Numerical simulation of river-bed variations: Sensitivity analyses with respect of the initial hydraulic roughness

Two elements affect the efficacy of the numerical models of mobile-bed evolution: the prediction of the hydraulic roughness, related with both sediment distribution on the bed and flow depth, and the prediction of the hydraulic sorting, when the distribution of sediments on the bed is not uniform. In this work the bed profile variation is simulated, by using a numerical model proposed in previous works, in function of the variation of the friction factor, that reflects the coarsening of median sediment diameter, due to armouring process, and the variation of flow conditions. The variation of grains-size distribution of the bed material is predicted by using a probabilistic approach and the median sediment diameter and the dimensions of bed forms are estimated at each time step. The paper is devoted to show, by means of computer-based numerical experiments, the sensitivity of the numerical results with respect to the initial hydraulic roughness used. The choice of the numerical space and time steps is discussed and the relation with the initial roughness coefficient is reported.