Overbank flow in compound channels with prismatic and non-prismatic floodplains

• Main Author: Rezaei, Bahram
• Published: University of Birmingham 2006
• Subjects: 627
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.564484

Two sets of experiments have been undertaken in compound channels with prismatic and nonprismatic floodplains with different floodplain widths and convergence angles in order to investigate flow behaviour in two-stage channels. Measurements were made of water surface profile, depth-averaged velocity, boundary shear stress distributions and velocity distributions and the results are presented in graphical and tabulated form. Empirical equations have been derived from each type of compound channel, allowing predictions to be made concerning the resistance parameters, n and f, the discharge, Q, the proportion of total flow in sub-areas, %Qj, the evolution of flow in the main channel and on the floodplain along the converging part of the flume, the boundary shear forces, SFj, and the apparent shear forces at a vertical and horizontal interface between the main channel and the floodplain, for a given stage, H. The percentage of flow in the main channel and on the floodplain, as well as the percentage of the boundary shear forces carried by each element in different sections of the compound channel with non-prismatic floodplains, were then compared with the prismatic floodplain cases with the same geometry. The results of these two sets of experiments indicate that they follow the same pattern, although there are some differences due to the effects of backwater and convergence in the flume. The apparent shear forces on a vertical interface, ASFv, are however remarkably different. Based on momentum balance, an analytical approach was developed to estimate the water surface profile in non-prismatic compound channels with different convergence angles, 9. The calculated water surface profiles compared well with the measured ones. It is shown that the Shiono and Knight Method (SKM) predicted lateral distributions of the depth-averaged velocity and boundary shear stress well, especially for prismatic compound channels with wide floodplains. Furthermore, there is a good agreement between the measured and predicted distributions in non-prismatic compound channels, provided the energy slope, Se,is used instead of the bed slope, So. The experimental results were then also used to calibrate the two parameter, ",t, and ",g, in the Exchange Discharge Model (EDM). This revealed that the calibration of the turbulent exchange parameter, wt strongly depends on the Manning's roughness coefficient. In term of compound channels with non-prismatic floodplains it is also shown that the EDM always overestimates the flow discharge for a specific stage.