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

Pebble clusters are common small-scale morphological features in gravel-bed rivers, occupying as much as 10 per cent of the bed surface. Important lin ks exist between the presence of pebble clusters and the development of flo w structures. These links are poorly understood at the three-dimensional le vel. Particularly neglected has been the effect of clusters on the lateral flow characteristics. A laboratory study was conducted using a hydraulic fl ume, within which simulated pebble clusters were superimposed onto a plane bed of gravel material. High-resolution three-dimensional flow data were co llected above the bed at two different flow depths using an acoustic Dopple r velocimeter. The results present evidence of the importance of lateral fl ow in the development of turbulent flow structure. Narrow regions of high l ateral 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 ident ified 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 th e cluster in the wake region; vortex shedding from the pebble crest and she ar layer; flow reattachment downstream of the cluster; upwelling of flow do wnstream of the point of reattachment; and recovery of flow. Copyright (C) 2001 John Wiley & Sons, Ltd.