Hyporheic exchange induced by channel-spanning obstacles in a coarse, highly-permeable laboratory streambed

Abstract:

Knowledge of flow over and through a porous streambed is essential to understanding hyporheic exchange in coarse gravel-bed rivers, where turbulence in the stream flow can penetrate significantly into the streambed. To study the turbulent momentum exchange between a stream and gravel streambed, laboratory experiments were conducted in a 2.5 m long refractive-index matching (RIM) laboratory flume. A flat gravel bed was simulated using cubically packed acrylic spheres (D_s = 1.27 cm) with a refractive index, RI = 1.495, that matched that of the NaI solution in the flume. Thus, optical access to the pore spaces could be gained, and the flow field from the near-bed and into the pore spaces could be measured with particle image velocimetry (PIV).

Dense 2-D velocity vector fields were measured for two bed configurations: (1) a flat, porous, bed composed of three layers of spheres and (2) a flat bed with a cylinder (D_c = 1.27 cm) placed atop it, to induce hyporheic exchange in the manner of a channel-spanning large woody debris. The flow over and through the bed produced by the cylinder is found to be dramaticallydifferent from that associated with a flat bed. The mean velocity field produced by the cylinder exhibited strong flow downwelling in the pore space immediately upstream of the cylinder and upwelling several pore spaces downstream. In particular, the shear layer separating from the cylinder remained parallel to the bed from the point of flow separation to the edge of the field of view, instead of reattaching several grain diameters downstream. The cylinder also promoted increased vertical momentum exchange as suggested by turbulent kinetic energy maps.