Sediment transport and channel adjustments associated with dam removal
Dam removal has recently become one of the most controversial issues among the environmental society community in the United States. Release of reservoir sediment is the main uncertainty associated with this issue. This study has extensively documented the short-term changes in channel geometry, bed level profile, size distribution of substrates, and their relations with the sediment transport at the reach scale after a low-head dam removal. Before the complete removal, the structure was breached to de-water the reservoir. The suspended sediment concentration (SSC) and discharge downstream of the dam showed nearly no change during the breach. In contrast, the complete removal of the structure produced a sudden increase in downstream discharge and SSC. However, the amount of increase was insignificant compared to the annual rainfall event. The SSC within 10 months after the removal showed no significant difference with that of SSC before the removal. No bank erosion was evident at both upstream and downstream of the dam; while net deposition occurred downstream and net erosion occurred in the reservoir. Deposition and scouring were evident in the reservoir with overall decrease by 30% in bed slope. Deposition measurements showed up to a 20 cm increase in bed level downstream of the dam. Downstream bed materials size became at least 40% finer for post-removal conditions than for pre-removal conditions as a result of deposition of finer materials. High resolution Digital Elevation Models (DEMs) of the river bathymetry were generated from differential GPS surveys. The volume of deposition and erosion was computed from differencing the DEM for post-removal from the DEM for pre-removal. Compared with the observations, the transport rate estimated from the DEMs overestimated the transport rate within an order of magnitude. A 1-D hydrodynamic model, calibrated with the observations showed promising simulation results on the dynamics of dam removal and sediment transport. The major features described by the model were: 1) the magnitude and timing of the flood wave during the dam removal; and 2) the bed level adjustments after dam removal. Sensitivity analysis showed that the model was strongly dependent on the size distribution of the reservoir substrates.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:dam removal channel adjustment sediment transport
Date of Publication:01/01/2005