Turbulent Hyporheic Exchange in Permeable Sediments

Monday, October 5, 2015
Kevin R Roche, Northwestern University, Civil and Environmental Engineering, Evanston, IL, United States, Antoine F Aubeneau, University of Notre Dame, Notre Dame, IN, United States and Aaron Ian Packman, Northwestern University, Evanston, IL, United States
Abstract:
Solute delivery from the water column into a streambed strongly influences transport in rivers and groundwaters. Current hydrological models simplify surface-subsurface (hyporheic) exchange by treating surface and groundwater flow fields as explicitly separate domains. This approach justifies use of the uncoupled continuous time random walk (CTRW) model framework, given the significant timescale separation between free-stream velocities and subsurface Darcy velocities. In very permeable sediments, however, subsurface flows are strongly coupled to the free stream via (1) a slip velocity at the sediment-water interface (SWI) and (2) propagation of turbulent eddies into the streambed. More information about the nature of this exchange process is needed before an appropriate model choice can be considered.

We characterized turbulent solute exchange between surface and porewaters in a 2.5-m recirculating flume. The channel was packed with 3.8-cm PVC spheres to form a coarse gravel bed, with a total depth of 21 cm. We implanted microsensors onto an array of spheres to measure in situsalt concentrations within the streambed. Water was recirculated in the channel, and concentrated salt solution was continuously injected within the sediment bed upstream of the sensor array.

We observed solute exchange increased with free-stream Reynolds number and decreased with depth in the sediment bed. Mass of injected solute remaining in the bed decreased rapidly in all cases, with only 10-30% of mass recovered 50 cm downstream of the injection point at Re = 25,000. We observed high-frequency (1-10 Hz) concentration fluctuations at bed depths of at least 4.75 cm and sporadic low-frequency fluctuations at depths of 12.5 cm. Spectral analysis revealed increased filtering of high frequencies with depth. These results demonstrate that free-stream turbulence impacts hyporheic mixing deep into permeable streambeds, and mixing is strongly influenced by the coupled surface-subsurface flow field.

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