Effects of Momentum Transfer in Compound Channels

Abstract

Different methods of considering the shear stresses between a main channel and its flood plains are discussed. The methods considered include the simplistic method of considering the entire hydraulic cross section as one, dividing the cross section into independent sections, assuming limited shear stresses between main channel and the flood plain, trying to find an interface with zero shear stress, or allowing for the shear stresses from a momentum balance. Based on evaluation of alternative steady‐state models, it is concluded that the area method is the most promising for computation of discharge and that the Prinos‐Townsend equation gives accurate results for apparent shear stresses at the main channel‐flood plain interface. The results of the theoretical study are used in an unsteady‐flow model, prepared based on hydrodynamic principles. The model facilitates the investigation of the significance of the momentum‐transfer phenomenon between the main channel and its floodplains. The analysis demonstrates that momentum interchange results in a shift in the loop rating curve to the right, a delay in the falling of water levels, an increase in flood plain flow, and a decrease in the main channel carrying capacity.