Impacts of Channel-Spanning Large Wood Jams on Surface and Hyporheic Flows

Large wood jams (LW jams), such as those formed by fallen trees, can significantly alter the transport of water, solutes, organic matter, and sediment in river corridors. The presence of LW jams alters the surface flow by increasing flow resistance and generating backwater rise. In addition, LW jams generate pressure gradients at the water-sediment interface. Such gradients drive flow into the sediment bed and then back to the surface water, which is often referred to as hyporheic flow. While the impacts of LW jams on surface and hyporheic flows have been recognized, there is a lack of quantitative models to predict such impacts, especially on hyporheic flows. One major reason is a lack of direct visualization and measurement of hyporheic flow driven by LW jams. In this study, we quantified the impacts of LW jams on the surface and hyporheic flows by directly measuring the flows around a translucent log jam placed above a transparent sediment bed made from hydrogel beads. First, we measured the 3D distribution of surface flow velocity before and after the log jam using particle image velocimetry (PIV). Second, we quantified the velocity of the LW jam-induced hyporheic flow by tracking the trajectories of fluorescent dyes in the sediment bed. Finally, we derived a physical-based model and validated the model using laboratory results.