RIPRAP DESIGN FOR OVERTOPPED EMBANKMENTS
FINAL REPORTRIPRAP DESIGN FOR OVERTOPPEDEMBANKMENTSPrepared byS.K. Mishra and James F. RuffColorado State UniversityEngineering Research CenterPrepared forU.S. Bureau of ReclamationOctober, 1998
ABSTRACTRIPRAP DESIGN FOR OVERTOPPED EMBANKMENTSThe flood potential at a dam and the resulting likelihood of dam failure must beevaluated as part of the design flood selection process. The design flood must be selectedbefore requirements for additional spillway capacity or overtopping protection can bedesigned. Overtopping means using all, or a portion of, the dam crest length as anemergency spillway. Innovative overtopping protection systems for dams are being studiedthroughout the world. It is the hope that these studies will reduce cost and still provide areliable level of safety to both new and existing dams. Research programs are currentlyunderway or planned for evaluating overtopping protection concepts. If these newadvancements were better known, dam owners may be more inclined to improve the levelof protection of their dams at a reasonable cost.In the construction of the new dams and rehabilitation of existing dams, riprap isusually the most economical material for erosion control. Currently, testing of large riprapsizes is lacking. Experiments have been conducted in laboratories without producingresults allowing application to a full-scale dam with confidence. To address this problem,the United States Bureau of Reclamation and Colorado State University (CSU) built a nearprototype embankment overtopping facility at CSU's Engineering Research Center in 1991.Three sizes of large riprap were tested on the overtopping facility in the summersof 1994, 1995, and 1997. Riprap gradations were obtained. Interstitial velocities of waterthrough the rock layer as well as the flow depths were recorded for each test. These datawere compared against the available riprap design equations. The need to develop auniversal riprap design equation for any slope specifically steep slopes was established.The gathered data were analyzed. A new universal riprap design equation wasdeveloped. Finally, an innovative riprap design procedure was outlined.
ACKNOWLEDGMENTSOur appreciation to Ms. Kathleen Frizell and USBR Contract #1425-1-FC-81-17790and Contract #1425-96-FC-81-05026, and the Colorado State University AgriculturalResearch Center Contract #C0L00708 for the funding support of this research and thecompletion of this report.11
TABLE OF CONTENTSPageiiivviviiiABSTRACTACKNOWLEDGMENTSLIST OF TABLESLIST OF FIGURESLIST OF SYMBOLSChapter 1: Introduction1.1 The Purpose of the Current Study13Chapter 2: History of Embankment Protection Studies2.1 Embankment Overtopping Protection Methods2.2 Literature Review2.3 NRC/CSU Studies for Riprap Design2.4 ARS Riprap Tests2.5 Need for Future Research446121314Chapter 3: Experimental Facilities, Riprap Characteristics, and Data Acquisition . . 15153.1 Overtopping Facility193.2 Characteristics of the Bedding Material193.3 Characteristics of the Riprap Materials Tested213.4 General Operating Procedures223.5 Instrumentation and Data AcquisitionChapter 4: Flow Observation4.1 Riprap Flow Conditions4.2 Flow Observations in 1994 and 19974.3 Flow Observations in 199528282831Chapter 5: Data Analysis5.1 Analysis of Salt Injection Method5.2 Analysis of Piezometer Data5.3 Velocity Calculation From Piezometer Data of 1994 and 19955.4 Velocity Calculation From Piezometer Data of 19975.5 Comparison of Interstitial Velocity Calculated From Two Methods . . .353537434343iii
Chapter 6: Development of a Predictive Equation for Interstitial Velocity ofWater Through Riprap6.1 Need for Developing a New Predictive Equation for InterstitialVelocity of Water6.2 Results From the Current Test Program454545Chapter 7: Description of Riprap Failure and Toe Treatment7.1 Failure of Riprap7.2 Toe Treatment Test Program494953Chapter 8: Development of the Riprap Design Equation8.1 Development of Design Guidelines8.2 Shield's Parameter8.3 Derivation of the Universal Equation8.4 Sensitivity Analysis8.5 Riprap Layer Thickness and Slope Influence575757637373Chapter 9: Design Procedure and Examples9.1 Design Procedure9.2 Example 19.3 Step by Step Solution9.4 Example 29.5 Step by Step Solution797981818383Chapter 10: Summary, Conclusions, and Future Research10.1 Summary10.2 Conclusions10.3 Scope of Future Research85858687References88Appendix A: Interstitial Velocity Data90108Appendix B: Manometer Dataiv
