Monitoring surface flow velocities at hydraulic barriers in a bedrock canyon

Abstract

<p class="p3"><span class="s2">Several species of Pacific Salmon migrate upstream every year in the Fraser River (British Columbia, Canada) to reach their spawning grounds. A large portion of the fish get delayed in specific sections of the river, where the morphology and the flow create sections with high flow velocity, high turbulence and jumps in bed elevation that constitute hydraulic barriers. Several fish populations migrate along the Fraser River and any barrier for their passage (created by the river morphology, the flow structure or a localized landslide) can severely endanger the survival of such species, while also impacting indigenous communities for which fish is a fundamental element. In the Fraser River those hydraulic barriers have been identified but not yet thoroughly studied. Here we present results from an extensive monitoring campaign conducted in the last 3 years to measure surface flow velocities in previously identified hydraulic barriers. We selected segments of the Fraser Canyon where distributions of surface flow velocities due to specific river morphologies are potentially impacting fish passage. These include areas where series of constriction</span><span class="s2"><span lang="EN-US">-pool-widenings create plunging flows, bedrock step rapids and overfalls. </span>In such areas we collected video of surface flows (with fixed field cameras and drones) at high frequency during the freshet season and obtained surface flow velocity maps using Large Scale Particle Image Velocimetry (LSPIV) for different values of flow discharge.</span> <span class="s2">Using this extensive dataset, we can detect how stable coherent flow structures are for different flow depths and flow discharges and we are also able to identify which areas are most problematic for fish passage (and for which values of flow), helping fish management agencies and public authorities to better protect the survival of vital species in British Columbia.  </span></p>