cost, and availability.

12.6-2 INCLINOMETERSInclinometers are used to monitor internal dis-placements of the embankment or movementsin the foundation. The purpose and advantage of making measurements of internal movements arethat the movements will undoubtedly be detect-ed before the effects appear on the surface.The embankment and foundation deforms andcauses the surface to move. Therefore, surfacemonitoring of displacements does not alwaystell the entire story of “what is happening.”The inclinometer is designed to measure hori-zontal movements of the embankment and/orthe foundation at any depth below the surface.

The system consists of a special casing withgrooves on the inside at the quarter points.

FIGURE 12.6-2—INCLINOMETER CASINGThe casing is installed in a drilled hole and back-filled with material selected by the engineer.

FIGURE 12.6-3—INCLINOMETER—Detail at surfaceThen an instrument is used to traverse thelength of the casing and determine its profile intwo perpendicular directions.

FIGURE 12.6-4—INCLINOMETER AND CASING

All readings are compared with the initial pro-file to determine if changes have occurred. Theresults are usually presented on a graph asshown below.

FIGURE 12.6-5—PLOT OF INCLINOMETER READINGSIn the above graph the movement that took place between the first and second reading isless than 1 ⁄ 4 inch. The small displacement couldnot have been measured by conventional sur-veying methods on the surface. Inclinometersare most beneficial if installed before a surfacemovement is detected.



12.6-3 TRIANGULATION/TRILATERATIONFor dams with a long crest length, the line-of-sight can be excessively long. In this case, theline of sight is frequently moved downstreamof the dam and becomes a baseline. Displace-ments are monitored by turning angles fromfixed points on the baseline to points on thedam. The system forms triangles and is knownas triangulation. Instead of measuring angles,sometimes the horizontal distances between theend points of the baseline and points on the damare monitored using electronic distance meas-uring (EDM). The distance measuring schemeis known as trilateration. Triangulation and tri-lateration are strictly for horizontal control.It should be emphasized here that the moreelaborate the scheme for monitoring surfacemovements, the more important it is to consulta qualified engineer or land surveyor.

12.7 SUMMARYInstrumentation refers to the method and equipmentused to make physical measurements of dams. How-ever, instrumentation is not a substitute for inspec-tion. It is a supplement to the visual observations madeduring an inspection.

Instrumentation of dams can provide information aboutthe behavior of the structure under normal operatingconditions. More frequently though, the owner is facedwith special or unusual situations which develop.Whether the situation involves leakage or displace-ment, it calls for immediate attention. With a little bitof imagination and the suggested methods presentedin the chapter, the owner can obtain information toassess the performance of his dam better.

94

SPECIALIZED INSTRUMENTATION

PURPOSE AND GENERAL DESCRIPTION

The piezometer measures the pressure of water entering the porous stone. Inthe case of an earth dam the pressure is primarily due to the infiltration of water into the embankment from the reservoir. The pressure exerted on thepiezometer is a function of the height reached by the water in the embank-ment above the piezometer.

The advantage the piezometer has over the observation well is the ability tomeasure small changes in the water level above it. A piezometer also has amore rapid response to changes of water pressure in the embankment. Thepiezometer can also sense changes in the water pressure created by factors

other than increase in the water level in the embankment.A piezometer can be installed in the foundation under the embankment. Thefigure at left is a typical installation detail.

An engineer should be consulted to evaluate the need and supervise the instal-lation of piezometer.



SPECIALIZED INSTRUMENTATION


The inclinometer is a system used to measure the inclination of a special cas-ing installed in the embankment portion of a dam.

The inclinometer probe will detect small horizontal displacements of the cas-ing. Many times the displacements are small but can be a sign of internalmovement of the dam. Being able to detect small internal movements canwarn against a large movement before it is observed on the surface as a crack or slide.

An engineer should be consulted to determine the location and supervise theinstallation of inclinometers.


The instrument is positioned over the station. A target is placed on the otherinstrument station and at the point to monitor on the dam.

Once the sight on the baseline is established the angle between the baseline andthe line to the target is measured.

After measuring angles to all targets the instrument is moved to the other sta-tion and the other angles from the baseline to the targets are measured.

Probably the most common source of error is not having the instrument exact-ly plumb.



OBSERVATION WELL MEASUREMENTS

DATE _______________________

DAM NAME __________________________ DIV _________________________ DAM ID ______________________

Maximum Gage Rod Height ________________ ft. Corresponding Reservoir Water Surface Elevation __________________

Comments:

____________________________________________________________________________________________________

____________________________________________________________________________________________________

____________________________________________________________________________________________________

____________________________________________________________________________________________________

____________________________________________________________________________________________________

*If dry, write “DRY.” If frozen, write “FROZEN.”FIGURE 12.4-12



FIGURE 12.4-13—GRAPH OF OBSERVATION WELL READINGS



13.1 TREE, BRUSH AND WEED CONTROL13.1-1 INTRODUCTION

Periodic maintenance is required to keep anydam in safe operating condition. The firstmaintenance requirement is to keep all por-tions of the dam clear of unwanted vegetative

growth. Excessive growth is harmful in thefollowing ways:

a. It can obscure the view of the embankmentand prevent a thorough inspection for pos-sible cracks or other evidence of problemson the dam.

b. Large trees could be uprooted during astorm and the resulting large hole left bythe root system could lead to breaching of the dam.

c. Some root systems can decay and rot, pro-viding a tunnel for water to pass through(called piping).

d. Root systems can cause the uplift of con-crete slabs or structures.

e. Roots can loosen the compacted soil of thedam.

f. Weeds can discourage the growth of desir-able grasses.

g. A muskrat habitat is taken away when cat-tails are cleared from around a dam. Whentrees are too far away from a water source,beavers are discouraged from buildingdams in the spillway or outlet.

After the removal of brush, the cuttings mayneed to be burned. Take precautionary meas-ures and contact the fire department, forestservice, or respective agency responsible forfires. When brush is cut down, it should behauled off the dam to allow a clear view of the embankment.Additional follow-up work would include:

Excavate and remove left-over root sys-tems.Fill in resulting holes. See earthwork sec-tion.

The following charts show various removalmethods.

CHAPTER 13

MAINTENANCE 10

METHOD

1. Repeated Cutting

PURPOSE

Eliminate food producingleaves, then roots starve.

TIMING

Do several times each year;control of growth takes oneyear.

ACTION REQUIRED

Hand cut, machine cut, mow,if the growth is palatable;vegetation can be grazedheavily by livestock.

2. Sp raying fol iage withHerbicides

To kill the targeted plant. Follow label directions. ANALYZE THE PROB-LEM.

Identify the problem species;determine density, stage of growth. Select the herbicideconsidering safety, selectivi-

ty, and residues. Determineif chemicals are registered foraquatic use.

CHART 13.1-2

METHODS OF TREE, BRUSH, AND WEED CONTROL

FIGURE 13.1-1—Effective portable brush cutter

NOTE: For further information on the use of herbicides, contact Colorado State University Extension Service Director, Ft.Collins, Colorado, or your county extension agent.



equipment. The intent is to select a materialwhich, when compacted, forms a firm, solidmass, free of voids.The type of material selected depends uponthe type of repair, and the purpose of that por-tion of the embankment. If flow-resistant por-tions of the fill are being repaired, select mate-rials which are high in clay or silt content. If the repair is to be free draining (e.g., riprapbedding, etc.) the material should have a high-er percentage of sands and gravels. It is best toreplace or repair with soils of types similar tothose originally in place.

13.2-3 MOISTURE CONTENTAn important soil property affecting com-paction is moisture content. Soils which aretoo dry or too wet do not compact well. Bor-row area material may be tested by roughlychecking its ability to be compacted. Theinspector should grab a handful of loose earthand squeeze it into a tight ball. If the samplemaintains its shape without cracking andfalling apart (which means it is too dry), andwithout depositing excess water onto his hand(which means it is too wet), the moisture con-tent is probably near the proper level.A soil’s ability to be compacted can also bechecked in a manner similar to that shown inFigure 13.2-1. All that is needed is a coffeecan and a stick. Soils which are too dry fluff up and form dust when tamped, while thosewhich are too wet bulge up around the stick.

To see if the soil in Figure 13.2-1(c) is form-ing a tight matrix, remove it from the containerand break it apart to confirm that each soil par-ticle is fully surrounded by an earth mass (i.e.,no voids).

13.2-4 EARTH PLACEMENTPrior to placement of earth, the repair areamust be prepared by removing all non-suit-able material. Vegetation such as brush, roots,and tree stumps must be cleared and any largerock or trash removed. Also, non-suitable earth,such as organic or loose soils, should beremoved so that the work area surface consistsof exposed “clean” and firm embankmentmaterial.Following clean-up, the affected area shouldbe shaped and dressed so that the new fill canbe compacted and will properly tie in to exist-ing fill. If possible, slopes should be trimmedback to about a 2:1 inclination, and surfacesroughened by scarifying or plowing. This willimprove bond between the new and existingfill and provide a reasonable base to compactagainst. (See Figure 13.2-2(b).)Soils should be placed in loose lifts about 8

inches in maximum thickness and compactedby hand or with equipment to form a densemass, free of large rock, streaks, lenses, ororganic material. (See Figure 13.2-2(c).) Soilmoisture content must be maintained in theproper range. The fill should be watered andmixed if too dry, and allowed to dry out if toowet.During the backfilling operation, care must beexercised so the fill does not accidently becometoo wet due to rainstorm runoff. Runoff watersshould be directed away from the work area,and the fill brought up so that it maintains acrown which will shed water. (See Figure13.2-2(d).)

10

(a) Soil is too dry. Itfluffs up and formsdust when compact-ed with tamper.

(b) Soil is too wet. Itbulges up aroundtamper when com-pacted.

(c) Soil moisture isin proper range. Soilforms firm, smoothsurface when com-pacted.

FIGURE 13.2-1



(a) ORIGINAL CONDITION.Gull is about 12 inches depth, and has col-lected trash and debris.

(b) AFTER CLEAN UP. Brush and debris

removed and sides of the gully pulled back toa gentle slope.

