Livebed Scour At Bendway Weirs and Rock Vanes Formed of Loose Rock

Bendway weirs and rock vanes are transverse (across channel), instream structures primarily used to establish the thalweg alignment of flow and bed-sediment movement along a curved, alluvial channel. Typically built from loose, quarry rock, bendway weirs and rock vanes are intended to direct flow away from a channel’s outer bank by scouring the channel’s alluvial bed so that the thalweg moves and establishes itself in a path along the ends (tips) of theses instream structures. Ironically though, bendway weirs and rock vanes themselves may fail owing to scour. The present study involved series of curved-flume tests to investigate how scour at bendway weirs and rock vanes develops during livebed conditions. It was found necessary to use a hydrograph procedure to simulate discharge increases associated with the rising limb of a hydrograph of flow to avert the false influence that bedforms (dunes) could exert on the performance of bendway weirs and rock vanes. Results from the tests showed that bendway weirs and rock vanes experienced rock dislodgement primarily via contraction scour, which undermined the toe of the tip of these instream structures. Relatively modest scour caused the rock to destabilize and tumble from the tip slope into the scour zone along the channel’s re-establishing thalweg, partially armoring the zone of contraction scour (re-establishing thalweg). As flow depth increased above the mean elevation of the bendway weirs or rock vanes, the flow field at each bendway weir or rock vane changed and velocity across the tip slope increased. These changes caused contraction scour to reduce in maximum depth, but extend further along the curved channel, and commonly shortened the bendway weirs or rock vanes by crossflow further destabilizing rock seated on the tip slope. Results showed that bendway weirs and rock vanes function to establish the thalweg alignment of flow, and reduce the magnitude and alter the orientation of dunes in curved channels with beds of sand. This finding, heretofore undocumented, is important insofar that dunes affect flow resistance and flow distribution in curved channels, and dunes affect bank stability.