Channel Bed Response to an Increased Sediment Supply

This project presents a suite of field observations, flume measurements, and numerical models investigating the response of channel beds to an increased sediment supply. When the sediment and water supply to a river reach are altered, as might happen from a dam removal, the balance between supply and transport capacity is also changed and the channel will adjust. There is a need to be able to predict these changes that may occur as a result of management actions. Monitoring the Sandy River, Oregon following removal of the Marmot Dam provides measurements of response to a five-fold sediment supply increase. Where supply increase was the greatest, bed slope became steeper and bed topography became less variable. Reaches with less aggradation responded primarily with bed surface fining. During the initial stages of deposition the bed configuration bore little resemblance to the pre-removal configuration, however, after one year, the planform regained the pre-removal pattern. In a recirculating field-scale flume with alternate bar topography, sediment supply was increased by manually augmenting the sediment supply in two steps such that the final bed transported three times as much as the initial bed. The initial and final bed topography and texture were very similar and included long stationary alternate bars. The transient bed was very different, dominated by several scales of shorter wavelength migrating bedforms. Further, the adjustment in topographic and textural patterns continued after the bed slope and mean sediment transport had approached steady state. A one-dimensional (1-D) morphodynamic model predicted steady state slope and transport rates for the flume experiments, but it over-predicted the rate of adjustment. Comparison of 1-D model results with flume observations demonstrated the importance of 2-D adjustments related to the spatial variability of topography and texture. The ensemble of field, flume, and numerical models results demonstrate four bed adjustments - changes to the mean and distribution of bed topography and texture. The adjustments can occur on different time scales, with grain size most likely to respond first. Spatial patterns of topography and texture can adjust to convey an elevated sediment supply without an increase in bed slope. Where slope increases are the dominant response, spatial patterns of topography and texture may moderate the slope effects, introducing systematic errors in one-dimensional model predictions.