Hydrodynamics of hyporheic exchange for complex natural streambed topography, channel geometry, and sediment structure

Laboratory flume experiments were conducted to examine solute exchange with flat gravel beds and gravel beds covered by dune-shaped bedforms. Advective pumping exchange with bedforms was found to be enhanced by inertial effects, including non Darcy flow and turbulent diffusion. Exchange with flat gravel beds also occurred by a combination of advective pore water flow and turbulent diffusion near the streamsubsurface interface. Scaling arguments showed that exchange scaled with the square of the stream Reynolds number in all cases, and the exchange rate was proportional to the square of the characteristic bed sediment size for a wide range of bed sediments.; Salt and dye injection experiments were performed in a laboratory flume with a heterogeneous sediment bed designed as a correlated random hydraulic conductivity field. A new process-based numerical model was developed to simulate the induced pore water flows in the heterogeneous bed and to assess the resulting hyporheic exchange with explicit representation of the bed sediment structure. The model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange. Heterogeneity of bed sediments produced a greater average interfacial water flux, greater spatial variability in the local flux, and a smaller solute penetration depth with a shorter mean hyporheic residence time.; Solute injection experiments were conducted in a large laboratory flume to observe exchange with naturally formed alternate bar and meandering channel morphologies. The observed exchange with alternate bars was found to be consistent with two-dimensional advective pumping process only for deeper stream depths, while the stream and pore water flows became increasingly three-dimensional with shallower stream depths. A new scaling relationship was developed for the experiments with meanders, which showed that the observed exchange was consistent with lateral advective hyporheic exchange induced by the head variation over meander bends.; A semi-coupled approach to modeling hyporheic exchange with arbitrary channel geometries is presented, in which a computational fluid dynamics model is used to solve stream flow over streambeds, which is then coupled to the pore water flow using a process-based numerical exchange model to predict advective hyporheic exchange. Application of this approach to two-dimensional bedforms generally gave good results. (Abstract shortened by UMI.)...