Towards understanding sediment transport and deposition in a heavily sediment-impacted reservoir and on its upstream alluvial surface [electronic resource]
Keywords: Civil and Environmental Engineering.

The issues surrounding the management of sediments accumulating in reservoirs remain an ongoing challenge to scientists and practitioners. All reservoirs trap inflowing sediment to some degree, leading to a suite of problems including loss of water storage capacity, impairment of navigability, loss of downstream flood-control benefits, increased flooding upstream because of streambed aggradation in the deltaic region, and sediment entrainment in hydropower equipment. The loss of storage capacity, in addition to reducing performance for water supply, hydropower and recreation, increases the pressure to develop replacement storage, especially in the face of population growth and climate change. One aspect of reservoir sedimentation that has received relatively little attention in the literature is the effect of progressive reservoir filling over time on the processes influencing sediment deposition. For example, as reservoirs become more and more filled with sediment, an increasing amount of deposition can occur above the normal-pool elevation, as a consequence of shallow overbank flows during high flow periods spreading across the alluvial surface of the sediment deposit. Much of the literature on reservoir sedimentation focuses on deposition in a reservoir with substantial remaining capacity. The evolution of the processes as the reservoir fills is of interest as well, but little attention is paid to this in the existing literature, despite possible implications for ecological succession, reservoir management, and dam removal. This dissertation employs a combination of analysis of previously collected field measurements, new field measurements using synthetic turf sediment traps, and numerical modeling to study the magnitude and spatial distribution of sediment deposition in and around Searsville Lake. I show that the alluvial surface upstream of Searsville Lake is spatially complex, and while two simple analytical models (a one-dimensional advection model and a turbulent diffusion model) have significant predictive value in determining the amount of sediment deposition, small-scale effects also play a substantial role. Additionally, simulations based on the Searsville area show results qualitatively similar to the field data in terms of the spatial complexity of erosion and deposition, as well as the relationship between hydrograph volume and magnitude of alluvial surface deposition. Sensitivity analysis shows that changes in alluvial surface roughness discretization and channel bathymetry influence erosional and depositional patterns on the alluvial surface, with varying impacts on the overall magnitude. The ordering of hydrograph events has a significant effect on the locations on the alluvial surface which experience the largest bed level change, and a moderate effect on the overall amount of sediment deposited on the alluvial surface. Finally, simulations predict deposition will preferentially occur at topographic lows, suggesting that over long times, if preferential flowpaths sample the full alluvial surface area, a relatively flat sediment surface is likely to develop. This speculation is consistent with the observed behavior at Searsville. A full modeling study of the sediment processes around Searsville--or any similar environment--featuring a full calibration and verification of the model, would further add to the understanding of the behavior of this complex environment. Long-term simulations accounting for the full range of channel processes, including migration, would be of particular interest, allowing for testing of the theory that the long term behavior of the system will be to move towards a more uniform sediment surface.