From Surface to Substrate: Impact of Floating Vegetation Root-Canopies on Turbulence and Hydrodynamics in Streams

Abstract:

Floating vegetation with unanchored roots have been increasingly used to improve water quality in streams and for ecosystem restoration. They affect mixing and transport in open channels by altering the hydrodynamics of the flow. We investigate these effects through laboratory models that mimic such vegetation patches. We used particle image velocimetry (PIV) to characterize flows through surrogate models of floating vegetation patches, made of vinyl-coated steel nails and polystyrene foam boards, and measured drag on those patches using submersible load cells. We analyzed two relevant sections– first one focused on the developed flow within the patch, and the second one at the downstream end of the patches. High-resolution 2D velocity fields reveal turbulence structures of different temporal and spatial scales within and outside the canopies in the experimental domain. We develop simplified theoretical models to predict relevant flow parameters (mean velocities, Reynolds shear stresses, and turbulent kinetic energy) based on the characteristics of the floating patches, which allows us to assess the impact of turbulent events on sediment dynamics. Data of streamwise velocity, Reynolds shear stress and turbulent kinetic energy show the development of a shear layer near the tip of root canopies. Spectral analysis shows dominant frequencies in the turbulence generated by the root canopies. Mixing layer and boundary layer structures near the bed seemingly drive turbulence features in these types of flows. Based on our experimental data, a mixing layer approach is more effective for hydrodynamic and sediment transport predictions. Such an approach seems more accurate to account for increases in turbulence generated by interactions with unanchored rigid root canopies.