Dam Safety: Concrete Repair Techniques C · chipping and the patch area thoroughly cleaned. A sawed...

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Ohio Department of Natural ResourcesDivision of Water Fact Sheet

Fact Sheet 94–32

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Concrete is an inexpensive, durable, strong andbasic building material often used in dams forcore walls, spillways, stilling basins, control tow-

ers, and slope protection. However, poor workmanship,construction procedures, and construction materials maycause imperfections that later require repair. Any long-term deterioration or damage to concrete structures causedby flowing water, ice, or other natural forces must becorrected. Neglecting to perform periodic maintenance andrepairs to concrete structures as they occur could result infailure of the structure from either a structural or hydraulicstandpoint. This in turn may threaten the continued safeoperation and use of the dam.

ConsiderationsFloor or wall movement, extensive cracking, improper

alignments, settlement, joint displacement, and extensiveundermining are signs of major structural problems. Insituations where concrete replacement solutions are re-quired to repair deteriorated concrete, it is recommendedthat a registered professional engineer be retained to per-form an inspection to assess the concrete's overall condi-tion, and determine the extent of any structural damage andnecessary remedial measures.

Typically, it is found that drainage systems are needed torelieve excessive water pressures under floors and behindwalls. In addition, reinforcing steel must also be properlydesigned to handle tension zones and shear and bendingforces in structural concrete produced by any externalloading (including the weight of the structure). Therefore,the finished product in any concrete repair procedureshould consist of a structure that is durable and able towithstand the effects of service conditions such as weath-ering, chemical action, and wear. Because of their complexnature, major structural repairs that require professionaladvice are not addressed here.

Repair MethodsBefore any type of concrete repair is attempted, it is

essential that all factors governing the deterioration orfailure of the concrete structure are identified. This isrequired so that the appropriate remedial measures can beundertaken in the repair design to help correct the problemand prevent it from occurring in the future. The followingtechniques require expert and experienced assistance forthe best results. The particular method of repair will dependon the size of the job and the type of repair required.

1. The Dry-Pack Method: The dry-pack method can beused on small holes in new concrete which have a depthequal to or greater that the surface diameter. Preparationof a dry-pack mix typically consists of about 1 partportland cement and 2 1/2 parts sand to be mixed withwater. You then add enough water to produce a mortarthat will stick together. Once the desired consistency isreached, the mortar is ready to be packed into the holeusing thin layers.

2. Concrete Replacement: Concrete replacement is re-quired when one-half to one square foot areas or largerextend entirely through the concrete sections or wherethe depth of damaged concrete exceeds 6 inches. Whenthis occurs, normal concrete placement methods shouldbe used. Repair will be more effective if tied in withexisting reinforcing steel (rebar). This type of repair willrequire the assistance of a professional engineer experi-enced in concrete construction.

3. Replacement of Unformed Concrete: The replacementof damaged or deteriorated areas in horizontal slabsinvolves no special procedures other than those used ingood construction practices for placement of new slabs.Repair work can be bonded to old concrete by use of abond coat made of equal amounts of sand and cement.It should have the consistency of whipped cream andshould be applied immediately ahead of concrete place-ment so that it will not set or dry out. Latex emulsionswith portland cement and epoxy resins are also used asbonding coats.

4. Preplaced Aggregate Concrete: This special commer-cial technique has been used for massive repairs, par-ticularly for underwater repairs of piers and abutments.The process consists of the following procedures: 1)Removing the deteriorated concrete, 2) forming thesections to be repaired, 3) prepacking the repair areawith coarse aggregate, and 4) pressure grouting thevoids between the aggregate particles with a cement orsand-cement mortar.

5. Synthetic Patches: One of the most recent developmentsin concrete repair has been the use of synthetic materialsfor bonding and patching. Epoxy-resin compounds areused extensively because of their high bonding proper-ties and great strength. In applying epoxy-resin patching

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mortars, a bonding coat of the epoxy resin is thoroughlybrushed onto the base of the old concrete. The mortar isthen immediately applied and troweled to the elevationof the surrounding material.

Before attempting to repair a deteriorated concrete sur-face, all unsound concrete should be removed by sawing orchipping and the patch area thoroughly cleaned. A sawededge is superior to a chipped edge, and sawing is generallyless costly than mechanical chipping. Before concrete isordered for placing, adequate inspection should be per-formed to ensure that (1) foundations are properly preparedand ready to receive the concrete, (2) construction joints areclean and free from defective concrete, (3) forms are grout-tight, amply strong, and set to their true alignment andgrade, (4) all reinforcement steel and embedded parts areclean, in their correct position, and securely held in place,and (5) adequate concrete delivery equipment and facilitiesare on the job, ready to go, and capable of completing theplacement without addition unplanned construction.

Concrete Use GuidelinesIn addition to its strength characteristics, concrete must

also have the properties of workability and durability.Workability can be defined as the ease with which a givenset of materials can be mixed into concrete and subse-quently handled, transported, and placed with a minimalloss of homogeneity. The degree of workability requiredfor proper placement and consolidation of concrete isgoverned by the dimensions and shape of the structure andby the spacing and size of the reinforcement. The concrete,when properly placed, will be free of segregation, and itsmortar is intimately in contact with the coarse aggregate,the reinforcement, and/or any other embedded parts orsurfaces within the concrete. Separation of coarse aggre-gate from the mortar should be minimized by avoiding orcontrolling the lateral movement of concrete during han-dling and placing operations. The concrete should be de-posited as nearly as practicable in its final position. Placingmethods that cause the concrete to flow in the forms shouldbe avoided. The concrete should be placed in horizontallayers, and each layer should be thoroughly vibrated toobtain proper compaction.

All concrete repairs must be adequately moist-cured to beeffective. The bond strength of new concrete to old con-crete develops much more slowly, and the tendency toshrink and loosen is reduced by a long moist-curing period.

In general, the concrete repair procedures discussed aboveshould be considered on a relative basis and in terms of thequality of concrete that one wishes to achieve for theirparticular construction purpose. In addition to being ad-equately designed, a structure must also be properly con-structed with concrete that is strong enough to carry thedesign loads, durable enough to withstand the forces asso-ciated with weathering, and yet economical, not only infirst cost, but in terms of its ultimate service. It should beemphasized that major structural repairs to concrete shouldnot be attempted by the owner or persons not experiencedin concrete repairs. A qualified professional engineer expe-rienced in concrete construction should be obtained for thedesign of large scale repair projects.

Crack RepairThe two main objectives when repairing cracks in concrete

are structural bonding and stopping water flow. For a struc-tural bond, epoxy injection can be used. This process can bevery expensive since a skilled contractor is needed for properinstallation. The epoxy is injected into the concrete underpressure, welding the cracks to form a monolithic structure.This method of repair should not be considered if the crack isstill active (moving). For a watertight seal, a urethane sealantcan be used. This repair technique does not form a structuralbond; however, it can be used on cracks that are still active.Cracks should be opened using a concrete saw or hand toolprior to placing the sealant. A minimum opening of 1/4 inch isrecommended since small openings are hard to fill. Urethanesealants can be reapplied since they are flexible materials andwill adhere to older applications. As previously noted, all ofthe factors causing cracking must be identified and addressedbefore repairing the concrete to prevent the reoccurrence ofcracks.

Any other questions, comments concerns, or fact sheetrequests, should be directed to the Division of Water at thefollowing address:

Ohio Department of Natural ResourcesDivision of Water

Dam Safety Engineering Program1939 Fountain Square, Building E-3

Columbus, Ohio 43224-1336(614) 265-6731 (Voice) (614) 447-9503 (Fax)

http://www.dnr.state.oh.us/odnr/water/

Bob Taft Governor • Samuel W. Speck Director • James R. Morris, P.E. Chief

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Fact Sheet 95–38

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The principal spillway for dams in the State of Ohiocan be one of several designs. The proper opera-tion of these spillways is an important part of maintaining

the overall safety of the dam. Pipe and riser, drop inlet, and slant pipespillways are susceptible to obstruction and damage by floatingdebris such as leaves, branches, and logs. One device used to ensurethat these spillways operate correctly is a trashrack. Trashracks aredesigned to keep trash and other debris from entering the spillwayand causing damage.

Common problemsTrashracks usually become plugged because the openings are too

small or the head loss at the inlet causes material and sediment tosettle out and accumulate. Small openings will cause debris such astwigs and leaves to accumulate on the trashrack bars. This buildupwill cause progressively larger debris to accumulate against thetrashrack bars. Ultimately, this will result in the complete blockageof the spillway inlet.

Pipe and riser spillways can also become blocked by a build up ofdebris in the spillway. This type of blockage occurs when notrashrack is in place, or if the openings are too large.

In many spillway systems, the size of the outlet conduit is smallerthan the size of the inlet. Therefore, it is incorrect to assume thatdebris which passes through the inlet will not obstruct the flowthrough the outlet. Large debris, such as logs and tree limbs, canbecome lodged in the transitions in the spillway. This reduces thecapacity of the spillway and could cause damage. An obstructedoutlet pipe can be a major problem because removal of large debrisfrom inside the spillway can be very difficult.

A partially blocked spillway reduces the capacity of the spillwayand may also create a higher than normal pool level. The combina-tion of these two factors can dramatically reduce the discharge/storage capacity of the dam. A reduction in the discharge/storagecapacity of a dam increases the likelihood that the dam will beovertopped during a severe storm event. Overtopping for even ashort period of time can cause damage to the embankment andpossibly failure of the dam. If the dam has an emergency spillway,a blocked principal spillway will cause more frequent flows in theemergency spillway. Since emergency spillways are usually grasslined channels designed for infrequent flows of short duration,serious damage is likely to result.

Trashrack designA well-designed trashrack will stop large debris that could plug the

conduit but allow unrestricted passage of water and smaller debris.

The larger the outlet conduit, the larger the trashrack opening shouldbe. In the design of a trashrack the openings should be sized so thatthey measure one-half the nominal dimension of the outlet conduit.

For example, if the outlet pipe is 18 inches in diameter, the trashrackopenings should be the effective equivalent of 9 inches by 9 inches;if the outlet conduit is 3 feet by 5 feet, the trashrack openings shouldbe the effective equivalent of 18 inches by 18 inches. This ruleapplies up to a maximum trashrack opening of two feet by two feet.For an outlet conduit with a nominal dimension of 12 inches or less,the trashrack openings should be at least 6 inches by 6 inches. Thisprevents large debris from passing through the inlet and blocking theoutlet conduit while allowing smaller debris (leave, sticks, etc.) toflush through the spillway system. Another important design criteriais that the trashrack should be securely fastened to the inlet. Theconnection must be strong enough to withstand the hydrostatic anddynamic forces exerted on the trashrack during periods of high flow.

Fish protectionMany owners are concerned about losing fish through trashracks

that have large openings. If this is a concern, a metal plate surround-ing the riser or drop inlet which extends above and below the normalpool level should be installed. See Figure on back of sheet. On thebottom of the plate, a metal screen should be attached and connectedto the riser pipe. The solid plate at the water level will prevent the fishand floating debris from passing over the crest of the riser. Theunderwater screen will keep the fish from moving under the metalplate and through the spillway. The underwater screen will notbecome blocked because most of the debris floats on the watersurface. If this design is used, the area between the inside of thecylinder and the outside of the riser must be equal to or greater thanthe area inside the riser.

