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Practical aspects of dam break analysis
Louis C Hattingh
Hattingh Anderson Associates CC
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Dam break analysis
• It is a model
• You need to understand what you model & have an idea of the answers that you expect
• Very little known inputs & lots of assumptions
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Baldwin Hills Reservoir
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What is important?
• Mode of failure (including mechanism) – Breach width
– Time
• Assumptions – Reservoir water level
– Floods or sunny day
• Level of accuracy
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Failure modes
• Internal erosion
• Structural
• Hydrologic
• Hydraulic
• Seismic
• Operational
• Other
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Failure modes • Internal erosion • Structural
– Concrete gravity dams failures – Concrete arch dam failures – Concrete buttress dam failures
• Hydrologic – Overtopping
• Hydraulic – Failure due to erosion of rock – Failure due to overtopping of spillway walls and stilling basins – Stagnation Pressure Failure of Spillway Chutes – Cavitation Damage Induced Failure of Spillways
• Seismic – Failure of embankment dams during to seismic loads – Seismic failure of retaining walls
• Operational • Other
– Landslide failures and incidents – Trunnion Friction Radial Gate Failure – Drum Gate Failures
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El Guapo Dam, Venezuela
• Built 1975 to 1980 • No proper hydrologic studies - based on
similar basin • Spillway system
– Original uncontrolled ogee with downstream chute
– Tunnel spillway added after chute wall overtopping during construction
• Failure in 1999
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El Guapo Dam, Venezuela
Is not a flip bucket but a hydraulic jump basin
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Flow outside the spillway chute
Walls Began to overflow at 1:15 am on 12/16/1999
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Water level behind dam decreased at 9:00 am on 12/16/1999
12/1/2014 ENTRO Dam Safety Training
Module Louis
Hattingh
10
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Water level rose again – erosion had undercut basin, chute and spillway weir at 4:00 pm on 12/16/1999
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Approach channel collapsed at 5:00 pm on 12/16/1999
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Flood wave reached 1st village at 6:00 pm on 12/16/1999 – reservoir lowered 30 meters in 40 minutes
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Zoeknog Dam
• Weathered granites
• No provision for dispersive material during construction
• Incorrect blanket drain position
• No attention to piezometer warning during first filling
• Failed on 25 January 1993
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Gleno Dam, Italy
• 50 m high multiple concrete arch dam 213 m long
• Masonry gravity plug built in deep central valley gorge (use lime mortar instead of cement mortar)
• Original concrete gravity • Changed to multiple arch but not
approved
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Gleno Dam, Italy
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Gleno Dam, Italy
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Gleno Dam, Italy
• 1923: – Failure of one of the buttresses leading to
multiple arch failure – 356 fatalities
• Change in design • Iffy concrete quality • Inappropriate material
– Lime mortar for masonry section • Settlement of masonry plug?
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Malpasset Dam, France
• Thin arch dam 66.1 m high and 6.7 m thick at base
• Foundation = gneiss
• No foundation grouting or drainage features
• Designed by Andre Coyne
• Completed 1954
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Malpasset Dam, France
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Malpasset Dam, France
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Malpasset Dam
• 1959: – Failure
– 421 fatalities • View of left
abutment and thrust block following the failure
• Thrust block moved about 1 m into abutment and slightly d/s
• FAILURE MODE?
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• U/S dipping fault and D/S dipping foliation shear formed lt. abut block
• Arch thrust in direction of foliation decreased permeability
• Tensile stress at u/s face opened foliation shear
• Nearly full uplift developed on foliation
• Block slid out on fault (phi = 30o) and dam went with it
After P. Londe
Malpasset Dam, France
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Kariba Dam, Zambia/Zimbabwe
• 128 m high concrete arch • Built between 1956 & 1959 • World’s largest artificial lake • Gated spillway sill = 33 m below crest • Spillway use created 80 m deep eroded
plunge pool over 20 years • Geological feature (discontinuity) in the
river section that was not picked up during planning and design
• Plans are abreast to deal with the issue
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Kariba Dam, Zambia/Zimbabwe
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Taum Sauk Dam, USA • Concrete-faced
earthfill “ring-dike” structure
• Upper reservoir of pumped-storage project
• Water routinely stored on 3 m high parapet
• NO SPILLWAY!!!
