Shear Layer Development and Fully Developed Flows in Compound Channels

In alluvial river reaches, flood events are characterized by the inundation of the adjacent land near the main channel course. In these compound channel or overbank configurations, the flow structure is much more complex than in a single channel due to interactions between faster and deeper flow in the main channel and the slower and shallower flow in the lateral floodplains. Moreover, the interactions may be enhanced by the floodplain roughness as this region is usually covered by vegetation. The present study aims at understanding the streamwise flow development and the turbulent flow structure in such conditions. An experimental campaign was carried out in a laboratory compound channel. An iterative procedure to obtain the uniform flow was followed by successive change of the discharge distribution and downstream levels. In total, nine uniform flows were studied, namely (i) six with floodplains made of polished concrete and (ii) three with synthetic grass covering the floodplains. The boundary and mixing layers are presented and the effects of the shallowness and of the floodplain roughness on main flow mechanisms are assessed.