GEOLOGY 351 IMPACTS OF DAMS AND CHANNELS

Text Reading: Pages 155 - 165

TERMINOLOGY: aggradation, armoring, “hungry” water, knickpoint, mitigation

URBANIZATION OF AREAS – SEDIMENT LOADING

· During construction, sediment loads to streams are high leading to stream aggradation

· Once urbanization complete, erosion rates stabilize to normal or less than normal due to stable land surface and collection of runoff in sewers

· Some areas have construction codes to control erosion and sediment runoff (Lake Tahoe) but not always enforced

URBANIZATION – STORM-WATER RUNOFF

· Dust, dirt and pollutants collect on impervious urban surfaces

o Fallout from vehicle exhaust

o Street litter

o Animal feces

o Fertilizers

o Petroleum residues from tire wear

o è during rainfall, pollutants runoff into sewers and local streams, oceans

o shock pollution in local streams kills aquatic life

URBANIZATION – EFFECT ON STREAM DISCHARGE

· lower infiltration capacity due to impervious surfaces

· rapid runoff on smooth surfaces

· eliminate depression storage – excess water into sewers and culverts

· increased flows due to imported water use for irrigation (overwatered lawns)

· effect on stream hydrograph

o decreased lag time of peak discharge from end of rainfall

o increase in the peak flow

· mitigation measures – attempt to increase lag time and dampen the peak discharge of runoff

o retention ponds

o settling basins

o debris basins

· will also alter the flood-frequency curve (increase the probability of low and intermediate flows)

LOS ANGELES RIVER

· concrete-lined ditch

· from flood-frequency curve of 1942 to 1980, 1-yr flow (100 % chance of occurring in 1 year) was 5,000 cubic feet per second (cfs)

· adjusted flood-frequency curve for 1971-1980, 1-yr flow is 30,000 cfs

CHANNELIZATION

· attempt to increase the amount of discharge (volume of water per time) the river channel can handle

· want channel to carry more water through quickly to prevent increase in river stage and flooding

· Discharge = Water Velocity x Cross-Sectional Area

· Increase discharge by increasing cross-sectional area

o Dredging deeper or wider channel

· Increase discharge by increasing water velocity

o Decrease channel roughness (remove riparian vegetation, line with concrete)

o Increase channel slope by straightening channel

PROBLEMS WITH DREDGING

· River banks become steep and unstable

· Stabilizing vegetation removed

· è scouring and slumping of channel walls

· Try to minimize by

o creating trapezoidal-shaped channel (slopes less steep)

o line walls with cobble- to boulder-sized material (riprap)

o line walls with concrete or spray with grout

· During low flows, enlarged channel size can result in low water velocities, decreased competence and increased sediment deposition è channel fills in with sediment and has less area or capacity for the high flow è flooding

PROBLEMS WITH CHANNEL STRAIGHTENING

· river will attempt to restore its original meandering or braided pattern and will erode sides

· steeper slope of straight channel will be a knickpoint in the longitudinal profile

· attempts of the river to erode the knickpoint (to establish a new equilibrium profile) can result in scour upstream

MISSISSIPPI RIVER

· Leveed river with numerous flood control systems

· For many years, Mississippi has been attempting to cut through a narrow neck of land and shift its course to that of the Atchafalaya River

· If happened, the lower 300 miles of the Mississippi River would be abandoned (including New Orleans and Baton Rouge and their shipping routes)

· Dam-like structure built where channel is attempting to shift

· Floods of 1970s weakened the control structure

· Another (expensive!) auxillary dam finished in mid-80s

DAMS

California

o 181 federal dams and reservoirs

o 1,200 nonfederal dams and reservoirs

o reservoir capacity capable of collecting 60% of average annual runoff

o water captured and rerouted to

§ 80% agriculture

§ 20% urban (1/2 is residential)

Dam and reservoir produces a knickpoint or break in the longitudinal profile of a river

o è response of river is to try to smooth out the knickpoint

o wants to deposit material upstream and downstream

o wants to erode headward through knickpoint (dam structure)

Attempts to erode headward through knickpoint

o Most dam structures engineered to withstand erosive forces

§ Except San Francisquito Dam (LA)