(c) PLACE OF FIRST LIFT.Soil is placed in a loose layer about 8 inchesin thickness, and then thoroughly tampeduntil it forms a firm surface.

After compaction of first life.

(d) PLACEMENT OF LAST LIFT.Again, loose earth thickness is about 6 inch-es. Slightly overfill the gully so the finalground surface is a little above the sur-rounding terrain.

The final result. Slight camber shown at “a”.

104

FIGURE 13.2-2



13.2-5 QUALITY CONTROLThe prior two sections were written so the damowner could ensure a reasonably good job withmaterials and equipment on hand. If the repair

job is of large size or done by others undercontract, certain tests are recommended tomake sure the work is of good quality (i.e.,worth the money spent on it). A few of thesetests are listed in Table 13.2-1. These requirespecial equipment and should be performedby an engineer or technician familiar with soilstesting. For more information, see footnote 1.

TA3LLE 13.2-1EARTHWORK QUALITY CONTROL

TESTING

13.2-6 REPAIRING RODENT DAMAGEThe type of treatment depends upon the natureof the damage which has been done to theembankment by the rodents. In any case, exter-mination of the rodent population is therequired first step. (See Section 13.3.)If rodent damage consists of mostly shallowholes scattered across the embankment, repairmay be necessary for cosmetic reasons, to keeprunoff waters from infiltrating the dam, or todiscourage rodents from subsequently return-ing to the embankment to convenient homes.In these cases, tamping of earth into the rodenthole should be sufficient. Try to get soils asdeeply as possible into the dam and compactin place with a pole or shovel handle.Different repair measures are necessary wherea dam has been damaged by extensive smallrodent tunneling or by beaver or muskrat activ-ity. In these cases, it may be necessary to exca-

vate the damaged area down to competent filland repair as previously described under Sec-tion 13.2-4.Occasionally, rodent activity will result in pas-sages which pierce the embankment and leavetunnels resulting in leakage of reservoir waters.In these cases, plugging the downstream end of the tunnel should be avoided as it will add tosaturation of the dam. Holes which pierce theembankment are a significant hazard, as theyprovide a passageway which could eventuallydevelop into a piping failure of the dam. Tun-nels of rodents or ground squirrels will nor-mally be above the phreatic surface with pri-mary entrance on the downstream side of thedam, while those of beaver and muskrat nor-mally occur below or at the water surface withentrance on the upstream slope.If a rodent hole is found to extend through thedam, the best procedure is first to locate theupstream end of the passage. The area aroundthe entrance should be excavated and thenbackfilled with impervious material. This pro-cedure places a plug or patch at the passageentrance so that reservoir waters are kept fromsaturating the interior of the dam. (See Figure13.2-3.) This should be considered a tempo-

rary repair. Excavation and backfilling of theentire rodent tunnel may be the only feasiblemethod of repair.

10

NAME OF TESTOR PROCEDURE

FIELD DENSITY

TEST

PROCTORCOMPACTIONTEST

GRADATIONANALYSIS

DESCRIPTION AND USE

A test to determine the in place

(field) density of soil. This testis the only way to be sure thatearth has been properly placedand packed in.The field density test is onlymeaningful when used in com-parison to results of the proctorcompaction test.

This test determines what a soil’smoisture content should be forbest compaction and serves as astandard of comparison for thefield density test.

It is a laboratory test procedureto determine the relation betweena soil’s water content and its den-sity for a specified compactiveeffort.

A curve showing the particle sizebreakdown for a particular sam-ple of soil. Perform this test onselected borrow material to seeif it is mostly granular and freedraining (sands and gravels) orfine “rained and flow resistant(silts and clays).

1 Earth Manual, United States Bureau of Reclamation, pp. 424, 466, 613.



FIGURE 13.2-3

TEMPORARY REPAIR OF DAM WHEN RODENT TUNNEL

PIERCES THE EMBANKMENT

Filling of rodent holes by pressure grouting isan expensive and sometimes dangerous pro-cedure. Pressures exerted during grouting cancause additional damage to the embankmentdue to hydraulic fracturing. Grouting shouldbe performed only under the direction of anengineer.

13.2-7 FILLING MINOR CRACKSOccasionally, minor cracks will form in anearth dam due to surface drying. These arecalled dessication, or drying, cracks and arenot to be confused with structural or settle-ment cracking as previously discussed in thismanual. Drying cracks are usually parallel tothe main axis of the dam, typically near theup or downstream shoulders of the crest. Thesecracks often run intermittently along the lengthof the dam and have depths of up to 4 feet.The key to distinguishing drying cracks fromother more serious structural

cracking is that the former are usually no wider than a few inch-es and display no “vertical offset.” (See Figure 13.2-4 below andPhotograph 7.5-8 in Chapter 7.)

FiGURE 13.2-4—DRYING V.S. STRUCTURAL CRACK

As a precaution, suspected drying cracksshould initially be monitored with the samecare required for structural cracks. The areashould be marked with survey stakes, and mon-itoring pins installed on either side of the crack to allow recording of any changes in width orvertical offset. (Refer to Chapter 12 Section12.5-2 for discussion of monitoring methods.)Once satisfied that observed cracking is dueto shrinkage or drying, the owner may dis-continue monitoring.Often these cracks will close up as climactic orsoil moisture conditions change. If not, it maybe necessary to backfill the cracks to prevententry of surface moisture which could resultin saturation of the dam. This can be done by

simply filling the cracks with earth and tamp-ing it in place with foot or shovel. In addition,it is recommended that the crest of the dambe graded to direct runoff waters away fromareas damaged by drying cracks.

13.2-8 SEALING RESERVOIR BASINSOccasionally, a reservoir will be constructedon pervious terrain. Leakage into the floor of the basin or into abutments may make it diffi-cult to maintain full storage and could endan-ger the dam if it occurs through the dam’sfoundation or abutment contacts.There are two common methods of sealingreservoir basins: blanketing with a compact-ed clay layer and lining with plastic sheeting.Regardless of the selected method, the processrequires adherence to technical engineeringstandards to insure a valuable result. Thus, forlarge jobs, the assistance of an experiencedengineer is recommended during design andactual construction.If the basin is to be sealed by placement of anearth blanket, it should be constructed of

106

(d) Temporary repair is complete. Engineering assistance recommendedto backfill remainder of the tunnel.

(c) Dam owner notices the leak and quickly lowers the reservoir. Repairis performed as described in Section 13.2-6 and Figure 13.2-2, and a plugis formed in the tunnel.

(b) The water surface is later raised and a leak develops, threatening thedam.

(a) Rodents attack the dam and tunnel above the phreatic surface. Thisoccurs when the water surface is low.



compact material, otherwise suitable for con-struction of an impervious core of the embank-ment. Blanket thickness should be roughly10% of the water depth above the blanket and3 feet as a minimum’. If the earth blanket issubject to wave action, a covering of riprapplaced upon bedding is recommended.Preventing seepage by placement of plasticsheeting is possible, but difficult, particularlyif only a portion of the basin is to be protect-ed. Liners must be installed according to man-ufacturers’ recommendations if they are to per-form properly, and surface preparation isnecessary to insure they do not become punc-tured during installation or use. Portions of the liner subjected to wave action or mechan-ical injury must be protected with a layer of earth, usually topped with riprap. An earth cov-ering is also recommended to protect the linerfrom deterioration due to sunlight

The primary problem with using plastic sheet-ing to line portions of a reservoir basin is thatthe plastic must be bonded into the underly-ing earth so that a cutoff is formed around itsedges. If this cutoff is not provided, water mayget under the plastic, around its edges, andflow unrestricted to the original point of leak-age. (See Figure 13.2-5.)

FIGURE 13.2-5SEALING RESERVOIR BASINS AGAINST MINOR LEAKAGE

BY USE OF PLASTIC SHEETING

1Design of Small Dams, U.S. Bureau of Reclamation, p. 332.

13.2-9 SUMMARY—EARTHWORK MAINTE-NANCEThis section has provided information useful toaccomplish minor earthwork repair, usingmaterials and equipment normally on hand. Itis not intended as a technical guide. Seriousconditions which threaten the safety of thedam should be repaired under the supervisionof an engineer familiar with earthwork. If acondition appears hazardous, the dam ownermay request an inspection and guidance fromthe State Engineer’s Office.

13.3 RODENT CONTROLThe following chart lists the six rodents thataffect dams in Colorado and the methods thatcan be used to control the rodents. One of theanimals is not a rodent but is of the order car-nivore—the badger. Control of these animalsis important to avoid costly repairs. On somedams in the state beavers or muskrats have ini-

tiated tunnels that eventually passed throughthe entire dam.Assistance in controlling rodents may be avail-able through the Furbearer Control Land Refer-ral System. This system, sponsored by the Col-orado Trapper’s Association, is designed tominimize damage caused by water animals,predators, and rodents to dams, livestock, pas-ture, and croplands. The referral system cur-rently provides assistance to and works in con-

junction with the Colorado Catt lemen’sAssociation, Wool Grower’s Association, Divi-sion of Wildlife, and Department of Agriculture.Coordination for rodent control should be madewith the county extension agent or the Col-orado State University Department of PestManagement. When the rodents have beeneffectively removed and a program of contin-ued removal has been initiated, the resultingvoids must be backfilled and properly com-pacted. See earthwork section of this chapter.

10

(a) ORIGINAL CONDITIONA suspected point source of seepage is found in the reservoir basin.

(b) AFTER CLEANUPCutoff trenches are dug to accept the plastic sheeting (refer to Section13.2-1 and Figure 13.2-2.)

(c) AFTER BACKFILLA bed is placed or the plastic sheet to lie on, so it won’t be damaged.

(d) FINAL RESULTPlastic sheet is tied into the cutoff trench with impervious backfill, andcovered with a layer of earth.



Badgers—are 1 1 ⁄ 2 feetlong, weigh 25 to 30pounds, grey color,stripe on headand neck.

Dig after rodents andmake larger hole.

More prevalent insagebrush country. Donot like moisture.

Any time. Trap the badgers.