Dam Safety: Design and Maintenance of Trashracksfor Pipe and Riser Spillways

Anti-vortexplate

Trashrackbars

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Anti-vortex devicesAn anti-vortex device can easily be incorporated into most trashrack

designs. A common anti-vortex device is a flat metal plate which isplaced on edge and attached to the inlet of the spillway. See Figurebelow. The capacity of the spillway will be increased by equippingthe trashrack with an anti-vortex plate. The anti-vortex plate in-creases capacity by preventing the formation of a flow inhibitingvortex during periods of high flow.

MaintenanceMaintenance should include periodic checks of the trashrack for

rusted and broken sections and repairing as needed. Trashracksshould be checked frequently during and after storm events to ensurethey are functioning properly and to remove accumulated debris.Extreme caution should be used when attempting to remove accu-mulated debris during periods of high flow.

3/4 View Top View

Anti-vortex plate

Normalwater level

Trashrackbars

flow flow

Reinforcedconcrete

base

Concrete piperiser with tee

section.

Trashrackskirt

Fish screen

Fish screen Trashrackbars

Trashrackskirt

Anti-vortexplate

Pipe riser

ConclusionThe benefits of a properly designed and maintained trashrack

include the following:

1. Efficient use of the existing spillway system that will maintainthe design discharge/storage capacity of the dam and preventovertopping.

2. Prevention of costly maintenance items such as the removal ofdebris from the spillway, repair or replacement of damaged spillwaycomponents, and the repair of erosion in emergency spillway.

3. A reduction in the amount of fish lost through the spillwaysystem if a fish screen is used.

Any questions, comments, concerns, or fact sheet requests shouldbe directed to the Division of Water at the following address:

Ohio Department of Natural ResourcesDivision of WaterDam Safety Engineering Program

1939 Fountain Square Drive, Building E-3Columbus, OH 43224-1336

Voice: (614) 265-6731 Fax: (614) 447-9503http://www.dnr.state.oh.us/odnr/water

Trash Rack Design With Fish Protector Screen

Basic Anti-vortex Plate Design

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Fact Sheet 94–30

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trolled flow of water over, around, and adjacent tothe dam. Earth embankments are not designed to beovertopped and therefore are particularly susceptibleto erosion. Once erosion has begun during overtop-ping, it is almost impossible to stop. A well vegetatedearth embankment may withstand limited overtoppingif its top is level and water flows over the top and downthe face as an evenly distributed sheet without becom-ing concentrated. The owner should closely monitorthe reservoir pool level during severe storms.

Seepage FailuresAll earth dams have seepage resulting from water

percolating slowly through the dam and its foundation.Seepage must, however, be controlled in both velocityand quantity. If uncontrolled, it can progressivelyerode soil from the embankment or its foundation,resulting in rapid failure of the dam. Erosion of the soilbegins at the downstream side of the embankment,either in the dam proper or the foundation, progres-sively works toward the reservoir, and eventuallydevelops a "pipe" or direct conduit to the reservoir.This phenomenon is known as "piping." Piping actioncan be recognized by an increased seepage flow rate,the discharge of muddy or discolored water, sinkholeson or near the embankment, and a whirlpool in thereservoir. Once a whirlpool (eddy) is observed on thereservoir surface, complete failure of the dam willprobably follow in a matter of minutes. As withovertopping, fully developed piping is virtually im-possible to control and will likely cause failure.

Seepage can cause slope failure by creating highpressures in the soil pores or by saturating the slope.The pressure of seepage within an embankment isdifficult to determine without proper instrumentation.A slope which becomes saturated and develops slidesmay be showing signs of excessive seepage pressure.

Structural FailuresStructural failures can occur in either the embank-

ment or the appurtenances. Structural failure of a

Dam Safety: Earth Dam Failures

Owners of dams and operating and maintenancepersonnel must be knowledgeable of the poten-tial problems which can lead to failure of a

dam. These people regularly view the structure and,therefore, need to be able to recognize potential prob-lems so that failure can be avoided. If a problem isnoted early enough, an engineer experienced in damdesign, construction, and inspection can be contactedto recommend corrective measures, and such mea-sures can be implemented.

IF THERE IS ANY QUESTION AS TO THESERIOUSNESS OF AN OBSERVATION, ANENGINEER EXPERIENCED WITH DAMSSHOULD BE CONTACTED.

Acting promptly may avoid possible dam failure andthe resulting catastrophic effect on downstream areas.Engineers from the Division of Water, EngineeringGroup of the Department of Natural Resources areavailable at any time to inspect a dam if a seriousproblem is detected or if failure may be imminent.Contact the division at the following address andtelephone number:

Ohio Department of Natural ResourcesDivision of Water, Engineering Group1939 Fountain Square, Building E-3

Columbus, Ohio 43224

In an emergency, call 614/265-6731 or 614/265-7006.

Since only superficial inspections of a dam canusually be made, it is imperative that owners andmaintenance personnel be aware of the prominenttypes of failure and their telltale signs. Earth damfailures can be grouped into three general categories:overtopping failures, seepage failures, and structuralfailures. A brief discussion of each type follows.

Overtopping FailuresOvertopping failures result from the erosive action of

water on the embankment. Erosion is due to uncon-

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spillway, lake drain, or other appurtenance may lead tofailure of the embankment. Cracking, settlement, andslides are the more common signs of structural failureof embankments. Large cracks in either an appurte-nance or the embankment, major settlement, and majorslides will require emergency measures to ensure safety,especially if these problems occur suddenly. If thistype of situation occurs, the lake level should belowered, the appropriate state and local authoritiesnotified, and professional advice sought. If the ob-server is uncertain as to the seriousness of the problem,the Division of Water should be contacted immedi-ately.

The three types of failure previously described areoften interrelated in a complex manner. For example,uncontrolled seepage may weaken the soil and lead toa structural failure. A structural failure may shorten theseepage path and lead to a piping failure. Surface

erosion may result in structural failure.

Minor defects such as cracks in the embankment may bethe first visual sign of a major problem which could leadto failure of the structure. The seriousness of all deficien-cies should be evaluated by someone experienced in damdesign and construction. A qualified professional engi-neer can recommend appropriate permanent remedialmeasures.

Any other questions, comments concerns, or factsheet requests, should be directed to the Division of

Water at the following address:Ohio Department of Natural Resources

Division of WaterDam Safety Engineering Program

1939 Fountain Square, Building E-3Columbus, Ohio 43224-1336

(614) 265-6731 (Voice) (614) 447-9503 (Fax)http://www.dnr.state.oh.us/odnr/water/


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Fact Sheet 99–53

Dam Safety: Embankment Instabilities

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The dam embankment and any appurtenant dikesmust safely contain the reservoir during normaland flood conditions. Cracks, slides, and depres-

sions are signs of embankment instability and shouldindicate to the owner that maintenance or repair workmay be required. When one of these conditions is de-tected, the owner must retain an experienced profes-sional engineer to determine the cause of the instability.A rapidly changing condition or the sudden developmentof a large crack, slide, or depression indicates a veryserious problem, and the Dam Safety Engineering Pro-gram should be contacted immediately. A professionalengineer must investigate these types of embankmentstability problems because a so-called “home remedy”may cause greater and more serious damage to theembankment and eventually result in unneeded expendi-tures for unsuccessful repairs.

Cracks Short, isolated cracks are commonly due to drying and

shrinkage of the embankment surface and are not usuallysignificant. They are usually less than 1 inch wide,propagate in various directions, and occur especiallywhere the embankment lacks a healthy grass cover.Larger (wider than 1 inch), well-defined cracks mayindicate a more serious problem. There are generally twotypes of these cracks: longitudinal and transverse. Lon-gitudinal cracks extend parallel to the crest of the em-bankment and may indicate the early stages of a slide oneither the upstream or downstream slope of the embank-ment. They can create problems by allowing runoff toenter the cracks and saturate the embankment which inturn can cause instability of the embankment. Transversecracks extend perpendicular to the crest and can indicatedifferential settlement within the embankment. Suchcracks provide avenues for seepage through the dam andcould quickly lead to piping, a severe seepage problemthat will likely cause the dam to fail.

If the owner finds small cracks during inspection of thedam, he/she should document the observations, and sealthe cracks to prevent runoff from saturating the embank-ment. The documentation should consist of detailednotes (including the location, length, approximate eleva-tion, and crack width), photographs, sketches, and pos-

sibly monitoring stakes. The crack must then be moni-tored during future inspections. If the crack becomeslonger or wider, a more serious problem such as a slidemay be developing. Large cracks indicate serious stabil-ity problems. If one is detected, the owner should contactthe Dam Safety Engineering Program and/or retain anengineer to investigate the crack and prepare plans andspecifications for repairs. When muddy flow dischargesfrom a crack, the dam may be close to failure. Theemergency action plan should be initiated immediatelyand the Dam Safety Engineering Program contacted.

Slides A slide in an embankment or in natural soil or rock is

a mass movement of material. Some typical characteris-tics of a slide are an arc-shaped crack or scarp along thetop and a bulge along the bottom of the slide (seedrawing). Slides may develop because of poor soil com-

Shallow Slide Cross Section

Deep-seated Slide Cross Section

Bulge

Foundation

Bulge

Scarp

Scarp

Dam fillSlide plane

Slide plane

Lake

Dam crest

Downstreamslope

Slide

Arc-shapedscarp of crack

Foundation

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paction, the gradient of the slope being too steep for theembankment material, seepage, sudden drawdown of thelake level, undercutting of the embankment toe, or satu-ration and weakening of the embankment or foundation.

Slides can be divided into two main groups: shallowand deep-seated. Shallow slides generally affect the top2 to 3 feet of the embankment surface. Shallow slides aregenerally not threatening to the immediate safety of thedam and often result from wave erosion, collapsed rodentburrows, or saturated top soil. Deep-seated slides areserious, immediate threats to the safety of a dam. Theycan extend several feet below the surface of the embank-ment, even below the foundation. A massive slide caninitiate the catastrophic failure of a dam. Deep-seatedslides are the result of serious problems within theembankment.

Small slides can be repaired by removing the vegeta-tion and any unsuitable fill from the area, compactingsuitable fill and adding topsoil to make the embankmentuniform, and establishing a healthy grass cover. If ashallow or deep-seated slide is discovered, the DamSafety Engineering Program should be contacted and anengineer retained to investigate the slide. Plans andspecifications may need to be prepared for its repairdepending on the findings of the investigation.

Depressions Depressions are sunken areas of the abutment, toe area,

or embankment surface. They may be created duringconstruction, or may be caused by decay of buriedorganic materials, thawing of frozen embankment mate-rial, internal erosion of the embankment, or settlement(consolidation) of the embankment or its foundation. Toa certain degree, minor depressions are common and donot necessarily indicate a serious problem. (An embank-ment with several minor depressions may be described ashummocky.) However, larger depressions may indicateserious problems such as weak foundation materials,poor compaction of the embankment during construc-tion, or internal erosion of the embankment fill.

Depressions can create low areas along the crest, cracksthrough the embankment, structural damage to spillways orother appurtenant structures, damage to internal drainagesystems, or general instability of the embankment. They canalso inhibit maintenance of the dam and make detection ofstability or seepage problems difficult.

