Conveyance prediction for meandering two-stage channel flows
An examination was carried out to determine the conveyance behaviour of overbank flow in meandering two-stage channel systems. Eleven influential parameters were identified namely sinuosity, aspect ratio, cross-sectional shape and bank slope of the main channel, relative boundary roughness of floodplain to main channel, floodplain longitudinal slope, meander belt width relative to floodplain width, sinuosity of the floodplain bank, relative overbank flow depth, system scale and lateral slope of the floodplain. The first ten of the eleven parameters were examined. Observations were made of the coherent flow structure of the floodplain and main channel systems, which influences energy losses. The effect of each of the influential parameters is quantified through a non-dimensional discharge function F* which is the ratio of the actual discharge in a two-stage channel to a theoretical discharge aggregated for three cross-section zones, the main channel, the floodplain within the meandering channel and the floodplain outside the meander belt. The effect of each parameter on the energy losses in meandering two-stage channel flows for the case involving straight floodbanks and a floodplain without cross-fall are analysed in terms of Darcy-Weisbach resistance coefficients using the Prandt-von Karman resistance relationship and treating the system as a whole. Flow domains are defined in the first of which viscosity and roughness are influential and in the second flow resistance is independent of viscosity. A design approach is presented for predicting the conveyance capacity in each of these domains and is based on small-scale data obtained at the Universities of Aberdeen and Glasgow and large-scale data from the UK Flood Channel Facility (UK-FCF). This approach was applied to the available independent laboratory data along with the James and Wark (1992) method. The author's approach was found to give good predictions of conveyance capacity. This approach was also applied to River Roding field data. The floodplain roughness which varies throughout the year and is difficult to estimate, is shown to be the most significant source of energy loss and is environmentally sensitive in natural meandering two-stage rivers.