IMPACTS OF SAND TRANSPORT ON FLOW VARIABLES AND DISSOLVED OXYGEN IN GRAVEL-BED STREAMS SUITABLE FOR SALMONID SPAWNING

摘要

Increased sediment production resulting from the erosion of unprotected soil in a catchment may result in severe changes in the morphodynamics and hydrodynamics of mountain rivers. Sediment overfeeding can lead to the degradation of salmonids' spawning sites. The objective of this study is to evaluate the impacts of sand transport on flow hydrodynamics and on vertical dissolved oxygen (DO) distribution in gravel-bed rivers. To accomplish the proposed objective, conditions similar to those found in nature, in what concerns flow and bed material, were reproduced in a laboratory flume and a theoretical model of the vertical distribution of DO was developed. Three laboratory tests simulated (ⅰ) an undisturbed openwork gravel bed; (ⅱ) a framework-supported gravel bed with a sand matrix and (ⅲ) a framework-supported gravel bed with imposed sand transport at near-capacity conditions. Instantaneous velocity maps were obtained with Particle Image Velocimetry (PIV). The collected data were analysed and theoretically framed with double-averaged methods (DAM). Mean Reynolds stresses were unaffected by reduced bed porosity or sediment transport even in the near-bed region. Sediment transport affected longitudinal velocity profiles and eddy viscosity decreased. Numerical simulations of the vertical distribution of DO and of gaseous species were carried out under the hydrodynamic and sedimentological conditions of the flume work. The results of the numerical simulations show that, in the presence of sand transport, near-bed concentrations of DO are strongly reduced, mostly due to the reduction of downward diffusive fluxes of gaseous oxygen and DO.