The owner should monitor depressions during the regularinspection of the dam. All observations should be docu-mented with detailed notes, photographs, and sketches.Minor depressions can be repaired by removing the vegeta-tion and any unsuitable fill from the area, adding fill and thentopsoil to make the embankment uniform, and finally estab-lishing a healthy grass cover. An engineer should be retainedto investigate large depressions or settlement areas. Plansand specifications may need to be prepared for its repairdepending on the findings of the investigation.

Importance of InspectionStability problems can threaten the safety of the dam and

the safety of people and property downstream. Therefore,stability problems must be detected and repaired in a timelymanner. The entire embankment should be routinely andclosely inspected for cracks, slides, and depressions. To dothis thoroughly, proper vegetation must be regularly main-tained on the embankment. Improper or overgrown vegeta-tion can inhibit visual inspection and maintenance of thedam. Accurate inspection records are also needed to detectstability problems. These records can help determine if acondition is new, slowly changing, or rapidly changing. Arapidly changing condition or the sudden development of alarge crack, slide, or depression indicates a very seriousproblem, and the Dam Safety Engineering Program must becontacted immediately.

Any other questions, comments concerns, or fact sheetrequests, should be directed to the Division of Water atthe following address:





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Fact Sheet 99–54

Dam Safety: Ground Cover

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The establishment and control of proper vegetationare an important part of dam maintenance. Prop-erly maintained vegetation can help prevent ero-

sion of embankment and earth channel surfaces, and aidin the control of groundhogs and muskrats. The uncon-trolled growth of vegetation can damage embankmentsand concrete structures and make close inspection diffi-cult.

Grass vegetation is an effective and inexpensive way toprevent erosion of embankment surfaces. If properlymaintained, it also enhances the appearance of the damand provides a surface that can be easily inspected. Rootsand stems tend to trap fine sand and soil particles,forming an erosion-resistant layer once the plants arewell established. Grass vegetation may not be effectivein areas of concentrated runoff, such as at the contact ofthe embankment and abutments, or in areas subjected towave action.

Common ProblemsBare AreasBare areas on an embankment are void of protective

cover (e.g. grass, asphalt, riprap etc.). They are moresusceptible to erosion which can lead to localized stabil-ity problems such as small slides and sloughs. Bare areasmust be repaired by establishing a proper grass cover orby installing other protective cover. If using grass, thetopsoil must be prepared with fertilizer and then scarifiedbefore sowing seed. Types of grass vegetation that havebeen used on dams in Ohio are bluegrass, fescue, ryegrass,alfalfa, clover, and redtop. One suggested seed mixtureis 30% Kentucky Bluegrass, 60% Kentucky 31 Fescue,and 10% Perennial Ryegrass. Once the seed is sown, thearea should be mulched and watered regularly.

ErosionEmbankment slopes are normally designed and con-

structed so that the surface drainage will be spread out ina thin layer as “sheet flow” over the grass cover. Whenthe sod is in poor condition or flow is concentrated at oneor more locations, the resulting erosion will leave rillsand gullies in the embankment slope. The erosion willcause loss of material and make maintenance of theembankment difficult. Prompt repair of the erosion is

required to prevent more serious damage to the embank-ment. If erosion gullies are extensive, a registered profes-sional engineer may be required to design a more rigidrepair such as riprap or concrete. Minor rills and gulliescan be repaired by filling them with compacted cohesivematerial. Topsoil should be a minimum of 4 inches deep.The area should then be seeded and mulched. Not onlyshould the eroded areas be repaired, but the cause of theerosion should be addressed to prevent a continuedmaintenance problem.

FootpathsPaths from animal and pedestrian traffic are problems

common to many embankments. If a path has becomeestablished, vegetation in this area will not provideadequate protection and a more durable cover will berequired unless the traffic is eliminated. Gravel, asphalt,and concrete have been used effectively to cover foot-paths. Embedding railroad ties or other treated woodbeams into an embankment slope to form steps is one ofthe most successful and inexpensive methods used toprovide a protected pathway.

Vehicle RutsVehicle ruts can also be a problem on the embankment.

Vehicular traffic on the dam should be discouragedespecially during wet conditions except when necessary.Water collected in ruts may cause localized saturation,thereby weakening the embankment. Vehicles can alsoseverely damage the vegetation on embankments. Wornareas could lead to erosion and more serious problems.Ruts that develop in the crest should be repaired bygrading to direct all surface drainage into the impound-ment. Bare and eroded areas should be repaired using themethods mentioned in the above sections. Constructedbarriers such as fences and gates are effective ways tolimit access of vehicles.

Improper VegetationCrown vetch, a perennial plant with small pink flowers,

has been used on some dams in Ohio but is not recom-mended (see Figure 1). It hides the embankment surface,preventing early detection of cracks and erosion. It is noteffective in preventing erosion.

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Vines and woody vegetation such as trees and brushalso hide the embankment surface preventing early de-tection of cracks and erosion. Tall vegetation also pro-vides a habitat for burrowing animals. All impropervegetation must be removed from the entire embankmentsurface. Any residual roots that are larger than 3 inchesin diameter must be removed. All roots should be re-moved down to a depth of at least 6 inches and replacedwith a compacted clay material; then 4 inches of topsoilshould be placed on the disturbed areas of the slope.Finally, these areas must be seeded and mulched toestablish a proper grass cover.

MaintenanceEmbankments, areas adjacent to spillway structures, veg-

etated channels, and other areas associated with a damrequire continual maintenance of the vegetal cover. Removalof improper vegetation is necessary for the proper mainte-nance of a dam, dike or levee. All embankment slopes andvegetated earth spillways should be mowed at least twice ayear. Reasons for proper maintenance of the vegetal coverinclude unobstructed viewing during inspection, mainte-nance of a non-erodible surface, discouragement of burrow-ing animal habitation, and aesthetics.

Common methods for control of vegetation include the useof weed trimmers or power brush-cutters and mowers. Chemi-cal spraying to kill small trees and brush is acceptable ifprecautions are taken to protect the local environment. Somechemical spraying may require proper training prior toapplication. Additional information can be found on theTrees and Brush Fact Sheet.

Any other questions, comments concerns, or fact sheetrequests, should be directed to the Division of Water at thefollowing address:





Figure 1: Crown Vetch(Source: http://www.vg.com)

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Fact Sheet 94–33

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Dams, dikes, and levees must not be thought ofas part of the natural landscape, but as man-made structures which must be designed,

inspected, operated, and maintained accordingly.Routine maintenance and inspection of dams andappurtenant facilities should be an ongoing process toensure that structural failures do not occur which canthreaten the overall safety of the dam. The informationprovided in this fact sheet pertains entirely to theinspection of concrete structures used at dams. Theintention is to help dam owners become more aware ofcommon problems that are typically encountered withconcrete so that they can more readily address theseriousness of a particular condition whenever it arises.

Structural InspectionsConcrete surfaces should be visually examined for

spalling and deterioration due to weathering, unusualor extreme stresses, alkali or other chemical attack,erosion, cavitation, vandalism, and other destructiveforces. Structural problems are indicated by cracking,exposure of reinforcing bars, large areas of broken-outconcrete, misalignment at joints, undermining andsettlement in the structure. Rust stains that are notedon the concrete may indicate that internal corrosionand deterioration of reinforcement steel is occurring.Spillway floor slabs and upstream slope protectionslabs should be checked for erosion of underlying basematerial otherwise known as undermining. Concretewalls and tower structures should be examined todetermine if settlement and misalignment of construc-tion joints has occurred.

What to Look ForConcrete structures can exhibit many different types

of cracking. Deep, wide cracking is due to stresseswhich are primarily caused by shrinkage and structuralloads. Minor or hairline surface cracking is caused byweathering and the quality of the concrete that wasapplied. The results of this minor cracking can be theeventual loss of concrete, which exposes reinforcingsteel and accelerates deterioration. Generally, minorsurface cracking does not affect the structural integrityand performance of the concrete structure.

Cracks through concrete surfaces exposed to flowingwater may lead to the erosion or piping of embankmentor foundation soils from around and/or under theconcrete structure. In this case, the cracks are not theresult of a problem but are the detrimental conditionwhich leads to piping and erosion. Seepage at thedischarge end of a spillway or outlet structure mayindicate leakage of water through a crack. Properunderdrainage for open channel spillways with struc-tural concrete floors is necessary to control this leak-age. Flows from underdrain outlets and pressure reliefholes should also be observed and measured. Cloudyflows may indicate that piping is occurring beneath oradjacent to the concrete structure. This could bedetrimental to the foundation support.

Concrete surfaces adjacent to contraction joints andsubject to flowing water are of special concern espe-cially in chute slabs. The adjacent slabs must be flushor the downstream one slightly lower to prevent ero-sion of the concrete and to prevent water from beingdirected into the joint during high velocity flow. Allweep holes should be checked for the accumulation ofsilt and granular deposits at their outlets. These depos-its may obstruct flow or indicate loss of support mate-rial behind the concrete surfaces. Tapping the concretesurface with a hammer or some other device will helplocate voids if they are present as well as give anindication of the condition and soundness of the con-crete. Weep holes in the concrete are used to allow freedrainage and relieve excessive hydrostatic pressuresfrom building up underneath the structure. Excessivehydrostatic pressures underneath the concrete couldcause it to heave or crack which increases the potentialfor accelerated deterioration and undermining. Peri-odic monitoring of the weep hole drains should beperformed and documented on a regular and routinebasis to ensure that they are functioning as designed.

Structural cracking of concrete is usually identifiedby long, single or multiple diagonal cracks with ac-companying displacements and misalignment. Cracksextending across concrete slabs which line open chan-nel spillways or provide upstream slope wave protec-

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tion can indicate a loss of foundation support resultingfrom settlement, piping, undermining, or erosion offoundation soils. Piping and erosion of foundationsoils are the result of inadequate underdrainage and/orcutoff walls. Items to consider when evaluating asuspected structural crack are the concrete thickness,the size and location of the reinforcing steel, the typeof foundation, and the drainage provision for the struc-ture.

Inspection of intake structures, trashracks, upstreamconduits, and stilling basin concrete surfaces that arebelow the water surface is not readily feasible duringa regularly scheduled inspection. Typically, stillingbasins require the most regular monitoring and majormaintenance because they are holding ponds for rockand debris, which can cause extensive damage to theconcrete surfaces during the dissipation of flowingwater. Therefore, special inspections of these featuresshould be performed at least once every five years byeither dewatering the structure or when operatingconditions permit. Investigation of these featuresusing experienced divers is also an alternative.

Preparing for an InspectionBefore an inspection of the dam's concrete facilities

is performed, it is recommended that a checklist bedeveloped that includes all the different componentsof the spillway and/or outlet works. The checklistshould also include a space for logging any specificobservations about the structure and the state of itscondition. Photographs provide invaluable records ofchanging conditions. A rapidly changing conditionmay indicate a very serious problem. If there are anyquestions as to the seriousness of an observation theDam Safety Engineering Program, or a registeredprofessional engineer experienced with dams, shouldbe contacted.