§ Near disaster at Baldwin Hills Dam

Attempts to deposit material upstream

o As river waters empty into the still waters of reservoir, water velocity, competence and capacity all decrease and result in sediment deposition

o Dams release water but very little sediment downstream

o Trap efficiency (percentage of sediment trapped behind dam) generally between 90 and 100%

o Generally all the bed load (coarse material) is trapped behind dam

o Causes infilling of reservoir with sediment

o Decreases the water capacity of the reservoir and limits its useful lifetime

o Can impact hydroelectric dams if sediment infills near dam itself

o Many dams not infilling in California as have multiple dams upstream which limit the sediment load

What To Do With Infilled Reservoirs?

o Some are mined for aggregate (sand)

o Others which are infilled with mud cannot be mined

o If dismantle dam and allow large sediment load to move downstream è negative impacts on river

Attempts to Deposit Sediment Downstream

o To reestablish equilbrium, river would attempt to deposit sediment downstream

o Does not occur due to the trapping of sediment behind the dam

o What actually occurs in many cases is stream has an increased tendency to erode due to its lack of sediment load (“hungry” water)

o è channel downcutting and erosion downstream – can lower downstream groundwater levels

o è removal of fine sediments in downstream channel resulting in an upper layer of coarse sediments which limit erosion (armoring of the channel)

Effects of Changes in Water Discharge

o water storage reservoirs – can dry up rivers downstream

o hydroelectric and irrigation dams – will release water during peak power useage and water useage times

§ cause extreme variations in flow

DECLINE OF SALMON AND OTHER FISH

· dams prevent the upstream migration

· fish ladders are not as effective as hoped

· dams and channels alter the preferred channel bottom for spawning (fine materials dumped on top of preferred gravels and sands)

· decreased water quality

· introduction of nonnative species

GLEN CANYON DAM, ARIZONA

· 1963 Dam is located upstream of the Grand Canyon and created Lake Powell reservoir

· Usual summertime floods not experienced downstream due to the dam

· Trapped sediment behind dam

· Sand deposited on bottom of streambed and beaches eroded from the side of the river (used by rafters for overnight camping)

· 1996 Controlled-Flood Experiment

o For 6 days, discharge from the dam was increased from 8,000 cfs to 45,000 cfs (at peak)

o Sand caught in deep pools in the bottom of the main channel were scoured and carried in suspension downstream

o Suspended load deposited along the river banks downstream after flood (new sand beaches!)

o After flood, erosion of beaches continued

o è would need to periodically flood to maintain beaches

o Downstream of Lava Falls, boulders were radiotagged to monitor transport

NEW MELONES DAM, CALIFORNIA

· Would flood Stanislaus River Canyon

· Would inundate rapids

· Description by Marc Reisner, author of Cadillac Desert: “I sat on the bank and watched. One after another, the big waves flattened out, their booming stilled, their splashing stopped … then they disappeared under gurgling little whirlpools, and where there had been rapids minutes earlier the river went dead calm”

· 1979 Mark Dubois, environmentalist and rafter, chained himself to boulder at an unknown site in the river. Rising water level would drown him.

· After 1 week, an agreement was reached to keep water at an intermediate level in the “immediate” future

· Reservoir ultimately filled in a high runoff year (1982-1983)

ASWAN DAM, NILE RIVER

· Nile River has low flows except during annual summer flooding

· Dam built in 1960s for water supply, flood control and power

· 98% of Nile’s sediment load is suspended sediment (fine grained material)

· Prior to dam, 125 million metric tons of sediment moved downstream each year

· Trap efficiency of dam is 98%

· Only 2.5 million metric tons of sediment move downstream now

· Within a few hundred years, reservoir will be filled

· Farmers downstream must now add artificial fertilizers and soil amendments to increase nutrient level in soil

· Downstream scouring of “hungry” water has destroyed 3 old barrier dams and more than 500 bridges since 1953

· Soil salinity increases due to loss of water flushing through soils

· Nile delta in the Mediterranean Sea is undergoing shoreline erosion

THREE GORGES DAM, YANGTZE RIVER

· Floods along the Yangtze River have killed more than 300,000 people in the 20^th century

· Three Gorges Dam is currently being built

About 20,000 people are working nearly round the clock to complete the 1.24-mile-wide structure by 2009.
The lake that will form behind Three Gorges Dam will stretch for about 350 miles -- the distance from San Francisco to Los Angeles.

· When completed (2009), will be largest and most expensive dam ever built

· Will provide power and flood control

· Will submerge more than 450 towns and displace 1.5 million people (including farmers who will need new farmland)

· Will flood scenic and historic region and impact many species

· Potential for disaster if dam breaks!

· Weight of water in large reservoirs has been known to trigger earthquakes

· Runoff pollutants will accumulate in reservoir