LIST OF TABLESPageTable 1.1 Mean stone diametersTable 2.1 Riprap gradation for bedding layer of 1997 (D50 48.3 mm)Table 2.2 Riprap gradation for 1994 tests (D50 386 mm)Table 2.3 Riprap gradation for 1995 tests (D50 655 mm)Table 2.4 Riprap gradation for 1997 tests (D50 271 mm)Table 5.1 Comparison of interstitial velocities obtained by two methodsTable 6.1 Interstitial velocitiesTable 7.1Riprap failure characteristicsTable 8.1 Deviation factors cs21920202144465369
LIST OF FIGURESPage9Free body diagram of a rock in a horizontal plane10Free body diagram of a rock in water flow on an inclined plane15Overtopping facility at CSU17Riprap test layer configuration in 1994 and 199518Riprap test layer configuration in 199720Grid used for sampling rocksSchematic drawing of injectors, conductivity probes, and23piezometer locations24Plan view of the piezometers, injectors, and conductivity probesFigure 3.61994 layout with the piezometer towers on the riprap slopeFigure 3.725before testing1995 layout with the piezometer towers on the riprap slopeFigure 3.825before testing1997 layout of the riprap slope with the piezometer towers beforeFigure 3.926testing27Figure 3.10 Numbering of rocks in 1995290.23m3/s,1994Q TransitionofflowintothechuteatFigure 4.1Flow becoming turbulent as it encountered the rock layer atFigure 4.229Q 0.23 m3/s, 1994Visible surface water looking downstream at Q 0.28 m3/s, 1997 . . 30Figure 4.3Wet and dry areas of riprap surface at Q 0.42 m3/s, 1997Figure 4.430(flow from top to bottom)Wet and dry areas of riprap surface at window #2, Q 0.23 m3/s,Figure 4.5311994Flow (left to right) pattern due to obstruction of flow path seenFigure 4.632through window #2 (Q 0.23 m3/s)33Cascading flow (flow direction from top to bottom) (1994)Figure 4.7331.13m3/s)(Q WatersurfaceprofileovertheriprapgabionFigure 4.834Large plumes of water (1995) (Q 2.12 m3/s)Figure 4.934Figure 4.10 Schematic of the velocity zones (1995)Typical voltage drop indicating the interstitial flow velocity forFigure 5.136the testsTypical voltage drop seen in the bedding layer in 1997 or nearFigure 5.237the bottom layer for 1994 and 199537Schematic drawing of piezometer-manometer configurationFigure 5.3Figure 2.1Figure 2.2Figure 3.1Figure 3.2Figure 3.3Figure 3.4Figure 3.5vi
Variation of depth of flow with unit discharge in 1994 and 1995 . . . . 3940Variation of depth of flow with unit discharge in 1997Photograph showing the profile of water flow through the riprap41layer of 1994Figure 5.7Photograph showing the profile of water flow through the riprap42layer of 1995Comparison of current formula and formula by Abt et al.Figure 6.148with existing dataFailure of 1994 riprap layer located 19.3 m from the crest of theFigure 7.150flume down the slopeFigure 7.2Several rocks of 1995 riprap layer caught in the trap (looking51downstream)51Failure of 1995 layer showing removal of the top layerFigure 7.352Figure 7.4Channelization in 1997 riprap layer with dislodgement of rocks52Failure of 1997 layer at 12.1 m from the crest down the slopeFigure 7.554Schematic diagram of the toe shapes and sizes tested in 1997Figure 7.655Wire mesh covering the riprapFigure 7.756Catastrophic failureFigure 7.858Figure 8.1Shield's diagram (adapted from Julien, 1995)59Figure 8.2a Robinson et al. riprap design equation60Figure 8.2b Closeup view of Robinson et al. (1995) riprap design equation61Figure 8.3a Abt et al. (1991) riprap design equation62Figure 8.3b Closeup view of Abt et al. (1991) riprap design dindifferenttests.Figure 8.467Universal design equationFigure 8.568Universal design equation (normal axes)Figure 8.671Error analysis at 50% slope (USBRJCSU)Figure 8.772Figure 8.8Error analysis at 40% slope (ARS)Relationship between maximum allowable surface discharge andFigure 8.976D50 at various slopesFigure 8.10 Relationship between maximum allowable surface discharge and77the slope of the embankment at various D50'sFigure 8.11 Combined effect of slope and rock size on maximum allowable78surface dischargeFigure 5.4Figure 5.5Figure 5.6vii
