Momentum Exchange between Main Channel and Flood Plain Using Momentum and Energy Approaches

Most rivers have flood plains that extend laterally away from the main channel at a gentle gradient or in a series of terraces. Multistage channels are deliberately formed in certain cases in order to increase conveyance capacity in large floods، and to have recreational expanses available at other times of the year. Two-stage channels، thus consist typically of a main river channel in which there is some discharge all of the time and flood plains، which are dry for most of the time، yet perform a vital function in times of flood. Since flood alleviation schemes are the focus of much of engineering work، the prediction of the conveyance capacity، velocity distribution and boundary shear stress distribution in such channels is clearly important. The boundary shear stress distribution is a prerequisite for studies on bank protection and sediment transport. Prediction of these parameters in two stage or compound channels is complicated by the lateral exchange of momentum that takes place in the shear layer that forms between the generally faster moving water in the main river channel and the slower moving water on the flood plain. Superposition of high lateral shear on bed-generated turbulence and longitudinal secondary flow structures is a convoluted problem in fluid mechanics. In the context of the river channels with flood plains، the problem is usually further complicated even for moderately straight channels by the complex geometry of the cross-section and the heterogeneous nature of the boundary roughness.