Flow Resistance for a Varying Density of Obstacles on Smooth and Rough Beds

Abstract

High-return-period floods involve complex flows over large floodplains covered with different types of resistive elements, from bed roughness to large emerging obstacles. In order to characterize the flow resistance in such configurations, experiments are conducted in a flume with emergent aligned obstacles on smooth and rough beds. Under uniform flow conditions, the drag force is measured by a hydrodynamic force measurement device, thereby obtaining the relative contribution of the drag and bed friction forces to the total flow resistance. The results show a strong correlation between the reference surface ratio (λf), i.e., the obstacle frontal area divided by the cell bed surface, and the relative contribution of the drag to the flow resistance, with negligible influence of bed roughness and shallowness. Equal contributions of bed roughness and form drag to flow resistance occur for λf≈0.02. After showing that the drag coefficient is roughly constant and the bed friction is consistent with Darcy-Weisbach’s head-loss coefficient, a Chezy-type equation is proposed to predict the mean uniform velocity U as a function of the flow and land occupation parameters.