Effects of macro-scale bed roughness geometry on flow resistance

Flow field over macro-roughness elements, as, for example, in flow conditions such as of steep mountain streams, is investigated by means of a theoretical model and a laboratory study with the main aim of understanding the influence of concentration and planimetric disposition of grain roughness on flow resistance. A theoretical model for the average velocity distribution profile over a bed with macro-roughness elements is developed herein. The model is based on a two-layer approach which takes into account the displacement of the main flow due to the drag around the roughness. The laboratory study is composed of an extensive set of flume experiments in the case of steep slopes and large relative roughness conditions. For a given flow discharge and bed slope, various experiments have been carried out with different patterns and spacing of the roughness elements constituted by river pebbles. Application of the theoretical model to the experimental results enables the determination of how flow resistance is affected by the macro-roughness; in particular, it appears that when the concentration of the roughness is within an optimal range, flow resistance is maximum.