In an emergency, call(614) 265-6731 or (614) 799-9538


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Fact Sheet 93–26

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A lake drain is a device to permit draining a reservoir,lake or pond. Division of Water AdministrativeRule 1501:21-13-06 requires that all Class I, Class

II and Class III dams include a lake drain.

Types of DrainsCommon types of drains include the following:

◆ A valve located in the spillway riser.

◆ A conduit through the dam with a valve at either theupstream or downstream end of the conduit.

◆ A siphon system (Often used to retrofit existingdams).

◆ A gate, valve or stoplogs located in a drain controltower.

Uses of DrainsThe following situations make up the primary uses of lake

drains:

Emergencies: Should serious problems ever occur tothreaten the immediate safety of the dam, drains may beused to lower the lake level to reduce the likelihood of damfailure. Examples of such emergencies are as follows:clogging of the spillway pipe which may lead to high lakelevels and eventually dam overtopping, development ofslides or cracks in the dam, severe seepage through the damwhich may lead to a piping failure of the dam, and partialor total collapse of the spillway system.

Maintenance: Some repair items around the lake and damcan only be completed or are much easier to perform witha lower than normal lake level. Some examples are: slopeprotection repair, spillway repairs, repair and/or installa-tion of docks and other structures along the shoreline, anddredging the lake.

Winter Drawdown: Some dam owners prefer to lower thelake level during the winter months to reduce ice damage tostructures along the shoreline and to provide additionalflood storage for upcoming spring rains. Several repairitems are often performed during this winter drawdownperiod. Periodic fluctuations in the lake level also discour-age muskrat and beaver habitation along the shoreline.Muskrat burrows in earthen dams can lead to costly repairs.

Common Maintenance ProblemsCommon problems often associated with the mainte-

nance and operation of lake drains include the following:

◆ Deteriorated and bent control stems and stemguides.

◆ Deteriorated and separated conduit joints.

◆ Leaky and rusted control valves and sluice gates.

◆ Deteriorated ladders in control towers.

◆ Deteriorated control towers.

◆ Clogging of the drain conduit inlet with sedimentand debris.

◆ Inaccessibility of the control mechanism tooperate the drain.

◆ Seepage along the drain conduit.

◆ Erosion and undermining of the conduit dischargearea because the conduit outlets significantlyabove the elevation of the streambed.

◆ Vandalism.

◆ Development of slides along the upstream slopeof the dam and the shoreline caused by loweringthe lake level too quickly.

Operation and Maintenance TipsA. All gates, valves, stems and other mechanisms should

be lubricated according to the manufacturer’s specifi-cations. If you do not have a copy of the specificationsand the manufacturing company can not be deter-mined, then a local valve distributor may be able toprovide assistance.

B. The lake drain should be operated at least twice a yearto prevent the inlet from clogging with sediment anddebris, and to keep all movable parts working easily.Most manufacturers recommend that gates and valvesbe operated at least four times per year. Frequentoperation will help to ensure that the drain will beoperable when it is needed. All valves and gates shouldbe fully opened and closed at least twice to help flushout debris and to obtain a proper seal. If the gate getsstuck in a partially opened position, gradually work the

Dam Safety: Lake Drains

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gate in each direction until it becomes fully opera-tional. Do not apply excessive torque as this could bendor break the control stem, or damage the valve or gateseat. With the drain fully open, inspect the outlet areafor flow amounts, leaks, erosion and anything unusual.

C. All visible portions of the lake drain system should beinspected at least annually, preferably during the peri-odic operation of the drain. Look for and make note ofany cracks, rusted and deteriorated parts, leaks, bentcontrol stems, separated conduit joints or unusual ob-servations.

D. A properly designed lake drain should include aheadwall near the outlet of the drain conduit to preventundermining of the conduit during periods of flow. Aheadwall can be easily retro-fitted to an existing con-duit if undermining is a problem at an existing dam. Aproperly designed layer of rock riprap or other slopeprotection will help reduce erosion in the lake drainoutlet area.

E. Drain control valves and gates should always be placedupstream of the centerline of the dam. This allows thedrain conduit to remain depressurized except duringuse, therefore reducing the likelihood of seepage throughthe conduit joints and saturation of the surroundingearth fill.

F. For accessibility ease, the drain control platform shouldbe located on shore or be provided with a bridge orother structure. This becomes very important duringemergency situations if high pool levels exist.

G. Vandalism can be a problem at any dam. If a lake drainis operated by a crank, wheel or other similar mecha-nism, locking with a chain or other device, or off-sitestorage may be beneficial. Fences or other such instal-lations may also help to ward off vandals.

H. The recommended rate of lake drawdown is one foot orless per week, except in emergencies. Fast drawdowncauses a build-up of hydrostatic pressures in the up-stream slope of the dam which can lead to slope failure.Lowering the water level slowly allows these pressuresto dissipate.

MonitoringMonitoring of the lake drain system is necessary to detect

problems and should be performed at least twice a year ormore frequently if problems develop. Proper ventilationand confined space precautions must be considered whenentering a lake drain vault or outlet pipe. Items to beconsidered when monitoring a lake drain system includethe stem, valve, outlet pipe and related appurtenances.Monitoring for surface deterioration (rust), ease of opera-tion, and leakage is important to maintain a working lakedrain system. If the stem or valve appears to be inoperablebecause of deterioration or if the operability of the lakedrain system is in question, because the valve does notcompletely close (seal) and allows an excessive amount ofleakage, then a registered professional engineer ormanufacturer’s representative should be contacted. Photo-graphs along with written records of the monitoring itemsperformed provide invaluable information. For furtherinformation on evaluating the condition of the lake drainsystem see the “Spillway Conduit System Problems”, “Prob-lems with Metal Materials”, “Problems with Plastic (Poly-mer) Materials”, and “Problems with Concrete Materials”fact sheets.

ConclusionAn operable lake drain accomplishes the following:

1. Makes for a safer dam by providing a method tolower the lake level in an emergency situation.

2. Allows the dam owner to have greater control ofthe lake level for maintenance, winter drawdownand emergency situations.

3. Meets the requirements of the Ohio Dam SafetyLaws.







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Fact Sheet 98–49

Dam Safety: Open Channel Spillways(Earth and Rock)

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MaintenanceMaintenance should include, but not be limited to, the

following items:

•Grass-covered channels should be mowed atleast twice per year to maintain a good grasscover and to prevent trees, brush and weedsfrom becoming established. Poor vegetal covercan result in extensive and rapid erosion when thespillway flows. Repairs can be costly. Reseedingand fertilization may be necessary to maintain avigorous growth of grass. One suggested seedmixture is 30% Kentucky Bluegrass, 60% Ken-tucky 31 Fescue, and 10% Perennial Ryegrass.

•Trees and brush must be removed from thechannel. Tree and brush growth reduces the dis-charge capacity of the spillway channel. Thisincreases the lake level during large storm eventswhich can lead to overtopping and failure of thedam.

•Erosion in the channel must be repairedquickly after it occurs. Erosion can be expectedin the spillway channel during high flows, and canalso occur as a result of rainfall and runoff, espe-cially in areas of poor grass cover. Terraces ordrainage channels may be necessary in large spill-way channels where large amounts of rainfall andrunoff may concentrate and have high velocities.Erosion of the side slopes may deposit material inthe spillway channel, especially where the sideslopes meet the channel bottom. In small spill-ways, this can significantly reduce the dischargecapacity. This condition often occurs immediatelyafter construction before vegetation becomes es-tablished. In these cases, it may be necessary toreshape the channel to provide the necessary ca-pacity.

•All obstructions should be kept out of thechannel. Open channel spillways often are usedfor purposes other than passage of flood flows.

Open channels are often used as the emergencyspillway and sometimes as the principal spill-way for dams. A principal spillway is used to

pass normal inflows, and an emergency spillway isdesigned to operate only during large flood events,usually after the capacity of the principal spillway hasbeen exceeded. For dams with pipe conduit principalspillways, an open channel emergency spillway is almostalways required as a backup in case the pipe becomesclogged. Open channels are usually located in naturalground adjacent to the dam and can be vegetated, rock-lined, or cut in rock.

DesignFlow through an emergency spillway does not neces-

sarily indicate a problem with the dam, but high velocityflows can cause severe erosion and result in a perma-nently lowered lake level if not repaired. Proper designof an open channel spillway will include provisions forminimizing any potential erosion. One way to minimizeerosion is to design a flatter channel slope to reduce thevelocity of the flow. Earthen channels can be protectedby a good grass cover, an appropriately designed rockcover, concrete or various types of erosion control mat-ting. Rock-lined channels must have adequately sizedriprap to resist displacement and contain an appropriategeotextile fabric or granular filter beneath the rock.Guide berms are often required to divert flow throughopen channels away from the dam to prevent erosion ofthe embankment fill. If an open channel is used for aprincipal spillway, it must be rock-lined or cut in rockdue to more frequent or constant flows.

Ohio Administrative Code Rule 1501:21-13-04 re-quires that the frequency of use for an earth (grass-lined)or a rock-lined emergency spillway be less than:

• Once in 50 years for Class I dams;• Once in 25 years for Class II dams; and• Once in 10 years for Class III dams.

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Among these uses are reservoir access, parkinglots, boat ramps, boat storage, pasture and crop-land. Permanent structures (buildings, fences, etc.)should not be constructed in these spillways. Iffences, bridges or other such structures are abso-lutely necessary, they should cross the spillway farenough upstream or downstream from the controlsection so that they do not interfere with the flow.Construction of any structures in or across thechannel requires prior approval from the Divisionof Water.

•Weathering of rock channels can be a seriousproblem and is primarily due to freeze/thawaction. Deterioration due to the effects of sun,wind, rain, chemical action and tree root growthalso occurs. Weathered rock is susceptible to ero-sion and displacement during high flows; there-fore, rock channels are often designed with 1 to 3feet of earth with a grass cover over the rocksurface to help insulate the rock from the effects offreeze/thaw action.

MonitoringOpen channel spillways should be monitored for erosion,poor vegetal cover, growth of trees and brush, obstruc-tions, and weathering and displacement of rock. Moni-toring should take place on a regular basis and after largeflood events. It is important to keep written records ofobservations. Photographs provide invaluable records ofchanging conditions. All records should be kept in theoperation, maintenance, and inspection manual for thedam.

Any other questions, comments concerns, or factsheet requests, should be directed to the Division ofWater at the following address:





Downstream View of Open Channel Spillway


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Fact Sheet 99–51

Dam Safety: Outlet Erosion Control Structures(Stilling Basins)

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tioned in the opening paragraphs, or if the discharge spill-way conduit does not have a headwall/endwall, then aregistered professional engineer should be contacted toevaluate the stability of the outlet.

Impact BasinA concrete impact basin is an energy dissipating device

located at the outlet of the spillway in which flow fromthe discharge conduit strikes a vertical hanging baffle.Discharge is directed upstream in vertical eddies by thehorizontal por-tion of the baffleand by the floorbefore flowingover the endsill.Energy dissipa-tion occurs asthe dischargestrikes thebaffle, thus,performance isnot dependenton tailwater.Most impactbasins were de-signed by the United States Department of Agriculture,Natural Resources Conservation Service and the UnitedStates Department of Interior, Bureau of Reclamation. Ifany of the severe defects that are referenced in theopening paragraphs are observed, a registered profes-sional engineer should be contacted to evaluate thestability of the outlet.

