Three‐dimensional flow structure around small‐scale bedforms in a simulated gravel‐bed environment

Abstract

Pebble clusters are common small-scale morphological features in gravel-bed rivers, occupying as much as 10 per cent of the bed surface. Important links exist between the presence of pebble clusters and the development of flow structures. These links are poorly understood at the three-dimensional level. Particularly neglected has been the effect of clusters on the lateral flow characteristics. A laboratory study was conducted using a hydraulic flume, within which simulated pebble clusters were superimposed onto a plane bed of gravel material. High-resolution three-dimensional flow data were collected above the bed at two different flow depths using an acoustic Doppler velocimeter. The results present evidence of the importance of lateral flow in the development of turbulent flow structure. Narrow regions of high lateral and downstream turbulence intensity exist to both sides of clusters and in a three-dimensional separation zone in their lee. This may indicate the presence of horseshoe-type vortical structures analogous to those identified in less hydraulically rough environments. However, it is likely that these structures are more complicated given the mutual interference of the surrounding medium. The lateral flow was also identified as a key component in the upwelling identified by other authors in the lee of pebble clusters. The results of the vertical flow analysis confirm the hypothesis that six regions with distinct vertical flow characteristics exist above clusters: flow acceleration up the stoss-side of the cluster; recirculation behind the cluster in the wake region; vortex shedding from the pebble crest and shear layer; flow reattachment downstream of the cluster; upwelling of flow downstream of the point of reattachment; and recovery of flow.

Occurrence of cluster microforms in mountain rivers

“…Clusters have been observed to have an impact on the local dynamics of gravel bed rivers by introducing perturbations from a non-clustered state in bed stability Wittenberg and Newson, 2005), downstream particle movement (Billi, 1988), bedload transport rates (Strom et al, 2004), overall flow resistance (Hassan and Reid, 1990;, and local flow field characteristics (Buffin-Bélanger and Roy, 1998;Lawless and Robert, 2001;Strom et al, 2007b). In addition to studying the local dynamic environment created by clusters, it is also of interest to understand the conditions that are conducive to cluster formation within a stream reach and/or stream-network; such knowledge would be analogous to our understanding of the conditions that induce particular types of sand bedforms (e.g., Simons and Richardson, 1961).…”

Geostatistical analysis of the effects of stage and roughness on reach-scale spatial patterns of velocity and turbulence intensity

“…These macroturbulent structures play an important role in sediment transport (e.g., Shvidchenko and Pender, 2001;Wu and Yang, 2004) and persist in the presence of roughness transitions (Robert et al, 1996), protruding clasts (Kirkbride, 1993;Smart, 1994;Buffin-Belanger and Roy, 1998;Lawless and Robert, 2001b) scales of topographic variability (Clifford et al, 1992;Clifford, 1996;Lawless and Robert, 2001a).…”

“…Widely used electromagnetic current meters provide only two components of velocity, most often the streamwise and vertical, and field data sets typically consist of only a few profile measurements along downstream transects (e.g., Robert et al, 1996). While some researchers have addressed the lateral dimension (e.g., Lawless and Robert, 2001b;Roy et al, 2004), their measurements have not spanned the entire width of natural channels. In general, the difficulty of acquiring detailed measurements of flow velocity and bed elevation under field conditions has limited the spatial extent of previous studies, and our knowledge of the variability and spatial pattern of velocity and turbulence intensity at the reach scale remains incomplete.…”

Use of Ecohydraulic-Based Mesohabitat Classification and Fish Species Traits for Stream Restoration Design

“…Compared with biologists, hydraulic engineers view the fluvial environment through the application of computational fluid dynamic (CFD) models, routing water mass and momentum among a modeler defined numerical mesh space [99]. Turbulence modeled at a micro-scale are a function of Reynolds stresses of fluid motion and boundary layer resistance are scaled to topography and in stream structures [254][255][256][257][258][259]. Discipline differences are best exemplified by the use of the term "turbulence", where biologists refer to it as "wavy" surface waters as criteria for mesohabitat classification and hydraulic engineers apply CFD at the micro-scale noted above [58,63].…”