HR: 0830h
AN: H21D-0870    [PDF]
TI: The Influence of Ground Water on Stream Restoration Following Dam Removal
AU: * Constantz, J
EM: jconstan@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS496, Menlo Park, CA 94025
AU: Essaid, H
EM: hiessaid@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS496, Menlo Park, CA 94025
AB: With the exception of ground-water seepage beneath dams and the resulting impact on structural failure, there is a void of work directly examining the downstream impacts of dams from the perspective of ground-water/surface-water interaction. This work considers alterations in an alluvial basin caused by: (1) erection of a dam upstream, (2) followed by ground-water pumping in the basin, and (3) dam removal. Theoretical results predict that when dams are removed in developed ground-water basins, downstream baseflows may be greatly reduced relative to natural baseflows, as a result of lower water table elevations in the developed basin relative to the natural setting. Without the dam as a `safety valve' providing extra streamflow during low-flow seasons, there is a real potential for ephemeral conditions downstream of the previous location of the dam as the dry season progresses. MODFLOW simulations are used to test these theoretical results, by quantifying the impact of dam removal on downstream surface water and ground water. The simulations incorporate an improved stream-aquifer interaction and streamflow routing package to represent movement of water in the vadose zone between the stream and a lowered water table. An idealized MODFLOW model with the new stream package has been constructed, which extends from the upland bedrock headwaters of a stream to the downstream sediment-filled basin. The model domain is 180 km long, 15 km wide, and 2.2 km deep, including a stream with a potential length of 180 km. In the upper reaches, the stream is divided into a north, south, and main stem with their confluence upstream of a dam situated in the domain above the bedrock/basin contact. Horizontal discretization is 1000 m in the direction parallel to the stream, 200 to 600 m perpendicular to the stream, and vertical discretization is 100 m. This modeling framework affords the opportunity to examine a variety of cases with and without the presence of an upstream dam. Initial MODFLOW results are consistent with the theoretical results regarding the impact of dam removal in developed basins, indicating that dam removal is unlikely to regenerate natural streamflows in developed ground-water basins.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1860 Runoff and streamflow
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2003 Fall Meeting