U.S. Department of Interior, Bureau of Reclama-tion Type II and Type III Basins

Type II and Type III basins reduce the energy of theflow discharging from the outlet of a spillway and allowthe water to exit into the outlet channel at a reducedvelocity. Type II energy dissipators contain chute blocksat the upstream end of the basin and a dentated (tooth-like)endsill. Baffle piers are not used in a Type II basinbecause of the high velocity water entering the basin.

Water moving through the spillway of a damcontains a large amount of energy. This energycan cause erosion at the outlet which can lead to

instability of the spillway. Failure to properly design,install, or maintain a stilling basin could lead to problemssuch as undermining of the spillway and erosion of the outletchannel and/or embankment material. These problems canlead to failure of the spillway and ultimately the dam. Astilling basin provides a means to absorb or dissipate theenergy from the spillway discharge and protects the spill-way area from erosion and undermining. An outlet erosioncontrol structure such as a headwall/endwall, impact basin,United States Department of the Interior, Bureau of Recla-mation Type II or Type III basin, baffled chute, or plungepool is considered an energy dissipating device. The perfor-mance of these structures can be affected by the tailwaterelevation. The tailwater elevation is the elevation of thewater that is flowing through the natural stream channeldownstream during various flow conditions.

A headwall/endwall, impact basin, Type II or Type IIIbasin, and baffled chute are all constructed of concrete.Concrete structures can develop surface defects such asminor cracking, bugholes, honeycombing, and spalling.Concrete structures can have severe structural defects suchas exposed rebar, settlement, misalignment and large cracks.Severe defects can indicate structural instability.

Headwall/EndwallA headwall/endwall located at or close to the end of the

discharge conduit will provide support and reduce thepotential for undermining. A headwall/endwall is typicallyconstructed of concrete, and it should be founded on bed-rock or have an adequate foundation footing to providesupport for stability.A headwall/endwall can become dis-placed if it is not adequately designed and is subject toundermining. Displacement of the headwall/endwall canlead to separation of the spillway conduit at the joints which

could affect theintegrity of thespillway conduit.If a concretestructure devel-ops the structuraldefects men-

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Type III energydissipators can beused if the en-trance velocity ofthe water is nothigh. They con-tain baffle pierswhich are locatedon the stilling ba-sin apron down-stream of the

chute blocks. Located at the end of both the Type II andType III basins is an endsill. The endsill may be level orsloped, and its purpose is to create the tailwater whichreduces the outflow velocity. If any of the severe defectsassociated with concrete structures are observed, a regis-tered professional engineer should be contacted to evalu-ate the stability of the basin.

Baffled ChuteBaffled chutes require no initial tailwater to be effective

and are located downstream of the control section. Mul-tiple rows of baffle piers on the chute prevent excessiveacceleration of the flow and prevent the damage thatoccurs from a high discharge velocity. A portion of thebaffled chute usually extends below the streambed eleva-tion to prevent undermining of the chute. If any of thesevere problems associated with concrete that are refer-enced in the opening paragraphs are observed, a regis-tered professional engineer should be contacted to evalu-ate the stability of the outlet.

Plunge PoolA plunge pool is an energy dissipating device located at

the outlet of a spillway. Energy is dissipated as thedischarge flows into the plunge pool. Plunge pools arecommonly lined with rock riprap or other material toprevent excessive erosion of the pool area. Dischargefrom the plunge pool should be at the natural streambedelevation. Typical problems may include movement ofthe riprap, loss of fines from the bedding material and

scour beyond the riprap and lining. If scour beneath theoutlet conduit develops, the conduit will be left unsup-ported and separation of the conduit joints and undermin-ing may occur. Separation of the conduit joints andundermining may lead to failure of the spillway andultimately the dam. A registered professional engineershould be contacted to ensure that the plunge pool isdesigned properly.

Additional information about related topics can befound on the following fact sheets: “Inspection of Con-crete Structures,” “Spillway Conduit System Problems,”“Open Channel Spillways (Concrete Chutes and Weirs),”and “Problems with Concrete Materials.”

Any questions, comments, concerns, or fact sheet re-quests should be directed to the Division of Water at thefollowing address:

Ohio Department of Natural ResourcesDivision of WaterDam Safety Engineering Program

1939 Fountain Square Drive, Building E-3Columbus, OH 43224-1336

(614) 265-6731 (Voice)(614) 447-9503 (Fax)


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Baffeled Chute Basin

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Fact Sheet 99–56

Dam Safety: Problems with Concrete Materials

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thickness), and exposed rebar indicate serious concretedeterioration. If not repaired, this type of concrete dete-rioration may lead to structural instability of the concretestructure. A registered professional engineer must pre-pare plans and specifications for repair of serious con-crete deterioration. For additional information, see the“Concrete Repair Techniques” fact sheet.

Disintegration can be a result of many causes such asfreezing and thawing, chemical attack, and poor con-struction practices. All exposed concrete is subject tofreeze-thaw, but the concrete’s resistance to weatheringis determined by the concrete mix and the age of theconcrete. Concrete with the proper amounts of air, water,and cement, and a properly sized aggregate, will be muchmore durable. In addition, proper drainage is essential inpreventing freeze-thaw damage. When critically satu-rated concrete (when 90% of the pore space in theconcrete is filled with water) is exposed to freezingtemperatures, the water in the pore spaces within theconcrete freezes and expands, damaging the concrete.Repeated cycles of freezing and thawing will result insurface scaling and can lead to disintegration of theconcrete. Hydraulic structures are especially susceptibleto freeze-thaw damage since they are more likely to becritically saturated. Older structures are also more sus-ceptible to freeze-thaw damage since the concrete wasnot air entrained. In addition, acidic substances in thesurrounding soil and water can cause disintegration ofthe concrete surface due to a reaction between the acidand the hydrated cement.

CracksCracks in the concrete may be structural or surface

cracks. Surface cracks are generally less than a fewmillimeters wide and deep. These are often called hair-line cracks and may consist of single, thin cracks, orcracks in a craze/map-like pattern. A small number ofsurface or shrinkage cracks is common and does notusually cause any problems. Surface cracks can becaused by freezing and thawing, poor construction prac-tices, and alkali-aggregate reactivity. Alkali-aggregatereactivity occurs when the aggregate reacts with thecement causing crazing or map cracks. The placement of

Visual inspection of concrete will allow for thedetection of distressed or deteriorated areas. Prob-lems with concrete include construction errors,

disintegration, scaling, cracking, efflorescence, erosion,spalling, and popouts.

Construction ErrorsErrors made during construction such as adding im-

proper amounts of water to the concrete mix, inadequateconsolidation, and improper curing can cause distressand deterioration of the concrete. Proper mix design,placement, and curing of the concrete, as well as anexperienced contractor are essential to prevent construc-tion errors from occurring. Construction errors can leadto some of the problems discussed later in this fact sheetsuch as scaling and cracking. Honeycombing andbugholes can be observed after construction.

Honeycombing can be recognized by exposed coarseaggregate on the surface without any mortar covering orsurrounding the aggregate particles. The honeycombingmay extend deep into the concrete. Honeycombing canbe caused by a poorly graded concrete mix, by too largeof a coarse aggregate, or by insufficient vibration at thetime of placement. Honeycombing will result in furtherdeterioration of the concrete due to freeze-thaw becausemoisture can easily work its way into the honeycombedareas. Severe honeycombing should be repaired to pre-vent further deterioration of the concrete surface.

Bugholes is a term used to describe small holes (lessthan about 0.25 inch in diameter) that are noticeable onthe surface of the concrete. Bugholes are generallycaused by too much sand in the mix, a mix that is too lean,or excessive amplitude of vibration during placement.Bugholes may cause durability problems with the con-crete and should be monitored.

Disintegration and ScalingDisintegration can be described as the deterioration of

the concrete into small fragments and individual aggre-gates. Scaling is a milder form of disintegration wherethe surface mortar flakes off. Large areas of crumbling(rotten) concrete, areas of deterioration which are morethan about 3 to 4 inches deep (depending on the wall/slab

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new concrete over old may cause surface cracks todevelop. This occurs because the new concrete willshrink as it cures. Surface cracks in the spillway shouldbe monitored and will need to be repaired if they deterio-rate further.

Structural cracks in the concrete are usually larger than0.25 inch in width. They extend deeper into the concreteand may extend all the way through a wall, slab, or otherstructural member. Structural cracks are often caused bysettlement of the fill material supporting the concretestructure, or by loss of the fill support due to erosion. Thestructural cracks may worsen in severity due to the forcesof weathering. A registered professional engineer knowl-edgeable about dam safety must investigate the cause ofstructural cracks and prepare plans and specifications forrepair of any structural cracks. For additional informa-tion, see the “Concrete Repair Techniques” fact sheet.

EfflorescenceA white, crystallized substance, known as efflores-

cence, may sometimes be noted on concrete surfaces,especially spillway sidewalls. It is usually noted nearhairline or thin cracks. Efflorescence is formed by waterseeping through the pores or thin cracks in the concrete.When the water evaporates, it leaves behind some min-erals that have been leached from the soil, fill, or con-crete. Efflorescence is typically not a structural problem.Efflorescence should be monitored because it can indi-cate the amount of seepage finding its way through thincracks in the concrete and can signal areas where prob-lems (i.e. inadequate drainage behind the wall or deterio-ration of concrete) could develop. Also, water seepingthrough thin cracks in the wall will make the concretemore susceptible to deterioration due to freezing andthawing of the water.

ErosionErosion due to abrasion results in a worn concrete

surface. It is caused by the rubbing and grinding ofaggregate or other debris on the concrete surface of aspillway channel or stilling basin. Minor erosion is not aproblem but severe erosion can jeopardize the structuralintegrity of the concrete. A registered professional engi-neer must prepare plans and specifications for repair ofthis type of erosion if it is severe.

Erosion due to cavitation results in a rough, pittedconcrete surface. Cavitation is a process in which subat-

mospheric pressures, turbulent flow and impact energyare created and will damage the concrete. If the shape ofthe upper curve on the ogee spillway is not designed closeto its ideal shape, cavitation may occur just below theupper curve, causing erosion. A registered professionalengineer must prepare plans and specifications for repairof this type of erosion if the concrete becomes severelypitted which could lead to structural damage or failure ofthe structure.

Spalling and PopoutsSpalling is the loss of larger pieces or flakes of concrete.

It is typically caused by sudden impact of somethingdropped on the concrete or stress in the concrete thatexceeded the design. Spalling may occur on a smallerscale, creating popouts. Popouts are formed as the waterin saturated coarse aggregate particles near the surfacefreezes, expands, and pushes off the top of the aggregateand surrounding mortar to create a shallow conical de-pression. Popouts are typically not a structural problem.However, if a spall is large and causes structural damage,a registered professional engineer must prepare plansand specifications to repair the spalling.

