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.
Highlights
Hydraulic modelling requires that the friction created by river beds be well characterised.The formulation generally used is the Manning-Strickler formula, which allows a simple calibration of the hydrodynamic models
The hydraulic resistance of a bed comprising macro-roughness elements is dependent on a knowledge of the drag coefficient
Hydraulic resistance will be expressed in terms of the Darcy-Weissbach coefficient defined over the total depth of the flow h
Summary
The formulation generally used is the Manning-Strickler formula, which allows a simple calibration of the hydrodynamic models The limitations of this formulation have been exposed in cases where the sizes of the roughness elements are similar to the water depth (strong confinement), torrential flow [1], vegetated beds [2,3], unsteady flow, or hyperconcentrated flow [4]. The formulations set out often seem to be limited to their particular domains of investigation [5,6,7] and rarely allow treatment of flow over beds that may be emergent, confined, or completely submerged Such significant variations in water depth or slopes are common in models of fish habitat, floodplains, or mountain rivers. The hydraulic resistance of a bed comprising macro-roughness elements is dependent on a knowledge of the drag coefficient
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