A numerical investigation into the importance of bed permeability on determining flow structures over river dunes

Abstract

Although 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.

Effects of Turbulent Hyporheic Mixing on Reach‐Scale Transport

“…Experiments over a greater range of streamflows, channel geometries, and streambed sediment types are needed to generalize current model formulations and verify their applicability in natural streams. These efforts will help clarify the importance of turbulent interfacial mixing compared to other known transport mechanisms, such as advective pumping through bedforms (Blois et al, ; Packman et al, ; Sinha et al, ). Further, tracer injections combined with detailed stream and hyporheic measurements will enable identification of the lower limit of , which corresponds to the conditions at which streamwise hyporheic velocities can be considered negligible and classical mobile‐immobile transport models can be used (Boano et al, ; Haggerty et al, ; Schumer et al, ).…”

Does the permeability of gravel river beds affect near‐bed hydrodynamics?

“…() and Sinha et al . (). During sediment transport, heterogeneous particle sizes, shapes and angularity promote the development of complex bed structures comprising particle imbrication, clustering and other complex packing arrangements which have important consequences for permeability (Cooper and Tait, ; Haynes et al ., ).…”

“…Sinha et al . () examined these differences further by producing numerical simulations of these experiments. Their simulations revealed the size of the recirculation zone in the leeside of a dune was smaller over a permeable bed because of jets of flow leaving the hyporheic zone and entering the overlying flow.…”

“…The permeability of river beds is an important control on hyporheic flow (Sawyer and Cardenas, 2009) and the movement of fine sediment (Chen et al, 2010) and solutes (Marion et al, 2008) into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near-bed hydrodynamics, despite interfacial transport usually being driven by pore water advection caused by turbulent coupling with the overlying flow (Packman and Bencala, 2000;Cardenas and Wilson, 2007).…”

Modeling the Effects of Turbulence on Hyporheic Exchange and Local‐to‐Global Nutrient Processing in Streams

“…Stream turbulence can influence vertical exchange in at least two ways, by controlling (1) the rate nutrients are delivered from the bulk stream to the streambed (or vice versa) and (2) mixing and transport of nutrients within the streambed (Figure a). In both cases, the vertical transport of mass (and momentum) is facilitated by eddies in the stream that are spatially coherent; that is, the positive or negative velocity fluctuations are spatially correlated (Grant & Marusic, ; Sinha et al, ; Vollmer et al, ; Zhong et al, ). At the sediment‐water interface, coherent turbulence manifests as sweep and ejection events, in which water parcels in the bulk stream with nutrient concentration C B (units of moles per cubic meter) sweep down to the interface, while water parcels near the interface with interfacial nutrient concentration C S (units of moles per cubic meter) are ejected into the bulk stream.…”