LIST OF SYMBOLSa C1, C2 D50D, Fd k2ksrn Pnqf SsSF T7ave Dimensional constant, and coefficient which varies with the cross-sectionalgeometry of the flowCorrection factor due to the aeration effectsCoefficient of stability of the rock layerCoefficient of uniformity of the riprapConstantsDepth of water to be determined from a stage-discharge rating curve, andrepresentative dimension of the rockRoughness height given by 3D84Median stone sizeMedian boulder diameterAbsolute roughnessFriction factorDrag forceWeight of the stoneAcceleration due to gravityWater depthEnergy and hydraulic gradientFunction of the stone shapeVolume constant for the rockNikuradse roughness sizeHydraulic mean radius of rock voidsMarming's constantPorosityExponentRelationship between unit dischargeUnit discharge at failureHydraulic radiusSlope of energy grade line which can be approximated as the slope of the bedSlope of bed (channel), and gradient expressed in decimal formSpecific gravity of rockSafety factorThickness of the riprap layerMean seepage velocity, and velocity of waterBulk velocity (average velocity)Average interstitial velocityEmpirical constant for a given rockfill materialAverage depth of waterviii
aSsSt11(110v,,TsT„Angle of the embankment with the horizontalDistance between the two probesTime elapsed between the drops of voltage at probe 1 and probe 2Specific weight (density, gravity) of waterSpecific gravity of rockRoughness factor, and porosity of the rock layerAngle of repose of the riprapAngle of slopeAverage velocity of water through rock voidsAngle of repose of the riprap material, and angle of internal frictionShear stressNon-dimensional Shield's parameterCritical shear stress to cause the incipient motion yRS
CHAPTER 1INTRODUCTIONIn recent years, a number of state and federal agencies have inventoried all damswithin the United States to assess their overall safety. During this process, the designfloods were reevaluated and often recalculated.Previous design flood selection criteria included factors such as dam height, storagevolume, downstream development, and relationships between design floods anddownstream hazard classifications. Current practice is to classify dams based on theconsequences of dam failure. This requires the identification of potential failure modes andquantitative evaluation of the consequences of dam failure. In addition to the traditionaleconomic analysis of impacts such as lost project benefits and property damages, impactsfrom loss of life, and environmental impacts should be considered.The revised floods were generally larger than the floods developed for the originaldesigns because of an increased database and revised criteria for developing floods basedon refined versions of the National Weather Service hydrometeorological reports (Oswaltet al., 1994). In many cases, occurrence of the new or revised design floods will result inovertopping of dams due to insufficient storage and/or release capacity provided by theexisting reservoirs. Traditional modification alternatives for resolving this problem, suchas providing additional reservoir storage and increased spillway capacity are often verycostly. As a result, some dam owners are faced with either accepting greater risks thanwere indicated by the original design flood calculations, or incurring significant costs toprotect their facilities from failure during extreme flood events. For some time, the FederalEnergy Regulatory Commission (FERC) and dam owners have maintained that finding analternative to traditional spillway designs by protecting these embankments from failuredue to overtopping would represent an important advancement in public safety andeconomy. Overtopping essentially means using all, or a portion of, the dam crest length asan emergency spillway.There is an urgent need for research to develop design criteria for protectiveoverlays as an alternative to traditional spillway design where dam safety modifications areunder consideration at major embankment structures in the United States. New andinnovative designs for spillways and overtopping protection systems for dams that willreduce costs and still provide a reliable level of safety are being developed throughout theworld. Many of these alternatives are applicable to both new and existing dams. Inaddition, research programs are currently underway or planned for evaluating new spillwaydesigns and overtopping protection concepts. If these new advancements were verified,dam owners may be more inclined to improve the level of protection of their dams at areasonable cost. Such systems will not only save a large amount of money, but will alsoincrease public safety.
