Effects of a pebble cluster on the turbulent structure of a depth-limited flow in a gravel-bed river

Abstract

Pebble clusters are important morphological features of gravel-bed rivers and have a significant effect on the spatial and temporal response of the flow. This paper presents original results from measurements of velocity taken at a high spatial and temporal resolution in a turbulent flow upstream and downstream from a pebble cluster. Three electromagnetic current meters (0.013 m in diameter), mounted on a wading rod, were deployed upstream and downstream of an isolated pebble cluster along 31 vertical profiles that contained between 7 and 13 points in a 5-m long section. The high sampling resolutions in a natural environment permitted characterization of the complex dynamics of the turbulent flow in the vicinity of the cluster. The description of these dynamics is based on the interpretation of contour maps of several turbulence statistics, including moments of velocity distributions, Reynolds shear stress, quadrant analysis and autocorrelation function. Downstream from the cluster, flow is decelerated, the intensity of turbulence increases, and zones of strong vertical motions are present. The superimposition of the maps allowed us to decompose the field of turbulent flow around a pebble cluster into distinct regions. In a downstream direction from the cluster, we observed zones of flow acceleration, recirculation, vortex shedding, flow reattachment, fluid upwelling, and flow recovering. Results indicate the existence of highly dynamic boundaries between zones where strong intermittent events are generated. They also highlight the difference between shedding and upwelling motions of the flow.