hoover dam – colorado river. reasons for dams flood control 39,000 dams worldwide higher than 15 m...
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Hoover Dam – Colorado River
Reasons for Dams
• Flood Control
39,000 dams worldwide higher than 15 m (ICOLD, 1988)
Reasons for Dams
• Flood Control
• Irrigation
Reasons for Dams
• Flood Control
• Irrigation
• Power Generation
Reasons for Dams
• Flood Control
• Irrigation
• Power Generation
• Recreation
Reasons for Dams
• Flood Control
• Irrigation
• Power Generation
• Recreation
• Navigation
Impacts of Dams
- Cross section of a graded stream and tributary in equilibrium
- Dam creates reservoir for hydropower and flood control
- What happens?
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
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
Change in Flow Below Trinity Dam, CA
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)
Reduction in Sediment SupplyHoover Dam – Colorado River
Fro
m W
illia
ms
and
Wol
man
(19
84)
Reduction in Sediment Supply
Gavins Pt. Dam – Missouri River
From Williams and Wolman (1984)
From Williams and Wolman (1984)
Channel Narrowing
Dow
nst
ream
of
Bla
ck B
utte
Dam
, C
A
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
How Mitigate for These ChannelAdjustments and Habitat Losses?
Dam Mitigation
• Dam removal – 500 dams removed in U.S.
CNN – 7/19/2000:
White House gives thumbs down on removal of Pacific
Northwest dams
-Sediment accumulates behind dam limiting its water retention capacity
Impacts of Dam Removal
-Dam is removed-What happens?
-Erosion of sediment accumulated in reservoir is redeposited downstream
Impacts of Dam Removal
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
Issues Surrounding Removal
• Sediment management
• Release of contaminants
• Loss of benefits of dams
• Loss of established (nonnative) ecosystem
• Historic preservation
• Potential to damage infrastructure
Dam Mitigation
• Dam removal
• Provide fish passage – fish ladders
Dam Mitigation
• Dam removal
• Provide fish passage – fish ladders
• Sediment management downstream
Rhi
ne R
iver
- G
erm
any
Adding Gravel Below Dam
Dam Mitigation
• Dam removal
• Sediment management downstream
• Provide fish passage – fish ladders
• Artificial flood releases
Unaltered River and Floodplain
Levees
Breached Levees
Levelling Levees
Breached Levees
Overtopping Levees
Setback Levees
Buyouts
S. Fork Nooksack River , WA