Abstract
AbstractAlthough permeable sediments dominate the majority of natural environments past work concerning bed form dynamics has considered the bed to be impermeable, and has generally neglected flow between the hyporheic zone and boundary layer. Herein, we present results detailing numerically modeled flow which allow the effects of bed permeability on bed form dynamics to be assessed. Simulation of an isolated impermeable bed form over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside, in the form of returning jets that generate horseshoe‐shaped vortices. The returning flow significantly influences the leeside flow, modifying the separation zone, lifting the shear layer adjoining the separation zone away from the bed. Simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside. With two dunes placed in series the flow over the downstream dune is influenced by the developing boundary layer on the leeside of the upstream dune. For the permeable bed case, the upwelling flow lifts the separated flow from the bed, modifies the shear layer through the coalescence with vortices generated, and causes the shear layer to undulate rather than be parallel to the bed. These results demonstrate the significant effect that bed permeability has on the flow over bed forms that may be critical in affecting the flux of water and nutrients.
Highlights
Dunes are widespread in alluvial channels and generated from sediment that range in size from fine sands to gravels [Dinehart, 1992; Seminara, 1995; Best, 1996; Carling, 1999; Kleinhans, 2001, 2002; Carling et al, 2005; Best, 2005; Bradley et al, 2013]
3.2 Permeable dune over a porous bed The simulation of a permeable dune on a permeable bed (Figures 4 & 5) generally shows that all components of the flow are lower in magnitude than for the impermeable dune (Table 2). The reason for this decrease in the magnitude of the peak flow velocity is attributed to the observation that the permeable dune confines the flow and increases the velocity, the porous nature of the dune allows the passage of some flow through it, which subsequently reduces the peak velocity in the boundary layer above the dune
The flow is hydrodynamically forced into the bed on the stoss side of the dune by local velocity gradients that are generated by flow around, or over, the bedform and that result in momentum transfer into the bed [Boano et al, 2014]
Summary
Dunes are widespread in alluvial channels and generated from sediment that range in size from fine sands to gravels [Dinehart, 1992; Seminara, 1995; Best, 1996; Carling, 1999; Kleinhans, 2001, 2002; Carling et al, 2005; Best, 2005; Bradley et al, 2013]. The role of bedforms in both inducing and driving hyporheic flow has been recognized previously [Stonedahl et al, 2012, 2013] and investigated through the use of numerical models [Boano et al, 2007; Cardenas and Wilson, 2007 a&b; Packman et al, 2004; Hester et al, 2013] In these previous simulations neither turbulence in the Brinkman layer nor the advective (hydrostatic) pumping is correctly predicted from the pressure field generated in the boundary layer. The coupled approach presented allows both hydrostatically- and hydrodynamically- induced hyporheic flow to be modelled simultaneously, providing insight into the nature of turbulence-driven hyporheic flow
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