Flow Resistance and Stage-Discharge Relationships in Compound Channels with Composite Roughness

The present study discusses and analyses the flow resistance parameters and stage-discharge relationships in compound channels with different bed roughnesses. An analytical solution of depth-averaged velocity is developed from the shear forces acting on the channel beds and walls. In addition, an expression for non-dimensional coefficient, integration constants, and analytical solution to compute the composite roughness are also derived. Experiments were conducted in compound channels. The model was applied to compute composite roughness from the present experimental study and two other experimental data in the literature. The key parameters friction factor, composite roughness, roughness height, non-dimensional coefficient, and secondary flow term are discussed in the model application to different data sets. The results indicate that the composite roughness and friction factor are the key flow resistance parameters that can influence the depth-averaged velocity and boundary shear stress distributions. The result shows that, in a rough bed, the boundary shear stress in the floodplain was significantly higher in a heterogeneous bed as compared to a smooth bed. 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 mean relative errors of the present model and existing models were also discussed. It is concluded that the present analytical solution gives a good prediction of the flow resistance parameters, stage-discharge relationships, depth-averaged velocity and boundary shear stress distributions when compared with experimental data with different bed roughnesses.
Keywords: Flow Resistance, Stage-discharge, Composite Roughness, Compound channels, Rough, Heterogeneous, Smooth