Author(s): Colin D. Rennie; Michael Church
Linked Author(s):
Keywords: Dcp; Bedload; Shear stress; Spatial distribution; Wandering gravel-bed river
Abstract: Spatial distributions of depth averaged water velocity, shear velocity, and apparent bedload velocity are mapped for the first time in a long reach of the wandering gravel-bed Fraser River. Spatially intensive acoustic Doppler current profiler (aDcp) measurements were taken over two days on the falling limb of freshet. The flow was near the threshold for bed material particle motion. The width of the river main channel was 500 m, and transects were spaced an average of 110 m apart. Shear velocity was estimated from the log-law using 11 ping moving averages of the vertical distribution of streamwise velocity. Apparent bedload velocity was measured as the bias in bottom tracking due to near-bed sediment motion, determined using real-time kinematic differential global positioning system data for actual boat velocity. The spatial distributions were interpolated from the asynoptic point data using kriging. The resulting maps are remarkably coherent, with maximum depth average velocity, shear stress, and apparent bedload velocity following the thalweg, with largest values in channel bends at zones of flow convergence where the thalweg accelerates into the bank. However, the highest apparent bedload velocity was observed outside the thalweg in a deep pool downstream of a rapidly eroding cut-bank. Erosion at this site was related to a flow confluence with relatively low shear but highly turbulent three-dimensional separated flow.
Year: 2007