Inspection and MonitoringRegular inspection and monitoring is essential to detect

problems with concrete materials. Concrete structuresshould be inspected a minimum of once per year. Theinspector should also look at the interior condition ofconcrete spillway conduit. Proper ventilation and con-fined space precautions must be considered when enter-ing a conduit. It is important to keep written records of thedimensions and extent of scaling, disintegration, efflo-rescence, honeycombing, erosion, spalling, popouts, andthe length and width of cracks. Structural cracks shouldbe monitored more frequently and repaired if they are athreat to the stability of the structure or dam. Photographsprovide invaluable records of changing conditions. Arapidly changing condition may indicate a very seriousproblem, and the Dam Safety Engineering Programshould be contacted immediately. All records should bekept in the operation, maintenance, and inspection manualfor the dam.






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Fact Sheet 99–57

Dam Safety: Problems with Metal Materials

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Corrosion is a common problem for spillway con-duits and other metal appurtenances. Corrosion isthe deterioration or breakdown of metal because

of a reaction with its environment. Exposure to moisture,acidic conditions, or salt will accelerate the corrosionprocess. Acid runoff from strip-mined areas will causerapid corrosion of metal conduits. In these areas, con-duits made of less corrodible materials such as concreteor plastic should be used. Soil types also factor into theamount of corrosion. Clayey soils can be more corrosivethan sandy soils since they are poorly drained and poorlyaerated. Silts are somewhere in between clays and sands.Some examples of metal conduits include ductile iron,smooth steel, and corrugated metal. Corrugated metalpipe is not recommended for use in dams since theservice life for corrugated metal is only 25 to 30 years,whereas the life expectancy for dams is much longer. Inareas of acidic water, the service life can be much less.Therefore, corrugated metal spillway conduits typicallyneed to be repaired or replaced early in the dam’s designlife, which can be very expensive.

Conduit coating is an effective way of controllingcorrosion of metal conduits if used properly. It is rela-tively inexpensive and extends the life of the conduit.Some examples of coatings include cement-mortar, ep-oxy, aluminum, or polyethylene film. Asphalt (bitumi-nous) coatings are not recommended since their servicelife is usually only one or two years. Coatings must beapplied to the conduit prior to installation and protectedto ensure that the coating is not scratched off. Coatingsapplied to conduits in service are generally not veryeffective because of the difficulty in establishing anadequate bond.

Corrosion can also be controlled or arrested by install-ing cathodic protection. A metallic anode such as mag-nesium (or zinc) is buried in the soil and is connected tothe metal conduit by wire. Natural voltage current flow-ing from the magnesium (anode) to the conduit (cathode)will cause the magnesium to corrode and not the conduit.However, sufficient maintenance funds should be allo-cated for the regular inspection of this active system.

If corrosion is allowed to continue, metal conduits willrust out. The spillway must be repaired before waterflows through the rusted out portion of the conduit anderodes the fill material of the embankment. Continuederosion can lead to failure of the dam. Sliplining can bean economical and effective method of permanentlyrestoring deteriorated spillways. During sliplining, asmaller diameter pipe is inserted into the old spillwayconduit and then grout is used to fill in the void betweenthe two pipes. If sliplining the spillway is not feasible, thelake may need to be drained and a new spillway must beinstalled. A registered professional engineer must beretained to develop and submit plans and specificationsfor any major modifications such as spillway slipliningor replacement.

Corrosion of the metal parts of the operating mechanismssuch as lake drain valves and slu ice gates can be effectivelytreated by keeping these parts lubricated and /or painted. Ifthe device has not been operated in several years, a qualifiedperson (i.e. manufacturer’s representative or registeredprofessional engineer) should inspect it to determine its

Figure 1 – Example of a corrugated metal pipeand riser spillway.

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operability. Caution must be used to prevent the mecha-nism from breaking. A registered professional engineermay be needed to prepare plans and specifications forrepair if the device is determined to be inoperable.

Regular inspection and monitoring is essential to detectany problems with metal materials. Coatings on metalpipes should be inspected for scratched and worn areas.The inspector should also look for corrosion inside thespillway conduit. Proper ventilation and confined spaceprecautions must be considered when entering the spill-way conduit system. If using cathodic protection, regularinspections are required to verify that the system isworking properly. It is important to keep written recordsof the amount of surface rust, pitting, and corrosion onany metal surface. Areas of thin metal should be moni-tored more frequently and repaired or replaced if theyrust out. Photographs provide invaluable records of chang-ing conditions. A rapidly changing condition may indi-cate a very serious problem, and the Dam Safety Engi-neering Program should be contacted immediately. Allrecords should be kept in the operation, maintenance, andinspection manual for the dam.






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Fact Sheet 99–58

Dam Safety: Problems with Plastic (Polymer) Materials

Plastics are often used as spillway and lake drainpipes in dam construction and repair. The mostcommon plastic pipes are high-density polyethyl-

ene (HDPE) and polyvinyl chloride (PVC). The advan-tages of using plastic pipe include excellent abrasionresistance, chemical corrosion resistance, low mainte-nance, and long life expectancy. Naturally occurringchemicals in soils will not degrade plastic pipe and causeit to rot or corrode. Plastic pipes are also much easier tohandle and install compared to heavier concrete and steelpipes.

Plastic pipes are considered flexible, and they get theirstrength from the material and the surrounding backfillwhereas rigid pipes, such as concrete, get their strengthfrom the material and the pipe structure. Backfill aroundplastic pipes must be properly compacted and in fullcontact with the pipe. It is important to take special carein the haunch area to prevent the pipe from lifting off thesubgrade and disrupting vertical alignment. Symmetricbackfilling is also required to prevent the pipe frombeing out of lateral alignment.

Hand-Compactedbackfill

Pipe crown

Haunch

Pipe invert

11

When designing a new spillway system, a registeredprofessional engineer will be required to specify thecorrect type of pressurized plastic pipe that can be used.

The pipe must be able to withstand the pressures from theweight of the embankment without crushing or buckling.The joints must also be watertight. Not all plastic pipewill meet these requirements.

As with other plastic materials, ultraviolet light degra-dation can be a problem. Photo-degradation can causeplastic to become brittle and crack. Carbon black is themost effective additive to enhance the photo-degrada-tion resistance of plastic materials. Pipes containingcarbon black can be safely stored outside in most cli-mates for many years without damage from ultravioletexposure. Plastic pipes can be affected by liquid hydro-carbons such as gasoline and oil. If hydrocarbons comein contact with plastic pipe, they will permeate the pipewall causing swelling and loss of strength. However, ifthe hydrocarbons are removed, the effects are reversible.

Regular inspection and monitoring is essential to detectany problems with plastic materials. Plastic pipes shouldbe inspected for deformation and cracking. The inspec-tor should also look at the interior condition of thespillway pipe. Proper ventilation and confined spaceprecautions must be considered when entering the spill-way pipe system. It is important to keep written recordsof pipe dimensions to note deformation and the lengthand width of cracks. Photographs provide invaluablerecords of changing conditions. A rapidly changingcondition may indicate a very serious problem, and theDam Safety Engineering Program should be contactedimmediately. All records should be kept in the operation,maintenance, and inspection manual for the dam.

Figure 1 - Cross-section of plastic pipe in trench






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Fact Sheet 94–27

Rodents such as the groundhog (woodchuck), musk-rat, and beaver are attracted to dams and reser-voirs, and can be quite dangerous to the struc-

tural integrity and proper performance of the embank-ment and spillway. Groundhog and muskrat burrowsweaken the embankment and can serve as pathways forseepage. Beavers may plug the spillway and raise thepool level. Rodent control is essential in preserving awell-maintained dam.

GroundhogThe groundhog is the largest member of the squirrel

family. Its coarse fur is a grizzled grayish brown witha reddish cast. Typical foods include grasses, clover,alfalfa, soybeans, peas, lettuce, and apples. Breedingtakes place during early spring (beginning at the age ofone year) with an average of four or five young perlitter, one litter per year. The average life expectancyis two or three years with a maximum of six years.

Occupied groundhog burrows are easily recognized inthe spring due to the groundhog’s habit of keeping them“cleaned out.” Fresh dirt is generally found at the mouthof active burrows. Half-round mounds, paths leadingfrom the den to nearby fields, and clawed or girdled treesand shrubs also help identify inhabited burrows anddens.

When burrowing into an embankment, groundhogsstay above the phreatic surface (upper surface of seepageor saturation) to stay dry. The burrow is rarely a singletunnel. It is usually forked, with more than one entranceand with several side passages or rooms from 1 to 12 feetlong.

Groundhog ControlControl methods should be implemented during early

spring when active burrows are easy to find, younggroundhogs have not scattered, and there is less likeli-hood of damage to other wildlife. In later summer, fall,and winter, game animals will scurry into groundhogburrows for brief protection and may even take uppermanent abode during the period of groundhog hiber-nation.

Groundhogs can be controlled by using fumigants or byshooting. Fumigation is the most practical method ofcontrolling groundhogs. Around buildings or other highfire hazard areas, shooting may be preferable. Ground-hogs will be discouraged from inhabiting the embank-ment if the vegetal cover is kept mowed.

Gas cartridges may be purchased at garden supplyand hardware stores. Information about the use andavailability of gas cartridges may be obtained fromcounty extension offices, or the U.S. Department ofAgriculture at the following address:

The USDAAnimal and Plant Health Inspection Service

Wildlife Services200 North High Street, Room 622

Columbus, Ohio 43215(614) 469-5681

MuskratThe muskrat is a stocky rodent with a broad head, short

legs, small eyes, and rich dark brown fur. Muskrats arechiefly nocturnal. Their principal food includes stems,roots, bulbs, and foliage of aquatic plants. They also feedon snails, mussels, crustaceans, insects, and fish. Usu-ally three to five litters, averaging six to eight young perlitter, are produced each year. Adult muskrats averageone foot in length and three pounds in weight. The lifeexpectancy is less than two years, with a maximum offour years. Muskrats can be found wherever there aremarshes, swamps, ponds, lakes and streams having calmor very slowly moving water with vegetation in the waterand along the banks.

Muskrats make their homes by burrowing into thebanks of lakes and streams or by building “houses” ofbushes and other plants. Their burrows begin from 6 to18 inches below the water surface and penetrate theembankment on an upward slant. At distances up to 15feet from the entrance, a dry chamber is hollowed outabove the water level. Once a muskrat den is occupied,a rise in the water level will cause the muskrat to digfarther and higher to excavate a new dry chamber.Damage (and the potential for problems) is compoundedwhere groundhogs or other burrowing animals constructtheir dens in the embankment opposite muskrat dens.

Continued on back!

Dam Safety: Rodent Control

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Muskrat ControlBarriers to prevent burrowing offer the most practical

protection to earthen structures. A properly constructedriprap and filter layer will discourage burrowing. Thefilter and riprap should extend at least 3 feet below thewater line. As the muskrat attempts to construct a burrow,the sand and gravel of the filter layer caves in and thusdiscourages den building. Heavy wire fencing laid flatagainst the slope and extending above and below thewater line can also be effective. Eliminating or reducingaquatic vegetation along the shoreline will discouragemuskrat habitation. Where muskrats have inhabited thearea, trapping is usually the most practical method ofremoving them from a pond.

Eliminating a BurrowThe recommended method of backfilling a burrow in an

embankment is mud-packing. This simple, inexpensivemethod can be accomplished by placing one or twolengths of metal stove or vent pipe in a vertical positionover the entrance of the den. Making sure that the pipeconnection to the den does not leak, the mud-pack mix-ture is then poured into the pipe until the burrow and pipeare filled with the earth-water mixture. The pipe isremoved and dry earth is tamped into the entrance. Themud-pack is made by adding water to a 90 percent earthand 10 percent cement mixture until a slurry or thincement consistency is attained. All entrances should beplugged with well-compacted earth and vegetation re-established. Dens should be eliminated without delaybecause damage from just one hole can lead to failure ofa dam or levee.