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Taum Sauk Dam, USA
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Taum Sauk Dam, USA • Membrane liner installed in 2004 • Reservoir level instrumentation could not
be reinstalled properly due to liner warranty issues
• Instruments were loose and not reading reservoir level properly
• Resetting of reservoir sensors did not account for settlement of embankment
• Alarms wired so high level and high-high level sensors needed to trigger for alarm
• Over-pumping was not detected and dam overtopped and failed
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Taum Sauk Dam, USA
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Taum Sauk Dam, USA
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0.667
0.1000.100
0.0000.000
0.1000.100
0.667
0.831
0.658
0.6900.6200.580
0.845
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0.900
0.000 20.000 40.000 60.000 80.000 100.000 120.000
Re
lative
le
ve
l (m
)
Chainage (m)
Spillway
Right bank of embankment
Left bank of embankment
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Vaiont Dam, Italy • 265 m high concrete arch dam • Completed in 1960 • Left side reservoir foundation = steep
slopes in bedded limestone with clay interbeds
• 1 month after completion & after heavy rain = first landslide = 700 000 m3 & 2 m wave
• Exploratory adits, piezometers & level of reservoir adjusted to limit slide movement
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Vaiont Dam, Italy
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Vaiont Dam, Italy
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Vaiont Dam, Italy • 1963
– Massive slide of 267 million m3
– 100 m high over dam wall – 2 600 fatalities – Arch survived
• Dam abondoned • Low strength clay layers between
limestone beds • Reservoir geology not fully
understood
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Have you considered all relevant failure modes?
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Practical example: Gariep Dam will be
destroyed should either Katse or Mohale Dam fail
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Katse Dam
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Failure mode?
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How to check
• Classical dam break/pull the plug
• Maximum discharge for sudden failure – Q = v x A
– v = 2/3 x (g x y)0.5
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Mohale Dam
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Failure mode?
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Uncertainty for both width & time
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300
250
200
40%
50%
60%
70%
80%
90%
100%
Breach width (m)
Time of breach
(minutes)
Mohale dam break @ Mohale
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300
250
200
40%
50%
60%
70%
80%
90%
100%
Breach width (m)
Time of breach
(minutes)
Mohale dam break @ Gariep
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The bottom line
• Katse Dam: – Peak flow @ Katse ≈ 500 000 m3/s
– Peak flow @ upper end of Gariep ≈ 28 000 m3/s
– Peak flow routed through Gariep ≈ 5 400 m3/s
• Gariep SEF: – 26 600 m3/s (unrouted)
– 16 000 m3/s (routed)
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The bottom line
• Mohale Dam: – Peak flow @ Mohale ≈ 320 000 m3/s
– Peak flow @ upper end of Gariep ≈ 18 000 m3/s
– Peak flow routed through Gariep ≈ 2 000 m3/s
• Gariep SEF: – 26 600 m3/s (unrouted)
– 16 000 m3/s (routed)
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300
250
200
40%
50%
60%
70%
80%
90%
100%
Breach width (m)
Time of breach
(minutes)
Mohale dam break @ Mohale
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300
250
200
40%
50%
60%
70%
80%
90%
100%
Breach width (m)
Time of breach
(minutes)
Mohale dam break @ Gariep
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What about 60 minutes?
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300
250
200
30%
40%
50%
60%
70%
80%
90%
100%
Breach width (m)
Time of breach (minutes)
Mohale dam break @ Mohale
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300
250
200
30%
40%
50%
60%
70%
80%
90%
100%
Breach width (m)
Time of breach (minutes)
Mohale dam break @ Gariep
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Water levels & exceedance probabilities
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1 325
1 330
1 335
1 340
1 345
1 350
0.1
1
10
100
1000
10000
71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10 11
Re
lative
wa
ter
leve
l (m
)
Un
rou
ted
in
flo
ws (
m3/s
)
Time
Inflow (m3/s)
Water level
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0
10
20
30
40
50
60
70
80
90
100
1 330 1 332 1 334 1 336 1 338 1 340 1 342 1 344 1 346 1 348 1 350
rese
rvo
ir e
xce
ed
an
ce
%
Relative water level (m)
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Concluding remarks
• Understand your problem – failure modes
• Consider required accuracy level
• Knowledge of the uncertainties - use sensitivity analysis
• Check the model outputs
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Acknowledgements
• Dr Chris Oosthuizen
• Dam Safety Surveillance – present & past
• Gregg Scott – formerly USBR
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Good luck & enjoy