Intensive aDcp Survey of a Gravel-Bed River Confluence

An intensive survey of the confluence of the Fraser River Main Channel with Minto Side Channel was conducted using an acoustic Doppler current profiler (aDcp) during the freshet in June, 2008. This stretch of the Fraser River, in British Columbia, Canada, is characterized as a wandering river with mid-channel islands and an active bi-modal sand/gravel bed, with the surface d50 of the sediment varying between 40 mm and 75 mm. In the study reach Minto Island divides the river flow into two channels: the Main Channel and Minto Side Channel. During the measurements the average width and discharge of the Main Channel (Minto Side Channel) were 500 m and 5 000 m3/s (300 m and 2 500 m3/s). Spatial distributions of velocity, shear velocity, bed morphology, and apparent bed velocity were investigated in both channels and in their confluence using transects spaced 30 m apart, with measurements taken over 4 consecutive days of the falling limb of the freshet. Flow velocity reached 3.5 m/s in the channel thalweg. Apparent bed velocity was calculated as the difference between the boat velocity measured by bottom track pulse, biased by sediment movement, and the boat velocity measured by GPS. Average apparent bed velocity at the confluence was 4.1 cm/s, and upstream in the secondary channel, Minto Side Channel, it was 2.7 cm/s. Bed velocities were highest in the thalweg bend pools, and in a very deep pool located at the confluence, but upstream of the thalweg. Large boils were observed at the deep pool, indicating highly turbulent flow, and erosion of the bank flanking the deep pool was significant. It is surmised that the Main Channel thalweg confines the Minto flow, causing the Minto flow to recirculate into the outer bank at the confluence, resulting in rapid bank recession.