BeaverBeaver will try to plug spillways with their cuttings.

Routinely removing the cuttings is one way to alleviatethe problem. Trapping beaver may be done by the ownerduring the appropriate season; however, the nearestODNR, Division of Wildlife, District Office or stategame protector should be contacted first.

Hunting and Trapping RegulationsBecause hunting and trapping rules change from

year to year, ODNR, Division of Wildlife authoritiesat one of the following offices should be consultedbefore taking any action.

Rodent Burrow DamageCrest

Water Level

Scarp

Phreatic Surface

Muskrat BurrowGroundhog Burrow

DANGEROUSLY CLOSE BURROWS






DefianceHenry

WoodOttawa

LucasFultonWilliams

Sandusky

Paulding

Putnam Hancock

SenecaHuron

Erie Lorain

Cuyahoga

Medina Summit

Portage

Mahoning

Trumbull

Geauga

Lake

Ashtabula

Richland

Ashland

WayneStark Columbiana

Carroll

TuscarawasHolmes

Knox

Coshocton Harrison

BelmontGuernsey

Muskingum

Licking

Crawford

Morrow

Delaware

Franklin

MercerAuglaize

ShelbyLogan

Darke

MiamiChampaign

Hardin

Wyandot

Madison

Union

Marion

Clark

Preble Montgomery

Greene

Fayette

Pickaway

Warren ClintonButlerRoss

HighlandHamiltonClermont

Pike

SciotoAdams

Fairfield Perry

Morgan

Noble Monroe

Washington

Athens

Vinton

JacksonMeigs

Gallia

Lawrence

Jefferson

Van Wert

Allen

Brown

Hocking

Wildlife District One1500 Dublin RoadColumbus 43215Phone: (614) 644-3925FAX (614) 644-3931

Wildlife District Two952 Lima Avenue, Box AFindlay 45840Phone: (419) 424-5000FAX (419) 422-4875

Wildlife District Three912 Portage Lakes DriveAkron 44319Phone: (330) 644-2293FAX (330) 644-8403

Wildlife District Four360 E. State StreetAthens 45701Phone: (740) 594-2211FAX (740) 592-1626

Wildlife District Five1076 Old Springfield PikeXenia 45385Phone: (937) 372-9261FAX (937) 376-3011

In Sandusky305 East Shorline DriveSandusky 44870Phone: (419) 625-8062FAX (419) 625-6272

In Fairport HarborPhone: (440) 352-6100FAX (440) 350-0250

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Fact Sheet 94–31

Continued on back!

Contrary to popular opinion, wet areas downstream from dams are not usually naturalsprings, but seepage areas. Even if natural

springs exist, they should be treated with suspicion andcarefully observed. Flows from ground-water springsin existence prior to the reservoir would probablyincrease due to the pressure caused by the pool of waterbehind the dam.

All dams have some seepage as the impounded waterseeks paths of least resistance through the dam and itsfoundation. Seepage must, however, be controlled toprevent erosion of the embankment or foundation ordamage to concrete structures.

DetectionSeepage can emerge anywhere on the downstream

face, beyond the toe, or on the downstream abutmentsat elevations below normal pool. Seepage may vary inappearance from a "soft," wet area to a flowing "spring."It may show up first as an area where the vegetation islush and darker green. Cattails, reeds, mosses, andother marsh vegetation often become established in aseepage area. Another indication of seepage is thepresence of rust-colored iron bacteria. Due to theirnature, the bacteria are found more often where wateris discharging from the ground than in surface water.Seepage can make inspection and maintenance diffi-cult. It can also saturate and weaken portions of theembankment and foundation, making the embank-ment susceptible to earth slides.

If the seepage forces are large enough, soil will beeroded from the foundation and be deposited in theshape of a cone around the outlet. If these "boils"appear, professional advice should be sought immedi-ately. Seepage flow which is muddy and carryingsediment (soil particles) is evidence of "piping," andwill cause failure of the dam. Piping can occur along aspillway and other conduits through the embankment,and these areas should be closely inspected. Sinkholes

may develop on the surface of the embankment asinternal erosion takes place. A whirlpool in the lakesurface may follow and then likely a rapid and com-plete failure of the dam. Emergency procedures, in-cluding downstream evacuation, should be imple-mented if this condition is noted.

Seepage can also develop behind or beneath concretestructures such as chute spillways or headwalls. If theconcrete structure does not have a means such as weepholes or relief drains to relieve the water pressure, theconcrete structure may heave, rotate, or crack. Theeffects of the freezing and thawing can amplify theseproblems. It should be noted that the water pressurebehind or beneath structures may also be due to infil-tration of surface water or spillway discharge.

A continuous or sudden drop in the normal lake levelis another indication that seepage is occurring. In thiscase, one or more locations of flowing water areusually noted downstream from the dam. This condi-tion, in itself, may not be a serious problem, but willrequire frequent and close monitoring and profes-sional assistance.

ControlThe need for seepage control will depend on the

quantity, content, and location of the seepage. Reduc-ing the quantity of seepage that occurs after construc-tion is difficult and expensive. It is not usually at-tempted unless the seepage has lowered the pool levelor is endangering the embankment or appurtenantstructures. Typical methods used to control the quan-tity of seepage are grouting or installation of an up-stream blanket. Of these methods, grouting is probablythe least effective and is most applicable to leakagezones in bedrock, abutments, and foundations. Thesemethods must be designed and constructed under thesupervision of a professional engineer experiencedwith dams.

Dam Safety: Seepage Through Earthen Dams

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Controlling the content of the seepage or preventingseepage flow from removing soil particles is extremelyimportant. Modern design practice incorporates thiscontrol into the embankment through the use of cut-offs, internal filters, and adequate drainage provisions.Control at points of seepage exit can be accomplishedafter construction by installation of toe drains, reliefwells, or inverted filters.

Weep holes and relief drains can be installed torelieve water pressure or drain seepage from behind orbeneath concrete structures. These systems must bedesigned to prevent migration of soil particles but stillallow the seepage to drain freely. The owner mustretain a professional engineer to design toe drains,relief wells, inverted filters, weep holes, or relief holes.

MonitoringRegular monitoring is essential to detect seepage and

prevent dam failure. Knowledge of the dam's historyis important to determine whether the seepage condi-tion is in a steady or changing state. It is important tokeep written records of points of seepage exit, quantityand content of flow, size of wet area, and type ofvegetation for later comparison. Photographs provideinvaluable records of seepage.

All records should be kept in the operation, mainte-nance, and inspection manual for the dam. The inspec-tor should always look for increases in flow andevidence of flow carrying soil particles, which would

indicate that a more serious problem is developing.Instrumentation can also be used to monitor seepage.V-notch weirs can be used to measure flow rates, andpiezometers may be used to determine the saturationlevel (phreatic surface) within the embankment.

Regular surveillance and maintenance of internalembankment and foundation drainage outlets is alsorequired. The rate and content of flow from each pipeoutlet for toe drains, relief wells, weep holes, and reliefdrains should be monitored and documented regularly.Normal maintenance consists of removing all obstruc-tions from the pipe to allow for free drainage of waterfrom the pipe. Typical obstructions include debris,gravel, sediment, and rodent nests. Water should notbe permitted to submerge the pipe outlets for extendedperiods of time. This will inhibit inspection andmaintenance of the drains and may cause them to clog.

Any other questions, comments concerns, or factsheet requests, should be directed to the Division ofWater at the following address:






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Fact Sheet 99–55

Dam Safety: Spillway Conduit System Problems

Continued on back!

In addition, undermining can occur as the result oferosion due to inadequate energy dissipation or inad-equate erosion protection at the outlet. This underminingcan be visually observed at the outlet of a pipe system andcan extend well into the embankment. In this case,undermining can lead to other conduit problems such asmisalignment, separated joints and pipe deterioration.An extensive discussion on outlet erosion control as itrelates to undermining of the pipe outlet can be found inthe “Outlet Erosion Control Structures” fact sheet.

Installation of seepage control devices is required as apreventative measure to control seepage along the con-duit and undermining. Regular monitoring of conduitsystems must include visual observation and notation ofany undermining or any precursors. These precursorsusually include pipe deformation, misalignment anddifferential settlement, pipe deterioration, separated jointsand loss of joint material.

Pipe deformationPipe deformations are typically caused by external

loads that are applied on a pipe such as the weight of theembankment or heavy equipment. Collapse of the pipecan cause failure of the joints and allow erosion of thesupporting fill. This may lead to undermining and settle-ment. Pipe deformation may reduce or eliminate spill-way capacity. Pipe deformation must be monitored on aregular basis to ensure that no further deformation isoccurring, that pipe joints are intact and that no under-mining or settlement is occurring.

Separated joints and loss of joint material:Joint Deterioration

Conduit systems usually have construction and/or sec-tion joints. In almost every situation, the joints will havea water stop, mechanical seal and/or chemical seal toprevent leakage of water through the joint. Separationand deterioration can destroy the watertight integrity ofthe joint. Joint deterioration can result from weathering,excessive seepage, erosion or corrosion. Separation at ajoint may be the result of a more serious condition such

Many dams have conduit systems that serve asprincipal spillways. These conduit systems arerequired to carry normal stream and small

flood flows safely past the embankment throughout thelife of the structure. Conduits through embankments aredifficult to construct properly and can be extremelydangerous to the embankment if problems develop afterconstruction. Conduits are usually difficult to repairbecause of their location within the embankment. Also,replacing conduits requires extensive excavation. Inorder to avoid difficult and costly repairs, particularattention should be directed to maintaining these struc-tures. The most common problem noted with spillwayconduit systems is undermining of the conduit. Thiscondition typically results from water leaking throughpipe joints, seepage along the conduit or inadequateenergy dissipation at the conduit outlet. The typicalcauses of seepage and water leaking through pipe jointsinclude any one or a combination of the followingfactors: loss of joint material, separated joints, misalign-ment, differential settlement, conduit deterioration, andpipe deformation. Problems in any of these areas maylead to failure of the spillway system and possibly damfailure.

UnderminingUndermining is the removal of foundation material

surrounding a conduit system. Any low areas or unex-plained settlement of the earthfill in line with the conduitmay indicate that undermining has occurred within theembankment. As erosion continues, undermining of aconduit can lead to displacement and collapse of the pipesections and cause sloughing, sliding or other forms ofinstability in the embankment. As the embankment isweakened, a complete failure of the conduit system and,eventually the dam may occur.

Seepage along the conduit from the reservoir can occuras a result of poor compaction around the conduit. Ifseepage control devices have not been installed, theseepage may remove foundation material from aroundthe conduit and eventually lead to undermining.

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as foundation settlement, undermining, structural dam-age or structural instability. Deterioration at joints in-cludes loss of gasket material, loss of joint sealant andspalling around the edges of joints. Separation of jointsand loss of joint material allow seepage through the pipe.This can erode the fill underneath and along the conduitcausing undermining, which can lead to the displacementof the pipe sections. Separated pipe joints can be detectedby inspecting the interior of the conduit. A regularmonitoring program is needed to determine the rate andseverity of joint deterioration. Joint separations shouldbe monitored to determine if movement is continuing.