In the construction of new darns and rehabilitation of existing darns, riprap isusually the most economical material for erosion control. The design of riprap to resistovertopping flow is dependent upon the material properties, the hydraulic gradient, and theunit discharge. Several researchers such as Stephenson (1979), Abt et al. (1987 and 1988),and Robinson et al. (1995) have provided empirical design criteria which currently offersthe best approach for design. However, flow hydraulics on steep embankment slopesprotected with riprap can not be analyzed by standard flow and sediment transportequations. Furthermore, testing of large riprap sizes is lacking and extrapolation toprototype sizes is difficult. All previous experiments dealing with riprap protection onslopes with overtopping flow have been performed with stone diameters of 158 mm or lessand slopes of 40% or less. Experiments have been conducted in laboratory facilities and/orlarge-scale flumes, but have not produced results allowing application to a full-scale damwith confidence. Large-scale tests on steeper slopes where aeration effects and scale effectsmay not be neglected are lacking. In general, a comprehensive data set of riprap failure forD50 larger than 158 mm is not currently available.Shield's parameter is a dimensionless form of the shear stress that expresses theincipient motion of stones in flowing water. Mechanics of the riprap stability is governedby the Shield's parameter. For riprap in overtopping flow, rock movement that leads toexposure of the underlying materials is the primary failure mode. Because of the manyassumptions made to simplify the Shield's parameter for the present design use, it isinadequate for its applications to riprap design in overtopping flows. The simplified formof the Shield's parameter assumes the slope to be less than 10% and related forces to beneglected. This assumption is unacceptable when dealing with the steeper slopes of atypical embankment dam.Interstitial velocities were never measured with the exception of the tests conductedjointly by the Nuclear Regulatory Commission (NRC) and Colorado State University(CSU). Even though interstitial velocity was measured during the NRC/CSU riprap testing,it was not used as a parameter for designing the riprap. Parkin et al. (1966), Leps (1973),and others analyzed interstitial velocities of rockfill without looking into the mechanics offailure, since it was not an immediate concern for the type of studies they were conducting.Studies carried out by Robinson et al. (1995) of the Agricultural Research Service atStillwater, did not measure interstitial velocities.Existing equations used to predict stone sizes needed on overtopped embankmentsproduce widely varying results. Table 1.1 provides a sample of the various mean stonediameters that may be obtained by using some of the existing methods for a typicalembankment dam of 2:1 downstream slope that must pass an overtopping unit dischargeof 1.4 m3/s/m. These methods yield up to sevenfold difference in stone size.Table 1.1: Mean stone diameterPredicted D50 stone sizeMethod(mm)Abt et al. (1987, 1988)457Robinson et al. (1995)4883,124Stephenson (1979)1.1 The Purpose of the Current Study2
The purpose of placing riprap on an embankment is to prevent erosion of theunderlying earthen materials that constitute the embankment. The need to predict thebehavior of riprap protection is the very important first step in predicting the eventualbreach of the dam. Design of stable riprap protection should prevent dam breaching fordesigned overtopping flows. Over estimation of the material needed to protect a dam canlead to excessive costs that make the project prohibitively expensive. Under estimation ofthe size of the material can lead to catastrophic consequences including loss of life,economic losses, and destruction of infrastructure. As a result, further tests of large sizestones on steeper slopes as conducted by this project are needed to develop better designcriteria for overtopped riprap.The objective of the current investigation is to derive a universal formula fordesigning riprap for overtopping flows regardless of the slope of the overtoppedembankment. The formula