Flume study of flow above large-scale artificial roughness.

Flow above large-scale roughness, in which the size of bed elements is of the same order of magnitude as the depth of the flow, is of great interest in aquatic environment field, namely in fish passage technology. This article presents a synthesis of the experimental results of laboratory tests conducted in a flume at the Institute of Fluid Mechanics in Toulouse, using large roughness elements made of cylindrical blocks. Following a description of the observed flows, the influence of the main factors are examined and relationships between flow discharge and channel slope, relative submersion, transversal porosity and longitudinal distance between rows are proposed. Examples of vertical velocity profiles are presented. These results are discussed in a fish passage perspective and an example of dimensional application is given.