Spillway Design Issues - Association of State Dam Safety Officials

Spillway Design IssuesNational Dam Safety Program - Technical Seminar No. 20“Overtopping of Dams”National Emergency Training Center, February 20, 2013Tom Hepler, Technical Specialist

Conventional Solutions forHydrologic Deficiencies(Accommodating Larger Floods)

Problem IdentificationHydrologic deficiencies at an existing dammay be identified as a result of:• Regular dam safety program– Mandated by regulators (e.g. FERC, State agencies)– Comprehensive review of dam design and performance• Change in design criteria or guidelines– New design flood (change in hydrology or hazard class)• Availability of new information– Damage or poor performance at low discharges

Inspection of Non-Federal Dams• National Dam InspectionAct of 1972 authorized theCorps of Engineers toinventory (NID) and inspectnon-Federal dams• Over 9,000 high-hazarddams inspected 1978-81• About one-third found tobe unsafe, most due toinadequate spillwaycapacity

Problem AssessmentFlood routings may indicate:• Dam overtopping potential• Potential structural instability• Spillway chute overtopping potential• Potential for spillway failure due to:– Cavitation damage (due to high flow velocities)– Uplift failure due to stagnation pressures– Foundation failure from scour erosion– Tunnel pressurization (due to higher reservoir head)

Dam Overtopping Issues• Failure of dams due to overtopping is a commonfailure mode, accounting for 30 percent of thefailures in the U.S. over the last 75 years• Many older dams may have been designed for floodsthat no longer represent a remote flood event• Many dams can not pass the current ProbableMaximum Flood without overtopping• Most embankment dams would likely not withstandsustained overtopping of a foot or more without ahigh probability of failure

Embankment Dam OvertoppingSouth Fork Dam, PA – 1889 (Artist’s Rendition)

Embankment Dam OvertoppingAuburn Cofferdam, CA - 1986

Embankment Dam OvertoppingTaum Sauk Dam, MO - 2005

Concrete Dam OvertoppingGibson Dam, MT

Structural InstabilityAustin Dam, PA

Spillway Chute OvertoppingEl Guapo Dam, Venezuela

Spillway Cavitation DamageGlen Canyon Dam, UT

Spillway Chute Uplift FailureBig Sandy Dam, WY

Spillway Foundation ErosionHyrum Dam, UT15

Spillway Basin Sweepout16

Spillway Channel Toe Erosion

Unlined Spillway Erosion

Spillway Debris PluggingKerckhoff Dam, CA19

Folsom Dam, CASpillway Gate Failure

Conventional Solutions• Increase flood storage by:– Raising dam crest– Reducing reservoir level before flood season(non-structural alternative)• Increase spillway capacity by:– Modifying existing spillway– Constructing new spillway• Improve spillway performance

Embankment Dam RaiseExisting Crest ElevationNew Crest ElevationNew EmbankmentExistingEmbankmentEmbankmentRetaining StructureExisting Crest ElevationNew Crest ElevationExistingEmbankmentRetaining StructureLake Sherburne Dam, MT

Embankment Dam RaiseLake Sherburne Dam, MTReinforced Earth Raise• Lower cost ($3.6 million)• Much less material• Single constructionseason• No outlet worksmodification• No downstream impact toenvironmental area

Embankment Dam RaiseHorseshoe Dam, AZ

Concrete Dam RaiseTheodore Roosevelt Dam, AZBartlett Dam, AZ25

Structural Stability ModificationsStewart Mountain Dam, AZPost-tensioned AnchorsSanta Cruz Dam, NMRCC Buttress26

Spillway Modifications• Increase discharge capacityTheodore Roosevelt DamNew Gated Spillways, AZStewart Mountain Dam Gated AuxiliarySpillway (foreground), augmentingdischarge capacity of Gated ServiceSpillway (background)27

Spillway Modifications• Increase discharge capacity (continued)Bartlett Dam Fuseplug Spillway, AZNew Waddell Dam Fuseplug Spillway, AZ28Horseshoe Dam Fuseplug Spillway, AZ

Spillway Modifications• Increase discharge capacity (continued)Ute Dam Labyrinth Spillway, NM29

Spillway Modifications• Improve spillway performanceMcPhee Dam Chute Air Ramp, COHoover Dam Tunnel Air Ramp, AZ-NV30

Conventional SolutionsFeasibility Designs• Site investigations (geology, topography)• Design layouts – combinations of dam raiseand increased spillway capacity• Cost estimates and construction schedulesEnvironmental Analyses (normally EIS)• Evaluate range of reasonable alternatives• Identify environmental impacts and mitigation• Select preferred alternative or proposed action

General Design Guidelines• Provide a structural/non-structural solutionthat safely accommodates design flood• Consider discharge capacity of spillway(s)and increased flood loading on dam• Establish flood operating criteria• Identify potential failure modes and estimaterisks to downstream population• Consider reliability of flood release featuresand freeboard needed for dam

What if conventional solutionsare found to be impractical orcost prohibitive?

Overtopping Protection OptionsMost applicable for following situations:• Very low annual probability of operation– Generally 1 percent or lower (>100-year flood)• Physical or environmental constraints onother more conventional solutions– e.g. Deep canyon site with no reservoir saddle• Prohibitive cost of other alternatives– Expensive spillway tunnel or lake-tap alternatives– Massive open-cut channel excavation alternatives– Extensive dikes needed for crest raise

Concrete DamOvertopping Protection35Coolidge Dam, AZ

Embankment DamOvertopping ProtectionBlue Ridge Parkway Dams, NC

Overtopping Protection Benefits• Increasingly being viewed as a viablealternative to more conventional solutions,while maintaining hydraulic conditions at dam• Potential economic advantages• Wider variety of materials and methods beingdeveloped and tested for field use• Becoming more accepted by regulatoryagencies as experience grows• But be aware of potential limitations, failuremodes, and risks!

Questions?