will be based on the mechanics of the riprap mixture underflowing water observed while testing in the near-prototype Embankment OvertoppingResearch facility located at the Engineering Research Center of CSU.Background information for the embankment protection studies is presented inChapter 2. A brief comparison of the different methods of riprap design is made and theneed for large-scale testing and further investigation of the mechanics is established. Adetailed description of the experimental facility and the procedures of the tests is given inChapter 3. Data acquisition is also discussed in Chapter 3. Chapters 4 and 5 present flowobservations and analyzes of data, respectively.Design of riprap for embankment overtopping at a given flow condition consists ofa two-step procedure. The first step involves sizing the riprap. The second step deals withpredicting the minimum depth of the riprap. A fairly accurate prediction of the interstitialvelocity of water through the rock layer is necessary to achieve this goal. Chapter 6 of thisdissertation presents a formula to predict the interstitial velocity of water through aparticular rock layer. Failure of the riprap and the toe treatment are discussed in Chapter 7.The mechanics of riprap failure based on the complete form of the Shield'sparameter is illustrated in Chapter 8. A universal equation is proposed based upon theparticular flow conditions tested under this program. The universal equation is verifiedusing test results from other researchers. The predictive equation for the velocity of waterflowing through a riprap layer, obtained in Chapter 6, is used in Chapter 8 to design thethickness of the riprap layer.This investigation provides a complete set of guidelines for designing riprap forprotection of overtopped embankments. Chapter 9 shows the application of the completeriprap design guidelines through design examples. Chapter 10 summarizes the entire riprapinvestigation providing conclusions, recommendations, and the scope of proposed futureresearch work in this area.3
CHAPTER 2HISTORY OF EMBANKMENT PROTECTION STUDIES2.1 Embankment Overtopping Protection MethodsSince 1983, extensive testing has been conducted in the United States (U.S.), GreatBritain, and the former Soviet Union to develop alternatives for overtopping protection forembankment dams. Protection systems tested include vegetative covers (e.g. grass-linings),roller-compacted concrete and soil cement, precast concre
Chapter 8: Development of the Riprap Design Equation 57 8.1 Development of Design Guidelines 57 8.2 Shield's Parameter 57 8.3 Derivation of the Universal Equation 63 8.4 Sensitivity Analysis 73 8.5 Riprap Layer Thickness and Slope Influence 73 Chapter 9: Design
Erosion Mechanisms & Failure Modes for Bank Riprap Translational Slide Downslope movement of a mass of stones with fault line on a horizontal plane Initiated when channel bed scours and undermines toe of riprap blanket Causes: Bank side slopes too steep Presence of excess hydrostatic pressure Loss of foundation support at the toe of the riprap blanket caused
CONSTRUCTION OF ROAD EMBANKMENTS 1982 ISBN 0 7988 2272 4 TRH 9, UDC 625.731.2, pp 1-42 Pretoria, South Africa, 1982 . Published by the Department of Transport . construction of embankments, therefore deals primarily with the rule-of-thumb-designed embankments. It should, however, be stressed that despite the most conscientious site .
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SIMPLIFIED DESIGN GUIDELINES FOR RIPRAP SUBJECTED TO OVERTOPPING FLOW By Kathleen H. FrizeII1, James F. Ruff2, and Subhendu Mishra3 Abstract Riprap, or some type of rockfill, is commonly used to prevent erosion of the downstream face of dams during rainfall events. Often, it is expected to
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In our CFD modelling and simulation of overtopped hydraulic structures, the upstream boundary is set as a mass-flow inlet, and the downstream and upper boundaries are set as pressure outlets. The boundaries corresponding to the floor and structures are set as no-slip walls. All studied 3D models are symmetrical, so only half of the geometry is .
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