Intensive aDcp Survey of a Gravel-Bed River Confluence

Abstract

An intensive survey of the high angle asymmetric gravel-bed confluence of the Fraser River Main Channel with Minto Side Channel, downstream of a mid-channel island, was conducted using an acoustic Doppler current profiler (aDcp) during the freshet in June, 2008 at a combined discharge of 7 500 m 3/s. The confluence was characterized by a vertical mixing interface, with the Minto high velocity core being advected downwards in the upstream part of the scour zone; weaker helical flow is evident in the main channel flow as it confines the strongly helical left bank flow. Apparent bed velocity generally followed the thalweg, with the highest values near 0.15 m/s occurring at the edges of the scour zone and along the elongated bed step. In plotted spatial distributions, the areas of high primary bed velocity matched areas of high shear velocity and/or strongly upward vertical flow velocity.