Name of Animal andDescription

Beavers—a beaver canbe 4 feet long andweight 30 to 40pounds. A litter of 2 to6 young can occur inMarch or May.

Harm to Dam

Create dens at up-stream slope and intodam. They block spill-way with their owndams. The water levelcould be raised from ablocked spillway.

Characteristics

Beavers are active dayand night. Can stayunder water up to 15minutes. Will draglogs about 100 yardsfrom lake edge andthen float logs. Expo-sure time to predatorsis increased for longlog hauling distances.

Timing

Any time

Action Required

No baiting is used. Itis best to trap beaversor remove their habi-tat and food sourcessuch as aspen trees,cottonwoods, and wil-lows.

Muskrats—are 16inches long (includestail). Weight 4 to 5pounds.

Muskrats dig holes onupstream side wheregood vegetation islocated to include cat-tails.

Dig horizontally fromwater side of a ditchor dam. Dig at points

just above saturatedsoil.

Trapping season ismiddle of Novemberthrough March. Athigher altitudes trap-ping occurs in October.

1. Trapping is mosteffective. Contact pri-vate trapper and hewill trap muskrats fortheir pelts. Usuallyeffective method.2. Put out floater sta-tions with rodent baiton the platform. Userolled oats.3. Remove rodentsany time if they threat-en the safety of the

dam.

108

Richardson’s GroundSquirrel—is 10 to 14inches long, 2 to 4inches tall, very smallears, weighs 1 pound.Smoke-grey fur.

They dig a hole 4 to 5feet deep. Diameter of hole 1 1 ⁄ 2 to 2 inches.

Not found in moistareas. Like rockyplaces. Do not live intimber. Like sagebrushand open country.Feed on grass. Foundnear Steamboat, Glen-wood Springs, Salidaand eastern Colorado.AT T R AC T B A D-GERS.

Poison from openingof spring to middle of July. Short season.Could hibernate inJuly.

Use recommendedpoison per your coun-ty extension agent.

CHART 13.3-1DAMS RODENT CONTROL CHART



10

Name of Animal andDescription

Prairie dogs—10-12inches long, weigh 1-1 ⁄ 3 to 2 1 ⁄ 2 pounds.

Harm to Dam

May burrow at toe of dam causing concen-trated leakage of seep-age water. May diginto crest causingholes up to 5 inches indiameter.

Characteristics

Do not like moisture.Attract badgers whodig bigger hole.

ATTRACTS BAD-GERS.

Timing

Follow label direc-tions.

Action Required

Avoid random poison-ing, could harm otherwildlife.

Use oats as a bait—especially when greenfood is not available.This is called prebait-ing. Then allow lapsetime.

Two to ten days later.Repoison.

Thoroughly treat en-tire colony. Secondapplication. Fumigationcan destroy remaininganimals.Use recommendedpoison per your coun-ty extension agent.

Pocket gophers—About 9 inches longand weight 1 ⁄ 4 pound.

Dig shallow holes 3inches in diameter and4 to 24 inches deep.Nest chamber usually8 to 10 inches diameter.

Main food is the rootsof dandelions andother undesirableweeds.Don’t like daylight.

Follow label direc-tions.

Use recommendedpoison per your coun-ty extension agent.Open hole —open uphole and put poisonbait or trap gophers.Core to plug off day-light. P robe method—bait is placed on therunway through aprobe hole. Bait dis-penser — possiblyhand dispenser typelike corn planter.3. Trapping for smallareas.4. Exclosure—fenc-ing for at least 12inches deep for smallareas.



13.4 CONCRETE STRUCTURES

13.4-1 GENERAL INFORMATIONFor the purpose of this manual, we will beconcerned with the concrete structures whichare accessory to the principal dam structure,which may be an earthfill or rockfill

embankment.We will be primarily concerned with:Upstream slope paving.Outlet control structures.Spillways (all types).Miscellaneous small structures.Generally speaking, concrete is a reasonablydurable material. However, because of the envi-ronment in which it is used, concrete doesdeteriorate over the years, and this process isaccelerated by exposure to extreme weatherconditions. The most common form of failureis the breakdown of the surface layers of con-crete as evidenced by the scaling and pittingwhich becomes very noticeable.Another form of failure is indicated by theappearance of large cracks in the concrete. Themost common cause for this type of failure isthe increase in stress that the concrete is sub-

jected to and usually results from the unevensettlement of the structure or from unequal orexcessive earth pressures against the concrete.Generally, these cracks result from unantici-pated service conditions, and the concrete doesnot have adequate reserve strength to accom-modate the extra load.

Large structural cracks that develop in con-crete will normally require an inspection andevaluation by a qualified structural engineerto determine the cause of the cracking and rec-ommend the most efficient and practical repairprocedure.This manual is directed to the repair and main-tenance of concrete structures, but it is advis-able at this point to recommend strongly to alldam owners and operators that they get thebest possible concrete that they can. This callsfor the responsible person to make use of whatis generally termed “Quality Control.”

Do not shy away from this phrase “QualityControl.” The basic idea is to make sure bymeans of several tests on the fluid concretewhen it is delivered to the job site that it con-forms to the material as ordered. The tests arenot too complicated, but some require specialequipment and knowledge of the techniquesemployed to use them properly.

Following is a list of the concrete qualities thatshould be checked. However, it should be madequite plain that the actual test procedures arebeyond the scope of this manual but aredescribed in several of the publications listedin the bibliography.Gradation and durability of coarse and fineaggregates (can be certified by the concretesupplier).Proper water content (requires slump cone andtamping rod for slump test).Correct air content (requires air meter).Correct strength (requires casting test cylin-ders and breaking at a later date).These are the major factors to be considered inorder to get top quality durable concrete. Topquality concrete pays off in longer service lifeand reduced maintenance costs.The basic idea in attaining durable concrete is

stated above. Start with top quality concrete.However, concrete, once it is in place is underconstant attack by weather, chemical action,and wear. Good concrete properly placed andfinished should have a dense, durable surface,made as water-resistant as possible, which willhelp slow the deterioration process.When concrete in a very moist atmosphere isexposed to repeated cycles of low tempera-tures at night (freeze) and warming tempera-tures during the day (thaw), there is a goodchance for the surface of the concrete to startbreaking up. This condition results when anymoisture which has penetrated the porous con-crete surface freezes and turns to ice. Then theexpansion of the ice tends to loosen or dis-lodge particles of the concrete. As thisfreeze/thaw action continues over a period of many years, the concrete slowly disintegrates.Several things can be done to slow or stop thisdisintegration. First, good quality concrete witha dense surface is the basic requirement. Next,if the concrete surface is showing signs of breakup, then one of several treatments shouldbe applied to the affected areas. There are var-ious products available that can be used to sealand protect the surface. Two general types of

coating materials are available for this pur-pose. The first type consists of single- com-ponent liquids which can be applied directly tothe concrete surface by brushing or spraying.The second type are the two component mate-rials which have to be job-mixed just prior toapplying and have to be used immediately.These generally are the epoxy compoundswhich require the addition of an activator (cat-alyst) just prior to

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Before applying the patching material, washthe area clean with clear water, leaving thearea in a surface-damp condition. Prepare thepatching material, which is usually a mixtureof sand and cement in proportions varyingfrom 1:1 to 3:1, with sufficient water addedto make a thick paste. The use of bonding addi-tives is strongly recommended. There are manybrands of these products available and theycan usually be obtained from hardware storesor concrete suppliers.The patch material should be well worked intothe repair area so as to assure complete fill-ing with full contact with the original, soundconcrete. The surface of the patch should befinished even or slightly above the surface of the original concrete.

13.4-3 BONDING ADDITIVESIn all concrete patching work, the use of bond-ing additives is strongly recommended. Theseproducts, when properly used, help to insure adurable, water-resistant patch that stays in placeand is highly resistant to weathering action.There are two general classes of bonding addi-tives: latex-based and epoxy-based. The latterare usually of the two- component formula-tion. For most work, the latex compounds arethe easiest and most convenient to use sincethey are non-toxic and do not require specialequipment for mixing and applying. Some of the tradenames under which these products aresold are Sikabond (Sika Chemical Corp.),Proweld (Protex Industries), Weldcrete, etc.

For special patching problems such as contin-uously wet areas, overhead areas, and others,the epoxy-type bond agents are far more sat-isfactory. There are certain problems in han-dling these epoxies, such as toxic fumes, mate-rials irritating to the skin and eyes, fast-setting,etc. However, the products can be handled safe-ly when the proper precautions are used. Infor-mation on available products can be obtainedfrom the dam owner’s concrete supplier.A very important part of the repair procedureis to keep the newly patched area moist for atleast three days. This is necessary in order toinsure full strength development of the cementin the patch material. The best way to accom-plish this is to coat the patch area with aspray-on curing compound which seals theexisting moisture into the patch. It is also pos-sible to cure the patch adequately by keepingit moist for the required period of time. Thiscan be done by covering the patch with wetburlap and keeping it wet, or by using a lawnsprinkler or soaker hose connected to a depend-able source of water. To insure a long, satis-factory, service life for the patch, all the stepsof the above procedure must be followed.

13.4-4 CONCRETE SLOPE PROTECTIONWhere dams are constructed with concretepaving on the upstream slope for wave protec-tion, it sometimes becomes necessary to repairor replace whole slabs or portions of slabswhich have been damaged or destroyed. Thebreakup of these slabs starts with the loss of the joint filler material which allows waveaction to wash out the underlying embankmentmaterial through the open joints. Repeated wavecycles with the additional loss of soil materialeventually leads to the complete underminingof the slab and its ultimate collapse.When the supporting soil becomes saturatedwith the water which enters through the open

joint, it has a tendency to flow downhill to themost convenient point of exit. This is whymany slabs fail in pairs, one on each side of theopen joint. The water flushes soil particlesfrom under the slabs in approximately equaldirections, forming cavities or pockets. Thetechnique for locating these cavities under theconcrete slab(s) is discussed in Chapter 4.

