EFFECTS OF A PEBBLE CLUSTER ON THE TURBULENT STRUCTURE OF A DEPTH-LIMITED FLOW IN A GRAVEL-BED RIVER

Pebble clusters are important morphological features of gravel-bed riv ers and have a significant effect on the spatial and temporal response of the flow. This paper presents original results from measurements o f velocity taken at a high spatial and temporal resolution in a turbul ent flow upstream and downstream from a pebble cluster. Three electrom agnetic current meters (0.013 m in diameter), mounted on a wading rod, were deployed upstream and downstream of an isolated pebble cluster a long 31 vertical profiles that contained between 7 and 13 points in a 5-m long section. The high sampling resolutions in a natural environme nt permitted characterization of the complex dynamics of the turbulent flow in the vicinity of the cluster. The description of these dynamic s is based on the interpretation of contour maps of several turbulence statistics, including moments of velocity distributions, Reynolds she ar stress, quadrant analysis and autocorrelation function. Downstream from the cluster, flow is decelerated, the intensity of turbulence inc reases, and zones of strong vertical motions are present. The superimp osition of the maps allowed us to decompose the field of turbulent flo w around a pebble cluster into distinct regions. In a downstream direc tion from the cluster, we observed zones of flow acceleration, recircu lation, vortex shedding, flow reattachment, fluid upwelling, and flow recovering. Results indicate the existence of highly dynamic boundarie s between zones where strong intermittent events are generated. They a lso highlight the difference between shedding and upwelling motions of the flow. (C) 1998 Elsevier Science B.V. All rights reserved.