AN EXPERIMENTAL STUDY OF TURBULENT FLOW OVER A SALMONID REDD AND THE IMPLICATIONS FOR THE GENERATION OF INTERSTITIAL CURRENTS THROUGH THE GRAVEL BED

Salmonid fish bury their eggs within riverbed gravel within a three-dimensional structure(redd)that consists of an upstream hollow with an accompanying downstream hump. Turbulent flow over this permeable bedform maintains an interstitial flow that supplies oxygenated water to the fish eggs and thus mediates the survival rates to hatching.

Brown Trout redds were constructed in a narrow flume and in a wide flume. The narrow flume allowed two-dimensional turbulent flow structure above the redd for comparison with other published 2-D studies. The wide flume allowed fully three-dimensional turbulent flow to develop above and around the redd. In addition, interstitial flow observations were made.

Laser Doppler Anemometry recorded the downstream, lateral and vertical components of turbulence at 3000 points. Isovel maps were constructed for the turbulent flow parameters. The results demonstrate that measurements of 2-D flow structure provide an inadequate description of the turbulent flow field. Rather 3-D measurements are required to describe the flow the relationship of this flow to the interstitial flow. Strong flow forcing occurs at the crossover between pot and tail. The zones of flow separation, greatest turbulence and shear are identified by plots of root mean square velocity, Reynolds stresses and quadrant event frequency. Together, these measures delineate regions that separate distinct bodies of low and high momentum fluid. The shape of these regions varies with mean flow velocity and the degree of freedom that the flow has to diverge laterally around the tail of the redd. The 3-D form of the hollow promotes firstly a significant flow convergence at the crossover between hollow and hump which generates high interstitial flow into the tail of the redd. Secondly, flow separation in the lee of the crest induces a low-pressure zone above the tail that promotes an upward flow of interstitial fluid. Thus acting in concert these two regions drive the interstitial flow patterns. The results demonstrate that the shape of the redd significantly enhances the pattern of flow through the redd such that the supply of oxygenated water is enhanced within the area of egg pockets.