MODELLING FLOW IN PRISMATIC AND NON-PRISMATIC COMPOUND CHANNELS

Abstract

The present research is performed to investigate the influence of channel roughness, floodplain widths and converging floodplain angles on flow characteristics in prismatic and non-prismatic compound channels. The experiments were conducted in 18 m length compound channels. Experimental measurements were made of water depth, depth-averaged velocity distribution, and boundary shear stress distributions. The analytical solution to predict composite roughness in compound channel with different channel roughness is presented. The model was applied to the experimental data observed in this stud and from the literature. To check performance of the present model, an error analysis in terms of Mean Absolute Error (MAE), Mean Absolute Percentage Error (MAPE), Root Mean Square Error (RMSE) and Nash-Sutcliffe Efficiency (E) was performed to compare the present model with existing models. The error value of the present model is the least compared to the existing models. These indicate that, the present model can predict the composite roughness in compound channels. The floodplain measurements also show that the boundary shear stress in a heterogeneous rough bed was significantly higher than that in a smooth bed. In addition, the non-dimensional coefficient and secondary flow term for rough bed are high at the interfaces between the main channel and floodplains due to velocity differences and developed shear layers between the main channel and floodplain. The analytical solution to predict the depth-averaged velocity distributions and boundary shear stress distributions in rectangular compound channels with different floodplain widths has also been investigated in this study. The experimental works were conducted in three different data sets of the present experiments with different floodplain widths. Different boundary conditions have been applied to determine the integration constants. An expression is proposed to model laterally varying non-dimensional coefficients. Those coefficients were constant regardless of spatial locations as suggested by the Shiono and Knight Method. Then, the model was verified against data sets of the present experiments and from the literatures. The results show that the analytical solution from present model is agreed with experimental data. The non-dimensional coefficient plays important in portraying the variation of depth-averaged velocity and boundary shear stress distributions, and it also affects the secondary flow terms in both the main channel and floodplain. The shear forces in compound channels with different floodplain widths have also been studied. In this study, two types of shear forces namely horizontal and vertical shear forces have been studied. The vertical and horizontal shear forces on the interfaces between the main channel and floodplain were discussed and analysed by applying external forces for the control volume in the main channel. The horizontal shear force at interface was negative in all configurations, indicating that the upper flow region accelerates the flow in the lower main channel. In contrast, the vertical interfaces were positive, suggesting that the slower floodplain flows retard the faster main channel flows. The analytical solution to predict the depth-averaged velocity and boundary shear stress distributions in non-prismatic compound channels with different converging floodplains has also been investigated in this study. The model was applied to different data sets of the present experiments with different floodplain converging angles ranging from 1.43 to 7.59 degrees. The results indicate that the floodplain converging angles can influence the distributions of depth-averaged velocity and boundary shear stress distributions.