Flow Resistance Caused by Large-Scale Bank Roughness in a Channel

Systematic experimental investigations have been performed under steady flow conditions in a channel whose banks are equipped with large-scale rectangular roughness elements. The purpose was to determine the flow resistance owing to such macrorough banks. The practical motivation of the study is to see how morphological restoration of banks in channelized rivers, such as lateral cavities, influences the steady flow characteristics. The experiments performed in 40 different geometrical configurations revealed various two-dimensional flow characteristics in the bank cavities created by the roughness elements compared to the prismatic reference channel. The overall head loss of the flow is governed by the existence of different phenomena, such as vertical mixing layers, wake zones, recirculation gyres, coherent structures, and skin friction. The analysis of the experiments for steady flow conditions showed that the flow resistance is significantly increased in the macrorough configurations because of the disturbance of the bank geometry inducing large-scale depressions. The additional flow resistance attributable to macroroughness has been related to the forms of the banks. By separating the observed flow behavior into a normal recirculating, a reattachment and a square-grooved flow type, macrorough flow-resistance formulas according two different approaches could be developed that are in good agreement with the laboratory experiments