The Impact of Biological Bedforms on Near-bed and Subsurface Flow

The pit and the mound are representative biogenic structures in rivers with gravel bed. Past work concerning the bed structures dynamics has generally considered the bed to be impermeable and smooth. Herein, a large-eddy simulation model is used to investigate the near-bed turbulence and hyporheic flow influenced by the pit and the mound on gravel beds. Both bed roughness and permeability are considered. A flume experiment is also used to validate the modelling results. Simulation of one pit shows that the length of downwelling region located at the downstream part of the pit is smaller than the upwelling region located ahead. The intrusion velocity is higher in downwelling region. Simulation of one mound reveals that the flow is forced into the bed upstream of the mound and returns to the boundary layer around the top of the mound. The recirculation zone is limited at the leeside of the mound. With the increasing Reynolds number, the depth of upwelling region at the leeside of the mound increase obviously. These results demonstrate the significant effect of biogenic structures on the flux of water.