A Semi-Analytical Model for the Hydraulic Resistance Due to Macro-Roughnesses of Varying Shapes and Densities

Abstract

A friction model resulting from investigations into macro-roughness elements in fishways has been compared with a broad range of studies in the literature under very different bed configurations. In the context of flood modelling or aquatic habitats, the aim of the study is to show that the formulation is applicable to both emergent or submerged obstacles with either low or high obstacle concentrations. In the emergent case, the model takes into account free surface variations at large Froude numbers. In the submerged case, a vegetation model based on the double-averaging concept is used with a specific turbulence closure model. Calculation of the flow in the roughness elements gives the total hydraulic resistance uniquely as a function of the obstacles' drag coefficient. The results show that the model is highly robust for all the rough beds tested. The averaged accuracy of the model is about 20% for the discharge calculation. In particular, we obtain the known values for the limiting cases of low confinement, as in the case of sandy beds.

Wave Height Attenuation and Flow Resistance Due to Emergent or Near-Emergent Vegetation

“…Despite the large amounts of theoretical and experimental results available in the literature, the interaction of waves with a vegetated canopy still deserves further study. In particular, the drag coefficient, which represents/includes the impact of several phenomena such as sheltering, bed interaction, and non-uniform velocity distribution [34], needs to be assessed.…”

Influence of Gravel-Bed Porosity and Grain Orientation on Bulk Flow Resistance

“…Rotating the cast-bed, these particles are directly exposed to the flow but cannot be eroded as they are part of the cast. As a consequence these areas are characterized by a less hydrodynamic shape than comparable frontal areas of real water-worked beds as the associated drag coefficient changes with the shape and orientation [38]. Consequently, the rotated cast-bed imposes a higher resistance on the flow than the cast-bed.…”

Effect of vegetation on flows and sediment transport

“…Alternatively, one could use bed friction laws, such as Manning friction loss equation [7] or other friction laws [10]. However, this would require considering the average velocity and friction coefficients calibrated from experiments without vegetation.…”