علوم و مهندسی آبیاری (Jan 2015)
Physically Based New Formula for Cohesive Bed Erosion Rate
Abstract
The currently most widely used erosion models are poorly based on physical principles and lack the ability to describe erosion of the consolidated sediment bed at increased shear stresses e.g. during a storm. There is a need for a new modeling approach that is both fast and accurate. A new erosion formulation is proposed that regards the bed shear stress as a stochastic variable and allows erosion only when the actual bed shear stress exceeds the actual yield strength at the interface. There is a formulation that relates the actual yield strength to the density and the cohesion. In the formula, is assumed that the potential erosion flux is the same as entrainment flux of fluid mud under turbulent flow. The formulation needs eight physical input parameters. The five shear strength parameters can be determined from standard consolidation experiments in a settling column as well as soil mechanical tests. The other two physical parameters have to be determined by means of relation concerning to erosion on fluid mud and shear parameter. For the last parameter, namely coefficient of the probability distribution of the bed shear stress, No standardized laboratory tests are yet available and should be estimated during calibration process. The formula has been validated on short term erosion experiments performed in a straight flume on the bed sediments of the junction of Sefidrud dam reservoir and Ghezel Ozan river. These erosion experiments have been performed on the same beds that were measured in the consolidation experiments used to validate the parameterization. In the straight flume seven experiments have been performed. The model can simulate the results from the flume experiment well. The simulations show the same characteristic exponential decay evolution of the eroded material height. Moreover, the model can be calibrated well in a qualitative sense.
