Upstream Hydrologic Effects of Dam Installation and Removal

Stanford University, Stanford CA

Investigators

Abstract

0438749 Loague The damming of a river causes profound changes to the hydrologic and geomorphologic regime of the river. Dam removal, a practice becoming increasingly common as our population of dams grows older, is similarly expected to cause major changes to the manner in which water and sediment move through the system. Dam construction causes a rise in base level for the upstream catchment, resulting in altered streamflow depths and velocities, higher water tables, and sediment deposition. In some cases the presence of a dam brings about changes to the ecohydrology of the catchment, including the creation of wetland ecosystems in the dam's immediate zone of influence. Downstream of the dam channel morphology is affected by the reduction in sediment load and the moderation of peak flows. This study proposes to use a comprehensive hydrologic model to simulate the effects of dams and dam removal on near surface hydrology and geomorphology. The model to be used, InHM, is well suited to the task as it simulates 3D variably-saturated subsurface fluid flow and 2D surface water flow using a fully-coupled approach. This makes it ideal for studying groundwater- surface water interactions, such as those caused by dams. As part of the research plan InHM is to be augmented with a physically based multiple species sediment transport algorithm, allowing simulation of upland erosion (both by rainsplash and overland flow), channel transport, and reservoir deposition, as well as erosion of reservoir sediments following dam removal. Driven by the fully-coupled hydraulics in the existing InHM the sediment transport algorithm will provide more accurate sediment flux estimates than models with simpler hydrologic underpinnings. The transport algorithm will take into account the affects of armoring and cohesion caused by, for example, vegetation. The expanded model will be tested at the R-5 catchment in Oklahoma, an experimental site chosen due to the wealth of long-term / spatially-variable data, including rainfall and climate records, water and sediment discharge data, and soil-water content data. The ability of InHM to simulate long-term hydrologic response with sediment transport will be proven at the R-5 site. InHM will then be used in an application mode to simulate the small watershed impounded by Searsville Dam in the coastal range of California. Searsville Dam, constructed in 1891, has suffered the fate of many old dams in that its reservoir is now almost completely filled with sediment. Consequently dam removal is being considered for this dam. This study aims to address questions related to the impact of Searsville Dam and its potential removal on the hydrologic regime, including the sustainability of a wetland ecosystem that has formed near the lake and sediment erosion and transport downstream. Using existing historical data on topography, land use, soils, geology, and climate, augmented with additional field data on sediment transport flux rates, channel hydrogeomorphologic parameters, and near-surface hydraulic properties, we will undertake a modeling effort to quantitatively assess the impacts of alternate dam management strategies, including an analysis of prediction uncertainty caused by model assumptions and data error. Additional simulations of generic hypothetical catchments with dams will broaden the scope of the study and provide a sound footing for assessing further research needs. The results from this study will be presented both in site- specific and broad- based terms so that they can reach a wide variety of interested parties including the general public.