Effect of permeability on hyporheic flows across a triangular dune

Despite the traditional separation between the studies of surface water and groundwater flows, for several decades, it has been recognized that rivers and aquifers are strongly connected and that their interaction gives rise to a continuous exchange of water and solutes, which exerts a significant influence on water quality. The analysis of this exchange has led to the identification of a zone, termed the hyporheic zone, where groundwater and stream water mix. The hyporheic zone has hydrodynamic, physiochemical and biotic characteristics different from those of both the river and the subsurface environments and controls the distribution of solutes, colloids, dissolved gases and biogeochemical reactions from ripple to global scales. Thus, this zone affects the distribution of benthic flora and fauna in lakes, oceans, bays and estuaries, as well as hyporheic and riparian organisms in fluvial systems. This chapter presents some results of a numerical study carried out to investigate the influence of the permeability of a porous medium on a coupled free fluid-porous medium flow. Laminar flow in the water column above triangular bedforms and Darcian flow in the underlying permeable sediments were considered. Numerical simulations were carried out under steady-state conditions using Comsol Multiphysics 3.5a in a range of bedforms height-based Reynolds number ReH from 6 to 4442 and for three different values of the permeability of the porous medium. First, numerical results are shown to confirm the close relationship between the characteristics of the separation region in the water column downstream of the bedform crest and those of the hyporheic zone in the porous medium. Second, numerical results highlight the influence of the permeability on the development of the hyporheic zone. Finally, the issues that arise in the application of these results to turbulent flows and to large scales are discussed.