Effect of bed permeability and hyporheic flow on turbulent flow over bed forms

Abstract

This paper uses particle imaging velocimetry to provide the first measurements detailing the flow field over a porous bed in the presence of bed forms. The results demonstrate that flow downstream of coarse-grained bed forms on permeable beds is fundamentally different to that over impermeable beds. Most significantly, the leeside flow separation cell is greatly modified by jets of fluid emerging from the subsurface, such that reattachment of the separated flow does not occur and the Reynolds stresses bounding the separation zone are substantially lessened. These results shed new light on the underlying flow physics and advance our understanding of both ecological and geomorphological processes associated with permeable bed forms. Water fluxes at the bed interface are critically important for biogeochemical cycling in all rivers, yet mass and momentum exchanges across the bed interface are not routinely incorporated into flow models. Our observations suggest that ignoring such exchange processes in coarse-grained rivers may overlook important implications. These new results also provide insight to explain the distinctive morphology of coarse-grained bed forms, the production of openwork textures in gravels, and the absence of ripples in coarse sands, all of which have implications for modeling and prediction of sediment entrainment and flow resistance.

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

“…However coarse‐grained fluvial deposits rarely have such a simple, regular geometry as examined by Blois et al . () and Sinha et al . ().…”

“…Blois et al . () examined the flow over a coarse‐grained 2‐D dune and revealed key differences depending on whether the dune sat on an impermeable or permeable bed (composed of cubically packed spheres). Using flume experiments they found the reattachment region to either move further downstream or to be absent on a permeable bed, plus the area of upwelling over the dune was larger, and the shear layer had a lower Reynolds stress.…”

Bottomset Architecture Formed In the Troughs of Dunes and Unit Bars

“…Higher pressures on the bedform's stoss drives water into the bed, most of which flows towards and exits the bed through the lee face and lee-side trough, where pressure is lower (Elliott and Brooks 1997b). Hyporheic flow influences bedform characteristics (Harrison and Clayton 1970), flow separation downstream, and, consequently, subsequent deposition in the trough (Blois et al 2014). The size, strength, and position of the flowseparation zone is different if there is flow through a gravel bedform.…”

“…The size, strength, and position of the flowseparation zone is different if there is flow through a gravel bedform. Vertical flow exiting the bed in a trough can displace the reattachment zone (Blois et al 2014, their Figs. 1 to 4), reducing the possibility of sediment reworking and hindering suspended-sediment deposition.…”

The importance and challenge of hyporheic mixing

“…At smaller spatial scales (e.g., millimeters to centimeters) and shorter time scales (e.g., milliseconds to seconds), turbulence, coherent flows, and non‐Darcy flow (Figure 1c) contribute to mixing in coarse sediments within a few grain diameters of the sediment‐water interface [ Nagaoka and Ohgaki , ; Packman et al ., ; Higashino and Stefan , ; Cardenas and Jiang , ; Menichino and Hester , ; Chandler et al ., ]. These processes, particularly ejection and sweep events near the bed, are enhanced by bed roughness elements of various sizes even in the absence of larger bed forms [ Boano et al ., ; Scalo et al ., ; Blois et al ., ]. These transient hydraulics induce flow reversals in the hyporheic zone that can facilitate mixing [ Blois et al ., ], which we discuss in more detail in section 4.…”