Spillway Design Issues
National Dam Safety Program - Technical Seminar No. 20
“Overtopping of Dams”
National Emergency Training Center, February 20, 2013
Tom Hepler, Technical Specialist
Conventional Solutions for
Hydrologic Deficiencies
(Accommodating Larger Floods)
Problem Identification
Hydrologic deficiencies at an existing dam
may 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 Inspection
Act of 1972 authorized the
Corps of Engineers to
inventory (NID) and inspect
non-Federal dams
• Over 9,000 high-hazard
dams inspected 1978-81
• About one-third found to
be unsafe, most due to
inadequate spillway
capacity
Problem Assessment
Flood routings may indicate:
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Dam overtopping potential
Potential structural instability
Spillway chute overtopping potential
Potential for spillway failure due to:
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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 common
failure mode, accounting for 30 percent of the
failures in the U.S. over the last 75 years
• Many older dams may have been designed for floods
that no longer represent a remote flood event
• Many dams can not pass the current Probable
Maximum Flood without overtopping
• Most embankment dams would likely not withstand
sustained overtopping of a foot or more without a
high probability of failure
Embankment Dam Overtopping
South Fork Dam, PA – 1889 (Artist’s Rendition)
Embankment Dam Overtopping
Auburn Cofferdam, CA - 1986
Embankment Dam Overtopping
Taum Sauk Dam, MO - 2005
Concrete Dam Overtopping
Gibson Dam, MT
Structural Instability
Austin Dam, PA
Spillway Chute Overtopping
El Guapo Dam, Venezuela
Spillway Cavitation Damage
Glen Canyon Dam, UT
Spillway Chute Uplift Failure
Big Sandy Dam, WY
Spillway Foundation Erosion
Hyrum Dam, UT
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Spillway Basin Sweepout
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Spillway Channel Toe Erosion
Unlined Spillway Erosion
Spillway Debris Plugging
Kerckhoff Dam, CA
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Spillway Gate Failure
Folsom Dam, CA
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 Raise
New Crest Elevation
Existing Crest Elevation
New Embankment
Existing
Embankment
Embankment
New Crest Elevation
Retaining Structure
Existing Crest Elevation
Existing
Embankment
Retaining Structure
Lake Sherburne Dam, MT
Embankment Dam Raise
Lake Sherburne Dam, MT
Reinforced Earth Raise
• Lower cost ($3.6 million)
• Much less material
• Single construction
season
• No outlet works
modification
• No downstream impact to
environmental area
Embankment Dam Raise
Horseshoe Dam, AZ
Concrete Dam Raise
Theodore Roosevelt Dam, AZ
Bartlett Dam, AZ
25
Structural Stability Modifications
Stewart Mountain Dam, AZ
Post-tensioned Anchors
Santa Cruz Dam, NM
RCC Buttress
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Spillway Modifications
• Increase discharge capacity
Theodore Roosevelt Dam
New Gated Spillways, AZ
Stewart Mountain Dam Gated Auxiliary
Spillway (foreground), augmenting
discharge capacity of Gated Service
Spillway (background)
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Spillway Modifications
• Increase discharge capacity (continued)
Bartlett Dam Fuseplug Spillway, AZ
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Horseshoe Dam Fuseplug Spillway, AZ
New Waddell Dam Fuseplug Spillway, AZ
Spillway Modifications
• Increase discharge capacity (continued)
Ute Dam Labyrinth Spillway, NM
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Spillway Modifications
• Improve spillway performance
McPhee Dam Chute Air Ramp, CO
Hoover Dam Tunnel Air Ramp, AZ-NV
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Conventional Solutions
Feasibility Designs
• Site investigations (geology, topography)
• Design layouts – combinations of dam raise
and increased spillway capacity
• Cost estimates and construction schedules
Environmental 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 solution
that 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 estimate
risks to downstream population
• Consider reliability of flood release features
and freeboard needed for dam
What if conventional solutions
are found to be impractical or
cost prohibitive?
Overtopping Protection Options
Most applicable for following situations:
• Very low annual probability of operation
– Generally 1 percent or lower (>100-year flood)
• Physical or environmental constraints on
other 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 Dam
Overtopping Protection
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Coolidge Dam, AZ
Embankment Dam
Overtopping Protection
Blue Ridge Parkway Dams, NC
Overtopping Protection Benefits
• Increasingly being viewed as a viable
alternative to more conventional solutions,
while maintaining hydraulic conditions at dam
• Potential economic advantages
• Wider variety of materials and methods being
developed and tested for field use
• Becoming more accepted by regulatory
agencies as experience grows
• But be aware of potential limitations, failure
modes, and risks!
Questions?