hoover dam – colorado river. reasons for dams flood control 39,000 dams worldwide higher than 15 m...
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![Page 1: Hoover Dam – Colorado River. Reasons for Dams Flood Control 39,000 dams worldwide higher than 15 m (ICOLD, 1988)](https://reader036.vdocuments.site/reader036/viewer/2022062320/56649d4f5503460f94a2ec0b/html5/thumbnails/1.jpg)
Hoover Dam – Colorado River
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Reasons for Dams
• Flood Control
39,000 dams worldwide higher than 15 m (ICOLD, 1988)
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Reasons for Dams
• Flood Control
• Irrigation
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Reasons for Dams
• Flood Control
• Irrigation
• Power Generation
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Reasons for Dams
• Flood Control
• Irrigation
• Power Generation
• Recreation
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Reasons for Dams
• Flood Control
• Irrigation
• Power Generation
• Recreation
• Navigation
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Impacts of Dams
- Cross section of a graded stream and tributary in equilibrium
- Dam creates reservoir for hydropower and flood control
- What happens?
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If sediment supply from tributary is low:- Deposition in reservoir and further upstream due to rise in base level and lowered slope
- Preferential erosion downstream due to decreased load
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If sediment supply from tributary is high:- Deposition in reservoir and further upstream due to rise in base level
- Deposition downstream due to decreased capacity of peak discharges – insufficient to remove tributary inputs
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Change in Flow Below Trinity Dam, CA
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Average Annual Peak Discharge
0
1000
2000
3000
4000
5000
6000
7000
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41
Dam
Dis
char
ge
(cm
s)
Pre-dam Post-damData from Williams and Wolman (1984)
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Reduction in Sediment SupplyHoover Dam – Colorado River
Fro
m W
illia
ms
and
Wol
man
(19
84)
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Reduction in Sediment Supply
Gavins Pt. Dam – Missouri River
From Williams and Wolman (1984)
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From Williams and Wolman (1984)
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Channel Narrowing
Dow
nst
ream
of
Bla
ck B
utte
Dam
, C
A
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Habitat Impacts of Dams• Lower flows
• Migration barriers – block fish passage
• New ecosystems develop in reservoirs behind dams – out compete native species
• Channel incision downstream – armoring
• Beach erosion
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How Mitigate for These ChannelAdjustments and Habitat Losses?
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Dam Mitigation
• Dam removal – 500 dams removed in U.S.
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CNN – 7/19/2000:
White House gives thumbs down on removal of Pacific
Northwest dams
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-Sediment accumulates behind dam limiting its water retention capacity
Impacts of Dam Removal
-Dam is removed-What happens?
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-Erosion of sediment accumulated in reservoir is redeposited downstream
Impacts of Dam Removal
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Habitat Impacts of Aggradation
• Shallower Flows– Increased water temperatures
• Pools Filled– Loss of cover and resting habitat
• Increased Fines– Lower egg survival and fry emergence– Loss of juvenile cover
• Destabilizes Banks– Bank erosion
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Issues Surrounding Removal
• Sediment management
• Release of contaminants
• Loss of benefits of dams
• Loss of established (nonnative) ecosystem
• Historic preservation
• Potential to damage infrastructure
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Dam Mitigation
• Dam removal
• Provide fish passage – fish ladders
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Dam Mitigation
• Dam removal
• Provide fish passage – fish ladders
• Sediment management downstream
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Rhi
ne R
iver
- G
erm
any
Adding Gravel Below Dam
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Dam Mitigation
• Dam removal
• Sediment management downstream
• Provide fish passage – fish ladders
• Artificial flood releases
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Unaltered River and Floodplain
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Levees
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Breached Levees
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Levelling Levees
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Breached Levees
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Overtopping Levees
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Setback Levees
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Buyouts
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S. Fork Nooksack River , WA