Monitoring river restoration structures through velocimetry: a field-based study on the use of submerged vanes for debris mitigation along a bridge

Accumulations of large wood along bridge piers can cause localized scour of the streambed, which can compromise the structural integrity of the bridge and increases the likelihood of catastrophic failure. They also suppress a variety of downstream ecological processes that require wood to occur. In-stream flow deflecting structures are a form of debris mitigation that work by deflecting debris-carrying flows away from piers so that they may pass unobstructed through the bridge. The efficacy of these structures has previously been demonstrated primarily through flume experiments; however, there is little field-based testing of these structures, and descriptions of the hydrodynamic and geomorphological interactions these structures induce in field settings are scarce. We investigated the use of an Acoustic Doppler Current Profiler (ADCP) and Large-Scale Particle Image Velocimetry (LSPIV) to monitor these interactions following the construction of flow-deflecting vanes in a large, urban river in Middletown, Ohio. We observed how the locations of greatest scour transitioned away from the banks and piers, suggesting that these structures may influence sedimentary dynamics in rivers. Additionally, we found that the vanes increased flow velocities in the spans between piers and decreased velocities at the piers, which suggests proper deflection from these structures. However, we noted that the vanes had a weaker influence on flow patterns during high-flow events, which suggests that the functioning of these vanes is highly dependent on channel stage.