Transient Responses of Gravel Bars to Increases in Sediment Supply - Field & Flume

Bedforms in a gravel-bed river respond to a combination of water discharge, the rate and size of sediment supply, and valley-scale geometry. This study investigates bar response to an increase in sediment supply. In a large flume (2.75 m wide) with a plane bed of mixed sand and gravel (gravel D50 = 10 mm; 15% sand), alternate bars were formed by inducing a slight perturbation in the flow at the upstream end. After the bars and sediment flux reached a steady state, sediment supply was increased (from 45 kg/min to 70 kg/min). After the bed and sediment flux reached steady state, sediment supply was again increased (from 70 kg/min to 140 kg/min). Throughout the experiment high-frequency (1 Hz) measurements of the sediment flux, as well as moderate frequency (every 90 minutes) measurements of the bed topography were made. As the channel increased transport capacity to match the increased sediment supply, the initial bed adjustment was an increase in slope with near uniform deposition in the cross-stream direction. The bed then evolved to a steady-state configuration in which the locations and dimensions of the bars and pools were very similar to the pre-augmentation condition. During the adjustment process, the cross-stream relief initially decreased, the bar wavelengths decreased, and the bar celerity increased. The evolution from the lower-relief interim state to the post-augmentation steady state was reminiscent of initial bar development from a plane bed. A similar sequence of bed adjustment was observed on the Sandy River, Oregon, following a large increase in sediment flux due to the 2007 Marmot Dam removal. Measurements of bedform evolution immediately downstream of the dam show a transition from a long high-relief lateral bar along the right bank, to a lower-relief multiple short wavelength mid-channel bars, finally back to a long high-relief lateral bar along the right bank, albeit 4-5 meters higher than the original. Previous work on bedform response to increases in sediment supply (from the results of landslides, dam removals, or hydraulic mining) have documented both increases and decreases in topographic relief. Both field and laboratory observations suggest that the variation in relief may be a transient phase as the bed adjusts to a new equilibrium configuration.