Conduit DeteriorationDeterioration of conduit material is normally due to the

forces of nature such as wetting and drying, freezing andthawing, oxidation, decay, ultra-violet light, cavitationand the erosive forces of water. Deterioration of pipematerials and joints can lead to seepage through andalong the conduit and eventually failure of conduit sys-tems. Additional information on deterioration can befound on the “Problems with Concrete Materials”, “Prob-lems with Metal Materials”, and “Problems with Plastic(Polymer) Materials” fact sheets.

Differential SettlementRemoval or consolidation of foundation material from

around the conduit can cause differential settlement.Inadequate compaction immediately next to the conduitsystem during construction would compound the prob-lem. Differential settlement can ultimately lead to under-mining of the conduit system. Differential settlementshould be monitored with routine inspections and docu-mentation of observations.

MisalignmentAlignment deviations can be an indication of move-

ment, which may or may not be in excess of designtolerances. Proper alignment is important to the struc-tural integrity of conduit systems. Misalignment can bethe direct result of internal seepage flows that haveremoved soil particles or dissolved soluble rock. Mis-alignment can also result from poor construction prac-tices, collapse of deteriorated conduits, decay of organicmaterial in the dam, seismic events or normal settlement

due to consolidation of embankment or foundation ma-terials. Excessive misalignment may result in other prob-lems such as cracks, depressions, slides on the embank-ment, joint separation and seepage. Both the vertical andhorizontal alignment of the conduit should be monitoredon a regular basis.

Monitoring and RepairFrequent inspection is necessary to ensure that the pipe

system is functioning properly. All conduits should beinspected thoroughly once a year. Conduits that are 24inches or more in diameter can be entered and visuallyinspected with proper ventilation and confined spaceprecautions. Small inaccessible conduits may be moni-tored with video cameras. The conduits should be in-spected for misalignment, separated joints, loss of jointmaterial, deformations, leaks, differential settlement andundermining. Problems with conduits occur most oftenat joints, and special attention should be given to themduring the inspection. The joint should be checked forseparation caused by misalignment or settlement andloss of joint-filler material. The outlet should be checkedfor signs of water seeping along the exterior surface ofthe conduit. Generally, this is noted by water flowingfrom under the conduit and/or the lack of foundationmaterial directly beneath the conduit. The embankmentsurface should be monitored for depressions or sink-holes. Depressions or sinkholes on the embankmentsurface above the spillway conduit system develop whenthe underlying material is eroded and displaced. Photo-graphs along with written records of the monitoringitems performed provide invaluable information.

Effective repair of the internal surface or joint of aconduit is difficult and should not be attempted withoutcareful planning and proper professional supervision.Various construction techniques can be applied for mi-nor joint repair and conduit leakage, but major repairsrequire a plan be developed by a professional engineerexperienced in dam spillway construction.






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Fact Sheet 94–28

These products should be painted, not sprayed, on the stumps.Other instructions found on the label should be strictly followedwhen handling and applying these materials. Only a few com-mercially available chemicals can be used along shorelines ornear water.

Embankment MaintenanceEmbankments, areas adjacent to spillway structures, vegetated

channels, and other areas associated with a dam require continualmaintenance of the vegetal cover. Grass mowing, brush cutting,and removal of woody vegetation (including trees) are necessaryfor the proper maintenance of a dam, dike, or levee. Allembankment slopes and vegetated earth spillways should bemowed at least once a year. Aesthetics, unobstructed viewingduring inspections, maintenance of a non-erodible surface, anddiscouragement of groundhog habitation are reasons for propermaintenance of the vegetal cover.

Methods used in the past for control of vegetation, but are nowconsidered unacceptable, include chemical spraying, and burning.More acceptable methods include the use of weed whips or powerbrush-cutters and mowers. Chemical spraying to first kill small treesand brush is acceptable if precautions are taken to protect the localenvironment.

It is important to remember not to mow when the embankmentis wet. It is also important to use proper equipment for the slopeand type of vegetation to be cut. Also, always follow themanufacturer’s recommended safe operation procedures.

Any other questions, comments, concerns, or fact sheetrequests, should be directed to the Division of Water at thefollowing address:





The establishment and control of proper vegetation is animportant part of dam maintenance. Properly maintained vegetation can help prevent erosion of embank-

ment and earth channel surfaces, and aid in the control ofgroundhogs and muskrats. The uncontrolled growth of veg-etation can damage embankments and concrete structures andmake close inspection difficult.

Trees and BrushTrees and brush should not be permitted on embankment

surfaces or in vegetated earth spillways. Extensive root systemscan provide seepage paths for water. Trees that blow down or fallover can leave large holes in the embankment surface that willweaken the embankment and can lead to increased erosion.Brush obscures the surface limiting visual inspection, providesa haven for burrowing animals, and retards growth of grassvegetation. Tree and brush growth adjacent to concrete walls andstructures may eventually cause damage to the concrete andshould be removed.

Stump Removal & Sprout PreventionStumps of cut trees should be removed so vegetation can be

established and the surface mowed. Stumps can be removedeither by pulling or with machines that grind them down. Allwoody material should be removed to about 6 inches below theground surface. The cavity should be filled with well-compactedsoil and grass vegetation established.

Stumps of trees in riprap cannot usually be pulled or grounddown, but can be chemically treated so they will not continuallyform new sprouts. Certain herbicides are effective for thispurpose and can even be used at water supply reservoirs ifapplied by licensed personnel. For product information andinformation on how to obtain a license, contact the Ohio Depart-ment of Agriculture at the following address:

Ohio Department of AgriculturePesticide Regulation8995 E. Main Street

Reynoldsburg, Ohio 43068Telephone Number (614) 728-6987

Dam Safety: Trees and Brush

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Fact Sheet 99–52

Dam Safety: Upstream Slope Protection

Continued on back!

Slope protection is usually needed to protect theupstream slope against erosion due to wave action.Without proper slope protection, a serious erosion

problem known as “beaching” can develop on the upstreamslope.

The repeated action of waves striking the embankmentsurface erodes fill material and displaces it farther down theslope, creating a “beach.” The amount of erosion dependson the predominant wind direction, the orientation of thedam, the steepness of the slope, water level fluctuations,boating activities, and other factors. Further erosion canlead to cracking and sloughing of the slope which canextend into the crest, reducing its width. When erosionoccurs and beaching develops on the upstream slope of adam, repairs should be made as soon as possible. However,an erosion scarp less than 1 foot high may be stable and notrequire repair.

The upstream face of a dam is commonly protectedagainst wave erosion by placement of a layer of rock riprapover a layer of bedding and a filter material. Other materialsuch as concrete facing, soil-cement, fabri-form bags, slushgrouted rocks, steel sheet piling, and articulated concreteblocks can also be used. Vegetative protection combinedwith a berm on the upstream slope can also be effective.

Rock RiprapRock riprap consists of a heterogeneous mixture of irregular

shaped rocks placed over gravel bedding and a sand filter orgeotextile fabric. The smaller rocks help to fill the spacesbetween the larger pieces forming an interlocking mass. Thefilter prevents soil particles on the embankment surface frombeing washed out through the spaces (or voids) between the

rocks. The maximum rock size and weight must be largeenough to break up the energy of the maximum anticipatedwave action and hold the smaller stones in place. If the rocksize is too small, it will eventually be displaced and washedaway by wave action. If the riprap is sparse or if the filter orbedding material is too small, the filter material will wash outeasily, allowing the embankment material to erode. Once theerosion has started, beaching will develop if remedial mea-sures are not taken. Technical Release No. 69 developed bythe USDA, Natural Resources Conservation Service can beused to help design engineers develop a preliminary or detaileddesign for riprap slope protection.

The dam owner should expect some deterioration (weath-ering) of riprap. Freezing and thawing, wetting and drying,abrasive wave action, and other natural processes willeventually break down the riprap. Its useful life varies withthe characteristics of the stone used. Stone for riprapshould be rock that is dense and well cemented. In Ohio,glacial cobbles or boulders, most limestone, and a fewtypes of sandstone are acceptable for riprap. Most sand-stones and shales found in Ohio do not provide long-termprotection. Due to the high initial cost of rock riprap, itsdurability should be determined by appropriate testingprocedures prior to installation. Vegetative growth withinthe slope protection is undesirable because it can displacestone and disturb the filter material. Heavy undergrowthprevents an adequate inspection of the upstream slope andmay hide potential problems. For additional information,see the “Trees and Brush” fact sheet.

Figure 1 - Beaching

Figure 2 - Rock Riprap

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Sufficient maintenance funds should be allocated for theaddition of riprap and the removal of vegetation. Severeerosion or reoccurring problems may require a registeredprofessional engineer to design a more effective slopeprotection.

Vegetated Wave BermVegetated wave berms dissipate wave energy and protect

the slope from erosion. Berms are constructed on theupstream slope at the normal pool level and should be noless than 20 feet wide. This method of slope protection willnot work well where the water surface fluctuates regularlyfrom normal pool. If improper or sparse vegetation ispresent, the wave berm may not adequately dissipate thewave energy, allowing erosion and beaching to develop onthe upstream slope. Technical Release No. 56 developedby the USDA, Natural Resources Conservation Serviceprovides design and layout information.

The vegetation on the wave berm should be monitoredregularly to verify adequate growth. Sufficient fundsshould be allocated for the regular maintenance of thevegetation. Severe erosion or reoccurring problems mayrequire a registered professional engineer to design a moreeffective slope protection.

Concrete FacingConcrete facing can be used if severe wave action is antici-

pated, however, settlement of the embankment must be insig-nificant to insure adequate support for the concrete facing. Aproperly designed and constructed concrete facing can beexpensive. This slope protection should extend several feetabove and below the normal pool level. It should terminate ona berm or against a concrete curb or header. Granular filter orfilter fabric (geotextile) is required under the concrete facing tohelp reduce the risk of undermining.

As with any type of slope protection, problems willdevelop if the concrete facing has not been properly de-signed or installed. Concrete facing often fails because thewave action washes soil particles from beneath the slabsthrough joints and cracks. This process is known asundermining, which will continue until large voids arecreated. Detection of voids is difficult because the voidsare hidden. Failure of the concrete facing may be suddenand extensive. Concrete facing should be monitored forcracks and open joints. Open joints should be sealed withplastic fillers and cracks should be grouted and sealed. Foradditional information, see the “Problems with ConcreteMaterials” fact sheet.

Inspection and MonitoringRegular inspection and monitoring of the upstream slope

protection is essential to detect any problems. It is impor-tant to keep written records of the location and extent of anyerosion, undermining, or deterioration of the riprap, waveberm or other slope protection. Photographs provide in-valuable records of changing conditions. A rapidly chang-ing condition may indicate a very serious problem, and theDam Safety Engineering Program should be contactedimmediately. All records should be kept in the operation,maintenance, and inspection manual for the dam.Figure 3 - Vegetated wave berm

Figure 4 - Concrete facing






Dam Safety: Concrete Repair Techniques C · chipping and the patch area thoroughly cleaned. A sawed...

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