Turbulent flow structure in a vegetated non-prismatic compound channel

Abstract

The presence of vegetation on the floodplains of compound channels affects the structure of mean flow, increases flow resistance and turbulence intensity, and impacts sediment transport. In prismatic compound channels, the shear stress at the interface between the flows in the main channel and lateral floodplains has a strong influence in the cross sectional velocity distribution and thus conveyance of the channel. Due to mass transfer between subsections, these effects are enhanced in non-prismatic channels. In the present study, the three-dimensional turbulent flow structure in a vegetated non-prismatic convergent compound channel is experimentally investigated. The main purpose is to clarify the explicit influence of the combination of two factors (floodplain vegetation and convergent floodplains) on mean flow and large-scale turbulence. The experiments were conducted for relative depths (i.e., the ratio between floodplain and main channel flow depths) equal to 0.21 and 0.31 and floodplain convergence angles equal to 7.25 and 11.3 . The results obtained in straight compound channel with vegetated floodplains were used as a reference. The analysis of the Acoustic Doppler Velocimeter measurements shows that due to convergent floodplains, a transverse current is directed from the floodplains to the main channel from both sides, generating two helical secondary currents. Investigations of the vegetation influence on Reynolds stresses shows that there is a significant vertical and lateral spreading of the shear region towards the main channel side slope. As the convergence angle increases, the floodplain flow proportion decreases. In this case, the presence of vegetation in the floodplain leads to a suppression of the momentum exchange between main channel and floodplains causing the discharge distribution between sub-sections to be significantly different from what would be without floodplain vegetation.