13.4-5 REPAIR PROCEDURESThe repair procedure for these damaged slabsstarts with the removal of all or part of theslabs affected. The cavities or pockets shouldthen be backfilled to the original embankmentslope line with well compacted layers of earth.The last step is to pour concrete for the newslabs, following the procedures for finishingand curing as outlined above.The following procedure is recommended forthe proper preparation and sealing of slab jointsand large slab cracks. First of all, check thearea adjacent to the joint or crack to make sureno subsurface cavities or pockets exist. Thenclean out all dirt and vegetation such as weedsand grasses from the joint or crack. Chip outall loose or disintegrated concrete. In extremecases where crack sealing compounds havebeen used previously, it may become neces-sary to chip out the old compound and finishcleaning by sandblasting the crack/joint andthe surrounding area. All loose material shouldbe removed from the crack or joint by blowingit out with compressed air. The clean dry crack

or joint should then be filled with a high qual-ity joint sealer compound. If possible, pick awarm, dry day to do the sealing operation.

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For dams with upstream concrete slope paving,the proper maintenance of the sealing materi-al in the slab joints and cracks is very impor-tant in preserving the structural stability andsafety of the dam. It is a very important part of any good comprehensive maintenance programfor a dam.

13.4-6 DRAINAGE FACILITIESDrains associated with concrete structures havetwo important basic functions. One is to preventor reduce the buildup of excessive ground waterpressure (hydrostatic pressure) and the second isto drain excess moisture from the backfill mate-rial around and behind structures and walls.Proper drainage helps to consolidate and stabi-lize these soil materials.Normally drains are usually installed as a part of the original construction work. Thus, theybecome a part of the routine inspection processwhen they are checked for proper operation.It is when these drains malfunction that itbecomes a critical maintenance problem. Thosedrains which are an integral part of the embank-ment (i.e. toe, finger, and chimney drains) can-not be repaired or replaced, except with greatdifficulty. However, drains which are used underand around structures such as spillway channels,retaining walls, and other minor structures canusually be re-excavated and repairs made. Also,it sometimes becomes apparent that additionaldrainage might be needed for a specific area. Allwork of this nature should be done under theguidance and control of a well-qualified profes-sional engineer registered in Colorado.

13.5 STEEL STRUCTURES AND METALCOMPONENTSAccessory structures at a dam site usually containmany metal components. Steel parts and structuresare by far the most common among metal compo-nents. Brass and aluminum are sometimes used in sec-ondary components. Examples of metal structuresinclude outlet pipes, operating and control structuressuch as gates; valves, trash racks, security structuressuch as railings, fence, etc.; maintenance and accessstructures such as ladders, bridges, etc. Accessorystructures are indispensable to the successful and safeoperation of the dam. They should be maintained in agood condition at all times. Causes for replacementor repair are discussed and useful maintenance pro-cedures are listed below.

13.5-1 CORROSIONThere are two basic types of corrosion. Thefirst is rusting, which means that the metalparticles combine with oxygen to form oxides.(This usually refers to iron or ferrous alloys.)The second is galvanic corrosion which meansthat the metal particles are loosened and car-ried away by electric currents.

Painting is the most commonly used method toprevent rusting. Paint should be applied onlyon a clean, dry surface. Sand blasting or gritblasting is recommended to remove rust andother tough deposits. Water, solvent, wirebrush, or flames may also be used to preparethe surface. Brief discussions on painting willbe presented in Sections 13.5-4 through 13.5-6.For more detailed information, consult thePaint Manual, which is published by the U.S.Bureau of Reclamation.Cathodic protection may be used to minimizeor prevent galvanic corrosion. Some specificideas will be discussed in Section 13.5-7. Actu-al application of cathodic protection should beperformed under the direction of a corrosionengineer. A dam owner can minimize galvan-ic corrosion by avoiding the mixing of differ-ent kinds of metal in the same structure. Forexample, aluminum or copper screws shouldnot be used to fasten steel parts. When con-tact between different metals is unavoidable,good insulation at the contact surface should beprovided.Rust scales and other impurities should bethoroughly cleaned from metal surfaces beforepaint is applied. Care should also be taken toprevent metal dust, sand, or other foreign mate-rial from being mixed with paints. Otherwise,the painted surface may develop pin-hole cor-rosion, especially when it is in contact withthe soil. The life of a buried pipe can be pro-longed if it is wrapped in moisture-imperme-able plastic sheets to insulate it from stray elec-

tric currents.13.5-2 REPLACEMENT AND REPAIR

Broken or deteriorated members of a metalcomponent should be replaced or repaired assoon as possible before the defective part hasa chance to cause subsequent damage to otherparts of the structure. Broken brass or alu-minum parts should be replaced. Steel partsmay be welded. Reinforcing or repairing a bro-ken part by fastening or screwing on an extrapiece of metal rod. or plate is not recom-mended because this is only temporary andwill soon fail.

Welding of steel parts may be achieved by thegas or arc welding process. A skillful andknowledgeable welder should be employed,because improper procedures will result notonly in a weak welded joint but also weak-ened strength of the steel in the vicinity of theweld.

13.5-3 GENERAL PRINCIPLE OF SURFACEPROTECTIONThe exposed surface areas of metal compo-nents are usually protected with an appropri-ate type of paint or other protective materials.The protective coating should be regu-larly inspected to determine that it is not

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suffering from blistering, flaking, pinholes,seediness, cracking, chalking, abrasion, ero-sion, or cavitation. Thorough inspection of protective coatings should be performed atleast once a year. Under ordinary circum-stances, a low power magnifying glass, aknife, a thickness gauge, a pit depth gauge,and a flash light are all the tools needed forinspection.During the inspection, note the number and dis-tribution of rust spots and pits. Measure the sizeand depth of the pits. Find out if the surfacecoating is still in good condition and has a goodbond to the protected surface. Special attentionshould be paid to hard-to-reach locations. Ade-quate lighting for the inspection is very impor-tant. Keep in mind that pitting-type corrosionis more damaging than generalized minor rust-ing. If the blistering, flaking, cracking, chalking,etc., do not penetrate to the lower paint coat,local touch-up or additional coats of paintingshould be enough to restore integrity and pro-long useful life of the protective coating. If themetal surface is exposed and the quality of theprotective coating has deteriorated, then com-plete removal of old paints may be necessaryand new protective coating reapplied.Before touch-up painting or an additional newcoat is to be applied, the old paint should bethoroughly cleaned with water and detergent.Just before the touch-up or repainting, the sur-face should be freed of oil, grease, dust, mois-ture, and other impurities. If large areas needto be cleaned, high-pressure water blasting or

steam cleaning may be employed. If detergentis used in the cleaning, thorough rinsing isrequired. The surface should be dried beforerepainting is undertaken.As a rule, the same type of paint material asthe existing coat should be used. Coal-tarenamel and vinyl resin should never beapplied over other types of coating, nor shoulda lacquer-based paint be used over a lin-seed-oil paint. Coal-tar epoxy paint may beused to repair defects in coal-tar enamel andvinyl paints.Touch-up or preventive maintenance painting

of metal structures should be performed asoften as is necessary. For example, after 10years of satisfactory service, a vinyl paint coat-ing should be repainted even though there isstill no serious deterioration. In the long run,this may prove to be more economical thanallowing the existing paint to deteriorate, whichwould call for a complete removal of the oldpaint and repainting in a few years. Whenpainting with volatile thinners or cleaning withsolvents in enclosed areas, adequate ventila-tion should be provided. Faulty electrical

equipment or sparking tools should never beused. Furnace pilot light should be turned off;otherwise, there may be an explosion.

13.5-4 PAINT PROTECTION FOR AIREXPOSUREMetal works, such as bridge components or

handrailings which are exposed to outsideatmosphere, can be protected with enamelpaints. Normally, a coat of red-lead primingpaint is applied to increase resistance to cor-rosion. A finish coat of aluminum paint, enam-el paint, or epoxy resin paint is then applied.For metal work exposed only to an indooratmosphere and not subjected to excessive con-densation, an enamel or aluminum paintapplied over a coat of suitable primer will pro-vide adequate protection. Priming paint usedunder enamel or aluminum paint is usually of the quick-drying type containing red-lead asrust inhibitor.

13.5-5 PAINT PROTECTION FOR AIR ANDWATER EXPOSURESpillway gates, checks, or turnouts, etc., areeither partially or intermittently exposed to theair or water. They can be protected with vinylresin paint of phenolic red-lead and aluminumpaint system. Cold-applied coaltar paint maybe used if there is only minimal exposure tosunlight. Zinc coatings may also be applied toprolong structure life. Zinc coating may beapplied by hot-dip galvanizing, metallizedspraying, organic zinc-pigmented coating orinorganic zinc-pigmented coating.

13.5-6 PROTECTION FOR UNDERWATEREXPOSURETrash racks, high-pressure gates, storage tank interiors, penstocks, outlet conduits, siphons,and other underwater metal works are nor-mally exposed to water. Exterior surfaces of buried pipes are also exposed to high mois-ture most of the time. These underwater metalworks may be protected by various protectivecoatings cited below:a. Hot-applied coal-tar coating. There are two

types in common use: Coal-tar enamel andcoal-tar pitch. A synthetic resin primer is

recommended for coating trash racks. Theeasiest method of application is by dip-ping, especially where many racks areinvolved. For coal-tar enamel, type B syn-thetic resin primer is recommended. (Referto American Water Works AssociationStandard C-203.) Coal-tar enamel is suit-able for use on the interior surfaces of pen-stocks and outlet pipes. In order to assurea smooth finish, the spinning process isrecommended.

