11. Boundary Shear Stress Distributions in Compound Channels Having Narrowing and Enlarging Floodplains

Compound channels are basically described as two stage open channels having main river and its adjoining floodplains. The momentum transfer phenomenon at the junction of main channel and floodplain is very crucial to be understood to estimate the discharge. The difference in water depth and roughness between the two zones generally causes momentum exchange at the interface. It is simple to quantify this momentum exchange for uniform flow conditions in river; however, for non-uniform flow condition the quantification is complex as the flow properties change in both longitudinal and lateral directions. Therefore, a study has been done on overbank flow with non-uniform flow condition, and stage discharge relationships are analyzed for accurate modeling. As natural rivers may have different configurations, so two different types of channels that are converging and diverging channels are considered and flow variables at different longitudinal positions are analyzed. Two discharge predicting models are developed which can be used for flow in natural rivers. These models depend on the non-dimensional forms of geometric and flow parameters and the percentage of the boundary shear forces carried by the adjacent floodplains. So it is required to analyze and estimate the boundary shear force distribution carried by main channel and non-prismatic floodplains before predicting flow. So, two different equations are proposed for percentage shear carried by converging and diverging floodplains. In addition, the developed models give simple ways for the quantification of percentage boundary shear force and provide accurate discharge result through non-prismatic channels. The predictions of the models are then compared with the models of other researchers. The prediction efficiency of the present model is found better than the models of previous researchers.