Author(s): M. J. Franca; J. L. C. Carvalho; A. M. Ricardo; R. M. L. Ferreira
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Keywords: No Keywords
Abstract: Boulders in rivers provide sheltering conditions to such species as Salmonidae. The knowledge of the flow structure around these is important to understand the dynamics of salmonidae communities. Although flow around obstacles is well documented, studies on the effects of sediment deposit and motion in the vicinity of boulders are scarce. This paper assesses how do sediment transport and the presence of sand affect flow dynamics around boulders. We present and discuss results from laboratory experiments of flow around an isolate protruding rock in a pebble-bed channel on both situations, with and without sediment transport. Instantaneous streamwise velocities were measured using Ultrasound Velocity ProfilerT D in longitudinal planes, upstream and downstream the protuberance, covering the entire flow depth. Experimental conditions, with two types of porous bed, included: fixed bed composed of gravel; and, bimodal bed composition (gravel and sand) with a mobile sand fraction transported under equilibrium conditions. Differences induced by the presence of a transport sheet near the bed in the time-averaged velocities and turbulent intensity are discussed as well as the influence of the sediment transport in the wake geometry and in the shielding effect provided by the boulder.
Year: 2009