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(This is a shop process whereby the pipe isspun while the coal-tar enamel is beingapplied.) For small structures, hand applica-tion may be acceptable if the job is donecarefully.

b. Hot-applied coal-tar enamel tape. The

coat-tar enamel tape is especially suitablefor coating joints and wrapping valves.c. Cold-applied tape. With this form of pro-

tection, it is not easy to assure voidfreewraps over rough surfaces. Extra care mustbe exercised to prevent damage to the tapeduring handling and backfilling.

d. Cold-applied coal-tar coating. When coal-tar pitch is mixed with solvents and fillers,the resulting coal-tar paint may be appliedwith a brush or a sprayer without heating.

e. Coal-tar epoxy paint. This is obtained bymixing coal-tar and epoxy. Immersion life

may be up to 20 years or longer. Exposureto sunlight, however, will cause chalking.The coal-tar epoxy can be applied to gates,valves, etc.

f. Vinyl resin paint. This may be used forinteriors of water tanks and steel pipe thatwill be empty in winter time and subject-ed to low temperatures.

g. Cement mortar. This may be used as a pro-tective interior coating for steel pipe, espe-cially where flow velocity is not expectedto be higher than 20 feet per second. Min-imum lining thickness of about 5/16 inch isrecommended.

h. Hot-dip asphalt coating. This is not gener-ally considered to be as effective as coal-tarcoating in protecting steel pipe from cor-rosion. But it is an inexpensive shop treat-ment.

i. Coatings applied under water. When waterand moisture cannot be excluded from apainting site, special epoxy type paint willhave to be used. Epoxy paints or gel forunderwater application are commerciallyavailable, but they are relatively expensive.For more specific information, talk to aspecialist or read the Paint Manual .

13.5-7 CATHODIC PROTECTIONIf the repainting of a steel structure is imprac-tical, as in the case of the exterior surface of aburied steel pipe, then cathodic protection maybe used to arrest or delay the progress of cor-rosion. Basically, cathodic protection consistsof making the electric potential of soil posi-tive to the potential of the steel work beingprotected.There are two methods of applying cathodicprotection to underground steel pipes. In thegalvanic anode method, zinc or magnesiumanodes are packed or buried with a backfill of

gypsum, bentonite clay, and sodium sulfate.The anodes and the steel pipe are then con-nected with copper wires to complete the cir-cuit. The zinc or magnesium will corrode awayand has to be replaced. The general arrange-ment is indicated by the accompanying sketch.

CATHODIC PROTECTION: GALVANIC ANODE(From: Joseph F. Bosich, CORROSION PREVENTION FOR PRAC-TICING ENGINEERS, Barnes & Noble, 1970)

It might be well to mention the conditionswhich determine which system to use.In the second method of cathodic protection(the rectifier-ground beds), a d-c current isimpressed on the structure to be protected sothat the structure becomes the cathode andgroundbed of the rectifier anode, as shown bythe following sketch. The groundbed consistsof an array of carbon, graphite, aluminum,high-silicon cast iron, or scrap iron anodes.The rectifier is used to transform a-c currentinto d-c current which is impressed into theearth at the groundbed. The structure (e.g., asteel pipe) is connected to the negative poleof the rectifier.

Field Installation of IMPRESSED-CURRENT (Rectifier-groundbeds)cathodic protection.(From: PAINT MANUAL, U.S. Bureau of Reclamation)

The rectifier-ground bed method is used whenan a-c power supply is available, and wherelarger current outputs are required for protec-tion of a large structure, or where high-resis-tivity soil is encountered.

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13.6 OUTLET GATES—MECHANICAL MAINTENANCE

13.6-1 INTRODUCTIONAs discussed in Chapter 9, the operability of adam’s outlet works is essential to the safe andsatisfactory operation of the dam. Release of

water is essential to realize the beneficial useof stored water. On reservoirs used for recre-ation, fish propagation, or other uses for whichrelease of water is not required, an operableoutlet, as with all dams, provides the onlymeans for the emergency drawdown of thereservoir, thereby being essential to the safe-ty of the dam.If the routine inspection of the outlet works,discussed in Chapter 9, indicates the need formaintenance, the work should be accomplishedas soon as access to the subject areas can begained. Postponement of maintenance couldcause damage to the installation, significantlyreduce the useful life of the structure, and resultin more extensive and more costly repairs whenfinally done.

13.6-2 CYCLING OF OUTLET GATESThe simplest procedure which can be used toinsure the continued operability of the outletgates is to cycle all gates through their fulloperating range at least once and preferablytwice annually. Many gate manufacturers rec-ommend cycling gates as often as four timeseach year. As cycling of gates under full reser-voir head could result in large outlet dis-charges, it is recommended that gate cycling be

scheduled during periods of low storage. If this cannot be done, cycling should be doneduring periods of low stream flows. If largereleases are anticipated, outlet testing shouldbe done only after coordinating releases withwater administration officials and notificationof downstream residents and water users.Cycling of the gates prevents the buildup of rust on contact surfaces of the operating mech-anism and the possible seizure of the operat-ing mechanism as contact surfaces rust togeth-er. During this cycling procedure, themechanical parts of the hoisting mechanism,including drive gears, bearings, and wearplates, should be checked for adverse or exces-sive wear. All bolts, including anchor bolts,should be checked for tightness. Worn and cor-roded parts should be immediately replaced.Mechanical and alignment adjustments shouldbe made as necessary.Note should be made of operating character-istics of the gate mechanism. Rough, noisy, orerratic movement of the gate or gate mecha-nism could be the initial signs of a develop-ing problem. The cause of detected operationalproblems should be investigated and correct-ed immediately.

13.6-3 OPERATING OUTLET GATEWhen operating the outlet, excessive forceshould not be needed nor should it be appliedto either raise or lower the gate. Most hoist-ing mechanisms are designed to operate sat-isfactorily with a maximum force of 40 poundson the operating handle or wheel. If excessiveforce is needed, problems may have developedwith the outlet installation which are causingbinding in the mechanical system. The appli-cation of excessive force may irreversibly bindthe gate or damage the outlet works. (See Fig-ure 13.6-1.)

PHOTO 13.6-1—BENT OUTLET STEMTwo-and-one-half inch diameter stainless steel stem removed from outletinstallation. Stem was tent due to excessive force applied to operatingmechanism while attempting to close outlet gate.

It is recommended that the gate be worked upand down repeatedly in short strokes until thebinding ceases. Investigations should also bemade to determine the cause of the operationalproblem. The cause of the problem should becorrected as soon as possible to assure the con-tinued operability of the gate.In cases where the gate does not properly sealin the closed position, debris may be lodgedunder or around the gate leaf or frame. Thegate should be raised a minimum of 2 to 3inches to flush the debris and an attempt madeto reclose the gate. This procedure should berepeated until proper sealing is achieved. Themanufacturer’s representative or an engineerexperienced in gate design and operationshould be consulted if operational problemspersist.

13.6-4 MAINTENANCE OF OPERATINGMECHANISMAs with any equipment, proper routine main-tenance and lubrication is essential for the con-tinued operability of the outlet and will sig-nificantly extend the useful life of theinstallation.

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1

FIGURE 13.6-4—WET WELL OUTLET INSTALLATIONThis sketch represents a properly designed and installed outlet mechanism.Adequate numbers of stem guides at the proper spacing assure the rigidi-ty of the control stem.

FIGURE 13.6-5—WET WELL OUTLET INSTALLATIONThis sketch represents an improperly designed and installed outlet gatemechanism because of the inadequate number of stem guides and improp-er spacing between guides. Excessive force used to attempt to close the gate(see Section 13.6b) has resulted in bending of the control stem because of inadequate stem support. Operational problems are further complicated asthe bent stem binds on the stem guides because of the misalignment of thestem relative to the guide bearing.

FIGURE 13.6-6.a—CHECKING STEM ALIGNMENT IN THEUPSTREAM/DOWNSTREAM DIRECTION

FIGURE 13.6-6.b—CHECKING STEM ALIGNMENT IN THE LEFT/ RIGHT DIRECTION.

A VIEW A-A



Excessive binding of the stem in the stemguides will result if the guides are not prop-erly aligned with the stem.

13.6-7 OUTLET GATE ADJUSTMENTSMany outlet gate installations are equippedwith wedge systems that allow the gate to be

adjusted to seal tightly. (See Figure 13.6-2.)These wedge systems force the gate leaf tight-ly against the gate frame as the gate is low-ered into a fully closed position. This causes atight contact between the gate seats on the gateleaf and the gate seats on the gate frame.Through years of use, the gate seats maybecome worn, causing the gate to leak anincreasing amount. If the installation isequipped with a wedge system, the leakagemay be substantially reduced or eliminated bythe proper readjustment of the wedges.Adjustment should never be attempted by anovice. As adjustment of these gates is com-plicated, inexperienced personnel can causeextensive damage to the gate installation byimproperly adjusting the gate. Improper adjust-ment could cause premature seating of the gate,possible scoring of the gate seats, binding of the gate, gate vibration, leakage, uneven clos-ing of the gate, or damage to the wedges orgate guides. Always employ experienced per-sonnel to perform these adjustments. Consulta gate supplier or manufacturer to obtain namesof people experienced in gate adjustment.

13.6-8 SEASONAL OPERATION OF THE

RESERVOIRIce can exert excessive force on, and causesignificant damage to, an outlet gate leaf. Stor-age levels in the reservoir during the wintershould be such that ice cannot develop againstthe gate. To prevent ice damage, storage shouldeither be at a level significantly higher thanthe gate if storage is carried through the win-ter, or if the reservoir is to remain empty overthe winter months, the outlet should be fullyopened. If operation requires the water levelto move across the gate, a bubbler or otheranti-icing system will be required.

13.6-9 SUMMARYRegular cycling of the outlet gates will pre-vent the accumulation of rust on contact orbearing surfaces and prevent the possibleseizure of the outlet operating system. Cyclingas well as periodic scheduled maintenance onvarious components of the outlet operatingsystem will assure the continuing satisfactoryand safe operation of the outlet and will sig-nificantly lengthen the useful life of the outletinstallation.

In many cases, operation of the mechanical por-tion of the outlet is dependent on hydraulic sys-tems and/or electrically powered drive units.Maintenance of the systems is important and iscovered in subsequent sections of this chapter.

13.7 ELECTRICAL SYSTEMS

13.7-1 INTRODUCTIONThe use of electrical devices on most of thedams in Colorado is limited. When a 1 2-inchdiameter slide gate is opened at night, the onlyelectrical item needed is a flashlight. There-fore, electrical systems will be discussedbriefly.

