Incorporating apparent shear stress in the roughness to improve the discharge prediction in overbank flows

Abstract

During flood events, the overbank flow is rather common in alluvial valleys. The so-called compound channel flow is characterized by the interaction between the deeper and faster main channel flow and the shallower and slower floodplain flow. The flow structure becomes much more complex than in a single channel. In the shear layer formed in a region near the interface, a lateral momen- tum exchange between the main channel and the floodplains and the secondary currents due to the non-isotropic turbulence lead to a 3D flow structure. Despite the availability of 2D and 3D flow models that may solve this question, 1D methods are often preferred due to the less amount of parameters required and to the shorter processing time. In the present paper, a new method to compute flow discharge in compound channels is proposed namely by incorporating the apparent shear stress in the Manning roughness. The method is calibrated and validated taking into account stage discharge relationships in several compound channel facilities.