Reservoir Sedimentation

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RESERVOIR AND LAKE TRAP EFFICIENCY S. W. Trimble1, B. Wilson2 1 Department of Geography, University of California, Los Angeles, Los Angeles, USA 2 Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, MN, USA

Introduction All streams transport sediment, and some of this sediment will be deposited in the relatively quieter waters of lakes and reservoirs (Morris and Fan, 1997). This accumulated sediment is detrimental to the lake or reservoir because it displaces the storage of water and often reduces the surface area. It is especially problematic in reservoirs designed for hydropower production: as useful storage volume is lost, increasingly only the flow of the river is available, and this may be insufficient during low-flow periods. In areas of extremely high sediment yields, smaller lakes and reservoirs may fill completely, but this is relatively rare. For some regions, reservoir sedimentation is no longer seen as a major problem. Indeed, in the USA, reservoir sedimentation surveys are now rarely performed. Basic processes As the sediment-laden stream flows into the relatively quiescent pool, the coarser particles are more rapidly deposited, while the finer particles are transported farther into the reservoir depending on the velocity and dynamics of the water. In some cases, especially where there is a subcrest outlet at the dam, a sediment-laden flow termed a turbidity current, made denser by the sediment or by thermal conditions, can transport fine sediment into the deeper portions of the pool and, on occasion, even through the sub-crest dam outlet. Indeed, some dams are designed and operated with this phenomenon in mind.

Sediment, and particularly coarse material, may also accumulate above normal pool level or crest level upstream of the reservoir or lake. This happens when the inflowing sediment forms a delta out into the pool. Because a stream must maintain a slope or gradient to furnish energy to flow, the slope must be extended upstream of the pool by aggrading or building up its bed with sediment. This process not only raises the elevation of the river but can also permanently flood the areas adjacent to the stream. Perhaps the most famous example is upstream of Elephant Butte Dam, completed in 1916 on the Rio Grande River in New Mexico. Located 2 km upstream of the reservoir pool and several meters above it, the small town of San Marcial was partially flooded and eventually evacuated by the 1930s (Eakin and Brown, 1939). This above-crest sediment creates a wedge and may extend many kilometers upstream. The trapping of sediment by reservoirs and lakes can create a sediment deficiency downstream. That is, stream flow deprived of its normal sediment load becomes “hungry” and tends to erode available sediment in streambeds downstream of the dam. In some cases, streams have cut vertically or “degraded” several meters over reaches up to several kilometers downstream of dams, often destabilizing banks and leaving water intakes stranded above water level (Williams and Wolman