|Abstract or Summary
- The decommissioning of dams, as an approach to restoring longitudinal connectivity and to managing aging infrastructure, presents valuable opportunities for organized study of channel responses to sediment pulses. Experiments with physical and numerical models suggest that rivers process coarse sediment pulses primarily through dispersion. In contrast, translation appears to be a more important process when the sediment pulse consists of finer material, particularly when the grain sizes are finer than is typically present in the river. While the reported physical and numerical experiments have provided valuable insight into expectations channel dynamics, they are largely unconfirmed by field observations. To explore whether dispersion dominates the processing of gravel pulses in natural rivers, we investigated channel changes associated with three barrier removals in Oregon, ranging from very small (Oak Creek culvert, height = 1.5m), small (Brownsville Dam, height = 2.5m), to medium (Savage Rapids Dam, height = 12m) in size. Each trapped coarse sediment initially after construction, after which bedload passed over or through the barriers. Material behind the barriers was finer than the dominant grains downstream at Oak Creek and Savage Rapids, but was coarser than dominant channel grains at Brownsville. We present results from post removal bathymetric and substrate surveys for two years at Brownsville and Oak Creek, and one year at Savage Rapids.
Net deposition and scour, with error estimates, were calculated from surface differencing, both in the reservoir and downstream of the former barriers. We also characterized features of the stored sediment (e.g. ratio of reservoir D50 to averaged surface D50 in downstream reach, sediment volume) and the channel (e.g. Froude number, slope) to place these sites in context with other analyses of sediment pulses. Our results suggest that, at all sites, sediment is processed by both dispersion and translation, though dispersion appears to be the more dominant process. Further, the channels processed sediments rapidly, eroding substantial portions of reservoir material within the first two years following removal. These results suggest that, in the case of small to medium reservoirs filled with non-cohesive material, substantial aggradation will likely be limited to local areas directly downstream of the dam.