Hyporheic exchange with gravel beds: Basic hydrodynamic interactions and bedform-induced advective flows

Stream-subsurface exchange processes are important because of their role in controlling the transport of contaminants and ecologically relevant substances in streams. Laboratory flume experiments were conducted to examine solute exchange with gravel streambeds. Two morphologies were studied: flat beds and beds covered by dune-shaped bedforms. High rates of exchange were observed with flat beds under a wide range of stream flow conditions, indicating that there was considerable turbulent coupling of stream and pore water flows. The presence of bedforms produced additional exchange under all flow conditions. The exchange with bedforms could be represented well by considering solute flux caused by bedform-induced advective pumping. Pumping exchange was enhanced by inertial effects, including non-Darcy flow and turbulent diffusion. For the flat bed case, dye injections showed that exchange also occurred by a combination of advective pore water flow and turbulent diffusion near the stream-subsurface interface. The relative effects of advective and diffusive transport processes could not be separated due to the complex nature of the induced flows in the gravel bed. However, exchange was found to scale with the square of the stream Reynolds number in all cases. Comparison of these results with those obtained with coarser and finer sediments demonstrated that the exchange rate is also proportional to the square of the characteristic bed sediment size. These scaling relationships can be used to,improve interpretation of solute transport observed in natural rivers.