13.7-2 USE OF ELECTRICITYIn general, electricity is used on dams for thefollowing:Provide lightingOperate outlet gates

Operate recording equipmentOperate spillway gatesOperate other electrical equipment like eleva-tors or cranesWhen demand is placed upon these pieces of equipment, it is hoped they will respond witha sound of a motor running, which is com-forting to the human ear during an emergency.

13.7-3 PREVENTIVE MAINTENANCEA quality preventive maintenance programwould include routine maintenance procedures,charts, wiring diagrams, and checklists thatare current. These would indicate that theowner was in charge of his dam. Items to look for during maintenance of electrical systemsare shown on the checklist. When items needto be corrected, it is best to make a list andgive priorities to the tasks for accomplishment.

13.7-4 ELECTRICAL CHECK LIST1. Check the fuses!2. Open Circuit—is there a broken wire or

defective switch?3. Short Circuit—one wire touches another

wire that it is not supposed to touch.4. Grounded Circuit—one wire or more is

grounded where it is not supposed to be.5. Is moisture kept out of the system?6. Is dust kept out of the system?7. Is there evidence of corrosion?8. Are there mineral deposits?9. Are there kinks in the wire?

10. Is the conduit pipe bent or crimped?

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11. Is support (telephone pole, brackets,anchorings) adequate?

12. Are there any fire hazards?13. Are people safe from a shock or electro-

cution?14. Is there a good dependable supply of

power?15. Will device operate when needed?16. If you can’t solve the problem call an

electrician.13.7-5 GENERATORS

Generators may be used for back-up when apower supply is cut off. Fuel availability, oilchanges, battery checks and antifreeze checksare part of maintaining these items before theyare needed for an emergency.

13.7-6 SUMMARYPeriodically, the electrical systems need to be:

CheckedMaintained in a clean, moisture-free and tightconditionTested for emergenciesEvaluated for regular replacementRepairedRecords should be kept noting the item, date,and what was done to the electrical device.The final questions thathelp guide good maintenance are:Do people know how to operate the equip-ment? Will the piece of equipment operate

when needed?13.8 HYDRAULIC SYSTEM

13.8-1 INTRODUCTIONA hydraulic control system is used to openand close the valves or sliding gates of the out-let or intake works. A hydraulic system is espe-cially desirable where the gate or valve requiresgreat force to operate, or the gate or valve islocated in an inconvenient place. The follow-ing is a schematic drawing of a typicalhydraulic system:

LEGENDBV BALL VALVECV CHECK VALVE—PILOT OPERATEDFL FILTERNV NEEDLE VALVEP PUMP, MANUALPG PRESSURE GAUGEFR PRESSURE RELIEF VALVE

FIGURE 13.8-1—HYDRAULIC OPERATING SYSTEM

The main component parts of a hydraulic sys-tem consist of hydraulic cylinders, hoses,pipelines, pumps (electric and / or hand pow-ered), and hydraulic fluid reservoirs. Control,operating, and auxiliary components includecontrol valves, safety (or overload) valves,pressure gauges, position indicators, etc.A hydraulic system for gate operation is essen-tially similar to a hydraulic jack that is usedto jack up cars. The main difference is that thehydraulic system usually has long hoses andpipelines to transmit hydraulic fluid to the gate-operating cylinders, and that there are gaugesto indicate hydraulic pressure in the system.

1

REGULATING GATE



Occasionally the State Engineer will requestthat the plotted or charted data be sent to theDam Safety Branch for review and to becomea part of the dam’s records. When requested,provision should be made for duplicating andforwarding the information.

14.2-5 MAINTENANCE INSTRUCTION—Any spe-cial instructions for performing periodic main-tenance should be given in detail. This will allownew personnel to understand the task and expe-rienced personnel to make sure they have com-pleted the work properly. All required mainte-nance work should be identified and listed. Thislisting may include:1. Cleaning brush and trees.2. Removing debris.3. Regrading the crest and / or access roads.4. Removing harmful rodents.5. Cycling and lubricating gates.6. Adding riprap when required.7. Sealing joints in concrete facings.8. Cleaning drain pipes and outfalls.9. Maintaining protection for monitoring

points.10. Mainta ining secur i ty for opera t ing

equipment.

14.2-6 SCHEDULE—Once the various required taskshave been identified, a schedule showing the fre-quency for each item needs to be drawn up.Suggested minimum frequencies for variousactivities are:HIGH HAZARD DAMS—Failure of the damwould cause extensive property damage andwould probably cause the loss of human life.Daily—Surveillance by the owner or caretaker.Weekly—Monitoring of seepage.Monthly — Thorough visual inspection. Gath-ering, immediately plotting, and interpretingobservation well and piezometer data.Annually—Reading horizontal and verticalcontrol monuments (more frequently if nec-essary).Routine maintenance as required.

Bi-Annually—Test operation of outlet andspillway mechanical components.SIGNIFICANT HAZARD DAMS—Failure of the dam would cause extensive property dam-age but is not expected to cause loss of humanlife.Weekly—Surveillance by the owner or care-taker. Monitoring of seepage.

Monthly—Thorough visual inspection whenstorage is in excess of one-half the maximumgage rod reading.Gathering, plotting and interpreting observationwell and piezometer data when storage is inexcess of one-half the maximum gage rod read-ing.Annually—Test operation of outlet and spill-way mechanical components.Bi-Annually—Reading of horizontal and ver-tical control monuments after a satisfactoryperformance record has been established forthe dam.Routine maintenance as required.LOW HAZARD DAMS—Failure would causelittle damage beyond the loss of the dam struc-ture itself.Monthly—Surveillance by the owner or care-taker.

Monitoring of seepage when storage is inexcess of one-half the maximum gage rod read-ing.Gathering, plotting, and interpreting observa-tion well and piezometer data when storage isin excess of one-half the maximum gage rodreading.Annually—Thorough visual inspection. (If thereservoir is full all year, visual inspectionshould be done quarterly.) Testing of the outlet.Every Five Years—Reading of horizontal andvertical control monuments after a satisfacto-ry performance record has been established.Routine maintenance as required.

14.2-7 RECORD—As part of the operating plan, thedata relating to the dam should be organized intoa permanent continuous record. All ongoingrepairs should be fully described and this infor-mation added to the record. This will help inassessing future changes especially when theyare viewed by personnel unfamiliar with the dam.

14.2-8 COMMUNICATION—A list of the agenciesinvolved in the administration of the dam and thenames of the people currently representing thoseagencies should be included in the operating planalong with identification of each agency’s involve-ment. This will help promote required communi-cation and a cooperative relationship with thoseagencies.

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14.3 ASSIGNING RESPONSIBILITYAfter the operating procedure has been written,reviewed,and found acceptable, the owner needs toidentify who will carry out the various duties. Copiesof the completed plan should be distributed to andreviewed with each participant. Each member of theboard of directors should also be familiar with theoperating plan. This will aid them in exercising theirresponsibilities to the shareholder, especially in thearea of funding routine maintenance and providingcontingency funds for non-routine repair work.

14.4 SUMMARYBy conscientiously following a well-thought-out oper-ating procedure the dam owner can expect:

Maximum assurance of a safe dam.Maximum assurance of uninterrupted servicefor the dam and reservoir.Reduced maintenance costs.An extended useful life for the dam.

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15.1 INTRODUCTIONGOAL—An emergency preparedness plan is a writtenprocedure for reacting to emergency situations causedby the threat of a dam failure. The goals of theplan are:

1 To prevent loss of life.

2. To minimize property damage.Keeping a dam from failing is the most assured wayto prevent loss of life and property damage. There-fore, emphasis must be placed on saving the dam fromfailure.RESPONSIBILITY—The dam owner is responsiblefor drafting the plan, providing copies to participants,coordinating emergency actions, and activating theplan immediately when a failure threatening situationis reported.REQUIREMENTHigh Hazard Dams—Sudden failure of the dam wouldcause the loss of life. In the interest of public safetythe State Engineer has requested written plans forthese dams. The plan includes evacuation of personslocated in the dam breach flood plain.Significant Hazard Dams—Sudden failure of the damwould not cause loss of life but would cause exten-sive property damage. Since the owner of the dam isresponsible for damages caused by the uncontrolledrelease of water from a dam failure, a written plan isrecommended. This plan usually does not includeevacuation.Low Hazard Dams—Sudden failure would cause lit-tle loss other than the structure itself. A written planis recommended to allow the best chance to save thedam and avoid costly reconstruction.RESULTBy planning in advance for quick and prudent actionand by devising an effective timely warning to down-stream residents, the disastrous results of a dam fail-ure can be avoided.

15.2 GENERAL INFORMATIONAddressing an emergency situation will require thecooperation of many persons and entities. The fol-lowing list is provided to alert the owner to those thatmay be involved.LOCAL PARTICIPANTS

The dam’s owners, shareholders, and beneficiariesOfficials of the nearby downstream cities and townsLocal police, county sheriffLocal fire departmentsCounty highway department personnelLocal construction companiesNews media serving the area (i.e., radio, TV, news-papers)Nearby engineering firmsProfessional diving serviceHelicopter serviceHospital and/or ambulance service

STATE AGENCIESState Engineer’s Office

State EngineerDam Safety BranchLocal Water CommissionerDivision Engineer

Colorado Office of Emergency Management(C.O.E.M.)

State PatrolDepartment of HighwaysDepartment of HealthFEDERAL AGENCIESU.S. Forest ServiceBureau of ReclamationNational Park ServiceNatural Resources Conservation ServiceU.S. Army Corps of EngineersFederal Bureau of InvestigationFederal Energy Regulatory CommissionUnited States Geological SurveyFederal Emergency Management AgencyA directory of these and any other agencies involvedshould be established which lists the home phone,address, and office phone of the primary and second-ary contacts for each, as well as a listing of the equip-ment and/or service they can provide and stating anestimate of their response time. This directory shouldbe updated as needed. An example will be found inSection 15.7.

