Turbulence Drives Hyporheic Exchange and Fine Particle Deposition across the Entire Riverbed Surface.

Fine particles (FP) in rivers play a key role in controlling rates of hyporheic exchange. FP are filtered within the riverbed, which leads to pore clogging that decreases the sediment permeability locally and redistributes hyporheic flow. Field and laboratory observations have shown that FP deposit everywhere on the riverbed, but the mechanisms responsible for the observed deposition are not entirely clear because currently available models do not fully resolve all relevant scales of coupling between hyporheic flow, particle filtration and riverbed clogging. While current models successfully represent FP exchange, deposition, and feedbacks from advective pumping, they have not been able to reproduce particle deposition in areas where hyporheic flow is exiting the riverbed. We hypothesized that particle deposition in these regions results from turbulence in and around the sediment-water interface, which has not been fully resolved in prior models. To evaluate this hypothesis, we extended an existing numerical model in OpenFOAM that simulates unsteady FP deposition and subsequent loss of connectivity between surface and groundwater. We adapted the model to include hyporheic exchange resulting from turbulent pressure fluctuations at the interface. The turbulent flow field was obtained from Large-Eddy Simulations of the surface water region. We found that turbulence explains the observed deposition patterns that have not been resolved with prior models. With our model, FP deposition is captured around both hyporheic flow entering and exiting regions, i.e., the stoss and lee sides of the riverbed dunes. In contrast, prior models only predicted deposition the hyporheic flow entering region, i.e., around the stoss side. This work demonstrates that turbulent hyporheic exchange in the upper layer of bed sediments is a ubiquitous and important mechanism of fine particle deposition that substantially modifies the hydrogeological properties of the sediment-water interface.