15.3 REPORTING INCIDENTSDam incidents should be reported immediately bythe person discovering the dangerous condition tothe person responsible for executing the emergency

plan (listed on the data sheet and in the EMER-GENCY PLAN).Generally, the person who first discovers what appearsto be a potentially hazardous condition at a dam sitewill have little or no background in dam design, con-struction, or safety. In order to be able to properlyidentify a potentially dangerous condition, it is nec-essary that dam tenders and others who visit the siteregularly are familiar with all features of the dam anddam site. This is especially true for dams with a his-tory of leakage, cracking, settlement, misalignment,and erosion from wave action. Also, it is necessary tohave a knowledge of measurements of sign)ficant drainand seepage outflows to act as a basis for meaningfulcomparisons.(When reporting a dam incident, remember that whenlocating problem areas, all directions, e.g., “left of,’ or“right from,” are taken while facing downstream.)Items that should be reported are:

1. Name of dam, lake, or reservoir, and river, stream,or tributary the dam is located on.

2. Location from highway or nearest town (U.S.,state, or county road numbers); also section, town-ship and range, and PM, if known.

3. Nature of the problem (e.g., excessive leakage,cracks, sand boils, slides, wet spots, etc.).

CHAPTER 15

EMERGENCY PLAN 1



4. Location of problem area in terms of embank-ment height, (e.g., about 1/3 up from the toe) andlocation along the dam’s crest (e.g., 100 feet tothe right of the outlet or abutment) and whether onthe upstream slope, crest, or downstream slope.

5. Extent of the problem area. This can be satisfac-

torily established by pacing.6. Estimated quantity of unusual flows as well as

whether the water is clear, cloudy, or muddy.

7. Water level in the reservoir below the dam’s crestor below the spillway, or the gauge rod reading.

8. Is water level in reservoir rising or falling?

9. Name and how to contact person making report.

10. Did the situation appear to be worsening whilebeing observed for this report?

11. Does the problem appear to be a containable prob-lem at this time or is it an emergency situation?

12. What are current weather conditions at the site?

13. Anything else that seems important.A reporting form will be found in Section 15.7.This list should be periodically reviewed by owners’representatives who frequently visit the dam site. Itwill alert them to make all these observations beforereporting the incident. An accurate report will allow anintelligent assessment of the situation and properimplementation of the plan.

15.4 POTENTIAL PROBLEMS AND IMMEDIATEDEFENSIVE ACTIONS (To be taken before or whilea detailed engineering assessment is made)The general approach to each threatening situation aslisted below should be reviewed and the actions imple-mented as soon as possible in each situation.OVERTOPPING BY FLOOD WATERS

1. Fully open outlet to reduce overflows.

2. Place sandbags along the crest to increase freeboardand force more water through the spillway and outlet.

3. Provide erosion-resistant protection to the down-stream slope by placing plastic sheets or othermaterials over eroding areas.

4. Divert flood waters around the reservoir basin, if possible.

5. Create additional spillway capacity by making acontrolled breach in a low embankment or dikesection where the foundation materials are ero-sion resistant.

LOSS OF FREEBOARD OR DAM CROSS SECTIONDUE TO WAVE EROSION CAUSED BY HIGH WINDS

1. Lower water level to an elevation below the dam-aged area.

2. Immediately place additional riprap or sandbagsin damaged areas to prevent further embankmenterosion.

3. Restore freeboard with sandbags or earth and rock-fill.

4. Continue close inspection of the damaged areauntil the storm is over.

SLIDES IN THE UPSTREAM OR DOWNSTREAMSLOPE OF THE EMBANKMENT

1. Lower water level at a rate and to an elevationwhich are judged to be safe under the slide con-dition. If the outlet is damaged or blocked thenpumping, siphoning, or a controlled breach maybe required.

2. Restore lost freeboard if required. This may entailplacing sandbags or fill on top of the slide.

3. Stabilize slides on the downstream slope by weigh-ing the toe area with additional soil material, rock,or gravel.

FLOWS THROUGH THE EMBANKMENT, FOUN-DATION, OR ABUTMENTS WHICH ERODE THE

MANMADE OR NATURAL MATERIALS CON-TAINING THE RESERVOIR

1. If the entrance area of the leak in the reservoirbasin can be found, try to plug it off with whatevermaterials are available such as hay bales, manure,mattresses, bentonite, plastic, etc.

2. Lower the water level until the flows decrease toa non-erosive velocity or until the flow stops.

3. Place a protective sand and gravel filter over theexit area to hold materials in place.

4. Continue lowering the water level until an eleva-tion judged to be safe is reached.

5. Continue operating at a reduced level until repairscan be made.

FAILURE OF APPURTENANT STRUCTURESSUCH AS THE OUTLET OR SPILLWAY

1. Implement temporary measures to protect the dam-aged structure, such as closing the outlet or pro-viding temporary protection for the damagedspillway area.

2. Experienced professional divers may be able toquickly assess the problem and possibly imple-ment repair.

3. Lower the water level to an elevation judged tobe safe. If the outlet is inoperable, then pumping,siphoning, or a controlled breach may be required.

MASS MOVEMENT OF THE DAM ON ITSFOUNDATION; i.e., SPREADING OR MASS SLID-ING FAILURE

1. Immediately lower water level until excessivemovement stops.

2. Continue lowering water until a level judged tobe safe is reached.


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HIGH LEVEL SATURATION OF THE ENTIREEMBANKMENT CROSS SECTION DUE TOEXCESSIVE SEEPAGE

1. Lower the water to a level which is judged to be safe.

2. Continue frequent monitoring for signs of slides,cracking, or concentrated seepage.

3. Continue operation at a reduced level until repairscan be made.

SPILLWAY BACKCUTTING THREATENS RESER-VOIR EVACUATION

1. Reduce flows over the spillway by fully openingthe outlet.

2. Provide temporary protection at the eroding sur-face by placing sandbags or riprap material.

3. When inflows subside, lower water level to a level judged to be safe.

4. Continue operating at a low water level in order toprevent spillway flows.

EXCESSIVE SETTLEMENT OF THE EMBANK-MENT

1. Lower water level by releases through the outletor by pumping, siphoning, or a controlled breach.

2. If necessary, restore freeboard, preferably by plac-ing sandbags.

3. Continue lowering water to a level judged to besafe.


LOSS OF ABUTMENT SUPPORT OR EXTENSIVECRACKING IN CONCRETE DAMS WHICHCOULD CAUSE SUDDEN FAILURE

1. Lower the water level by releases through the outlet.

2. Attempt to block water movement through thedam by placing plastic sheets etc., on the upstreamface.

3. Prepare to notify and evacuate downstreamresidents.

4. Continue lowering water to a level judged to besafe.

15.5 HELPFUL SUGGESTIONSFollowing are suggestions for making a controlledbreach, sandbagging, and plastic sheet placement:CONTROLLED BREACHOne method of making a controlled breach is to con-struct a small coffer dam upstream from the breacharea. Then excavate the breach through the embank-ment and place an appropriately sized pipe throughthe embankment and backfill around the pipe andre-establish to embankment freeboard. The coffer damcan then be removed and water released through thenewly installed pipe.

SANDBAGGINGWhen placing sandbags in high velocity flow water, itis difficult to keep the bags in place. In order to con-trol water in this situation it is advisable to:

1. Make sure the bags are securely tied so the mate-rial does not wash out of them.

2. Begin placement near the shore or in a quiet areaand work toward the higher velocity flow areas.

PLASTIC SHEET PLACEMENTPlastic sheets normally used in construction have beenemployed successfully to resist erosion of a dam’sdownstream slope or spillway channel during stormflows. The top end of the sheet must be securelyanchored in a nearly horizontal area such as the crestarea, where velocities are low. Closely spaced sandbagsor rocks will anchor the sheet and minimize flow underthe sheet. This protection should be extended beyondthe dam’s toe or the eroding area in the spillway byoverlapping with the upper sheet over the lower one

and anchoring successive sheets.15.6 ADDITIONAL CONSIDERATIONS

WARNING TO DOWNSTREAM RESIDENTSGenerally, in more densely populated areas, warningto downstream residents will be carried out throughthe county sheriff or town or city police department.This warning will be initiated by the person executingthe emergency plan.In rural locations the warning may have to be given byphone or direct contact with the nearest downstreamresidents. Where phone communication is not avail-able, the person observing the dangerous conditionmay have to provide the warning to the nearest down-

stream residents, campers, etc. A listing of the near-est downstream residents should be kept by the personresponsible for implementing the plan and an addi-tional copy kept at the dam site.RESPONSE TIMEMany of the dams within the state are located in areaswhere access is very time consuming. In order toassess the actual response to an incident at all damsproperly, it is important to establish a realistic esti-mate of the time that will be required for the variousparticipants in the plan to reach the dam site. This isespecially true for the engineers who will be respon-sible for an accurate evaluation of the situation andrecommending on-site remedial actions and subse-quent repairs.FLOOD ARRIVAL TIMEAn estimate of the time that it will take for the floodcaused by a dam failure to reach the nearest dwellingand downstream town should be made. This will aid inscheduling the sequence of warning and defensiveactions. Local water commissioners or the DivisionEngineer may be of assistance in making this estimate.

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SITUATIONS TO BE ADDRESSEDSeveral situations should be anticipated and therequired notification or actions specified. In general,they are:

A. Dam Emergency—structure may be saved withimmediate remedial action.

B. Dam Breach—no chance to save the structure.

C. Flooding expected or in progress upstream fromthe dam site.

D. Any other conditions peculiar to this dam.

The required notification and actions for each situationshould be thought out and planned in advance.NOTIFICATION TO THE DIVISION ENGINEERAND LOCAL WATER COMMISSIONERWhen unusual flow due to the controlled or uncon-trolled release of water is anticipated, the DivisionEngineer and / or Water Commissioner should be noti-fied as required in CRS 1973 as amended, 37-87-103.In addition to allowing for minimizing flooding andproviding protection of downstream water controlstructures, these administrative personnel may be ableto find a way to regain control of the released water forfuture use by the affected community.FLOODED AREA MAPSWhere the floodplain downstream from a large capac-ity reservoir is heavily populated, it is recommendedthat reasonably accurate floodplain maps be madewhich will show the limits of flow for full spillwaydischarge and the limits of flooding for failure of thestructure when filled to the spillway crest withoutstorm inflow. These will be very helpful in identify-

ing the actual areas where flooding is anticipated andwhere evacuation is required. Local flood control dis-tricts may provide assistance in the preparation of these maps.HELPFUL ITEMSItems that should be kept at the dam site, or at a home,cabin, or storage facility nearby are: a list of the near-est downstream residents affected by flooding and therequired method for warning them; a set of drawingsthat show the basic dimensions and typical cross sec-tions of the dam; a table showing spillway dischargecapacity and volume of surcharge storage for eachfoot of water above the spillway crest (this will beuseful in predicting the magnitude of flows that willbe experienced downstream); and a copy of recentleakage quantities or other pertinent monitoring data.Additional items that are recommended at the damsite are:A high intensity light source to facilitate night inspec-tion. A spotlight with a minimum of 200,000 candle-power with an appropriate battery-pack would be goodfor this.An annual log of repairs or major maintenance.

Photographs of snow drifts which accumulate on andmay saturate portions of the dam should be taker annu-ally and a file kept for comparison and referenceAn identification plate stating the dam name and nameof the owner should be displayed at the site. As anexample, this plate could be securely fastened to theoutlet control works.

15.7 MODEL PLANIn order to assist the owner in preparing an emer-gency preparedness plan, the following model is pro-vided. Contact your Dam Safety Engineer or theDenver Office of Dam Safety to obtain the instruc-tion manual, or you can find it on the web athttp:/water.state.co.us/.

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COLORADO DIVISION OF WATER RESOURCES,STATE ENGINEER’S OFFICE, MODEL FORPREPARING A DAM SAFETY EMERGENCYPREPAREDNESS PLAN

INTRODUCTIONThis model provides the directions for preparing anEmergency Preparedness Plan (EPP) in accordancewith the Rules and Regulations for Dam Safety andDam Construction, September 1988, Office of theState Engineer (SEO). It also conforms with theGuideline for Developing an Emergency Action Plan,FEMA, May 1996.It is assumed that the dam owner has an adequateoperation and maintenance plan that provides the mon-itoring (inspections and instrumentation) necessary to

detect emerging problems and emergencies at theirdams. In the case where a dam is in a remote location,Early Warning Systems should be provided to indi-cate that adverse conditions are occurring at the damthat require immediate response.The purpose of this model and the attached SAMPLEDAM plan is to aid the dam owner in assembling nec-essary information in an accessible format that assuresa timely response to emergencies at their dams.

REQUIREMENTSThe regulations require that the owners of Class 1(High Hazard) and Class 2 (Significant Hazard) dams:

• Prepare, maintain, and exercise EPP’s for immedi-ate defensive action to prevent the failure of theirdam, using the State Engineer’s model plan orequivalent.

• Have the Local Emergency Manager (LEM) orGovernment Official responsible for public safetyreview their plan, and make appropriate modifica-tions.

• Provide the principal persons and agencies respon-sible for executing the plan with copies, and dis-tribute the plan to all affected entities.

• Review, update, and exercise the plan periodical-ly.

CONTENT OF PLANSIn order for all plans to be thorough and consistent,they should include the following basic elements. (SeeSAMPLE DAM plan for further guidance.)

1. Cover sheet with name and identification data.

2. Notification procedures for both impending con-ditions (DAM EMERGENCY or DAMBREACH), and termination of the emergency.(Flowcharts are recommended as tools for quick response)

3. A description of the methods for detecting, evalu-ating, and classifying emergencies. (See Table 1 of SAMPLE DAM plan.)

4. A summary of the responsibilities of the principalparticipants in the plan.

5. A description of contingency plans for respondingto emergencies at the dam. (See Table 2 of SAM-PLE DAM plan.)

6. An Appendix which includes:• Owner’s plan for exercising the EPP.• Directory of holders of the plan.• Copies of the plans for the dam showing gener-

al details, including outlet works and spillways,and their capacities

• Copies of the inundation maps for Class 1 dams,or copies of topographic maps showing streamchannels affected.

PROCESS FOR PREPARING PLANSPreparation of an EPP requires coordinated planningwith all the principal parties responsible for emer-gency response and public safety. The recommendedprocess for developing a plan is as follows:Step 1:

For Class 1 dams that will affect urban areas,determine the potential inundated area by definingdam break flood profiles downstream from thedam.For Class 1 dams in rural areas, the evacuationarea may be determined by inspection.

Step 2:For Class 2 dams, only the drainage which willbe affected must be identified using a topograph-ic or geographic map.It is recommended that the failure of the dam dur-ing the Inflow Design Flood be used as a worst-case scenario. The area that will be inundated inthe reservoir should also be identified for evacu-ation if there are people at risk. Prepare inunda-tion maps which show the time and distance fromthe dam when the flood wave will arrive at criti-cal areas in relation to the beginning of the damfailure hydrograph, the maximum discharge (Qp),

and the depth of flow. Critical public facilities(eg water supplies, hospitals, electric utilities,etc.)that would be affected by the floodingshould be identified. (A note should be added tothe maps that the inundated areas are approxi-mate, and should be used with caution for evacu-ation purposes.)

Step 3:Develop emergency procedures, and who isresponsible for them. This includes evaluation of the problem, and classification of the emergency.See Tables 1 and 2 of the Sample Plan. Theseshould be incorporated in the plan.

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Step 4-:Contact the jurisdictions that are responsible forpublic safety and coordinate the preparation of the plan with them. This is normally the LEM orSheriff’s Office. Find out what their requirementsare.

Step 5:Determine the primary and auxiliary systems thatwill be available for communicating with the par-ticipants that need to be notified.

Step 6:Develop priority lists of the persons/agencies thatneed to be notified in accordance with the levelof urgency per Table 1. List the Name, Title,Address, and primary and auxiliary communica-tion systems.

Step 7:Draft Notification Flowcharts using the lists fromstep 5 for DAM EMERGENCY CONDITIONSand DAM BREACH CONDITION. These chartscan be used by the dam owner to quickly notifyother parties of the emergency conditions in rec-ommended order. The flowcharts in the SAM-PLE DAM represent the sequence for the damowner. There may be instances where the pub-lic discovers the problem and calls the sheriff.The dam owner should adapt the use of theflowchart accordingly.

Step 8:Prepare a draft of the EPP in accordance with thismodel, or equivalent, and submit it for review andcomment by the participants, including the State

Engineer’s Office and the Colorado Office of Emergency Management.Step 9:

Revise the plan as needed and distribute the finalplan to the participants. The plan must be updat-ed annually.

RESOURCE INFORMATIONThe following personnel and agencies may need tobe included in the planning and participation of mostemergency plans.

• Owner Personnel

• Local Emergency Managers and/or Sheriff’s Office.

• Colorado Office of Emergency Management

• Colorado Division of Water Resources (SEO)Division Engineer’s OfficeDam Safety Branch

• Federal AgenciesNational Weather ServiceFederal Dams

OWNER PERSONNELAll the key personnel of the dam owner should beinvolved in the planning, training, and exercising of anemergency plan. This includes the caretaker, super-intendent, engineer, or their representatives.

LOCAL EMERGENCY MANAGERS (LEM)Local government is responsible for protecting citi-zens from disasters. They are required to be preparedto respond when an emergency occurs. They will beresponsible for evacuating the flooded area. The nameand telephone number of the LEM or Sheriff may beobtained from your telephone book, or by calling theColorado Office of Emergency Management.

COLORADO OFFICE OF EMERGENCY MAN-AGEMENT (COEM)The COEM provides guidance to local government inthe prevention of, preparation for, response to, andrecovery from disasters. The COEM may also beinvolved in the establishment of an Emergency Oper-ations Center, and will mobilize other state agencieslike the State Patrol and Colorado Department of Transportation of the emergency in accordance withthe state’s emergency plan. Check your telephonebook for their location and number.

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COLORADO DIVISION OF WATER RESOURCES(DWR)The Colorado Division of Water Resources (DWR),also known as the State Engineer’s Office, is respon-sible for determining the amount of water which issafe to impound in the reservoirs in the state in accor-dance with the statutes and regulations promulgated bythe State Engineer. The personnel of the Dam Safe-ty Branch and the Division Engineer’s Office partic-ipate in the planning for EPPs by providing informa-tion about the dams, and consultations and review of the plans. They must also be notified of emergenciesso they may assist the dam owner in preventive actions,and to assure the owner is taking appropriate action forthe protection of the public safety. The Division Engi-neer’s Office, Resident Dam Safety Engineer may becontacted to obtain the following:

1. Data about the dam, location, etc. for the cover.

2. Flood inundation maps that were prepared by

DWR in accordance with HB-1416. These may beused if available.

3. Names of the participants in the SEO to include inthe notification lists/Flowchart.

4. Plans of the dam, outlet works, and spillways.

5. Information on dam inspection training and exer-cising EPPs.

Following are the locations for your Division Engi-neers Office:

South Platte River DrainageDivision 1, Greeley

Arkansas River DrainageDivision 2, Pueblo

Alamosa River DrainageDivision 3, Alamosa

Gunnision River DrainageDivision 4, Montrose

Colorado River DrainageDivision 5, Glenwood Springs

White-Yampa River DrainageDivision 6, Steamboat Springs

San Juan River DrainageDivision 7, Durango

FEDERAL AGENCIESThe National Weather Service (NWS) is responsible for issuing flood warnings. In order to predict theflooding from dam failures the NWS needs to knowthe National identification number (NATID) for thedam, or its height and present capacity. Check yourtelephone book for your local office.Where other Federal Dams are affected by the failureof a dam, they should be contacted so they can includethe potential incident in their EAP.

OTHER RESOURCESOther persons and agencies that may need to be iden-tified as a resource for carrying out the emergencyplan are:

• News media in the area(Radio, Television, Newspapers).

• Local construction companies.

• Engineering Consultants.

• Helicopter service.

• Professional Diving Service.

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