Author: Fooman Forough
One of the important aspects of flow in natural rivers is the occurrence of a particular hydraulic condition when the water level may reach the bankfull level in a compound channel, and any further rise would result in water moving into the floodplains. In this condition, due to the interaction between the flow in main channel and floodplains, the hydraulic behavior of flow is more complex than the flow in single channels. Although compound channels with non-prismatic floodplains are particular type of sections in natural rivers, only a few studies have been conducted on these sections. The main objective of this study is to compute water surface profiles in non-prismatic compound channels based on the energy and momentum equations and using lateral depth-averaged velocity distribution (Shiono-Knight method-SKM) for estimating the conveyance of the channel. The computed water surface profiles are compared with the corresponding observed water surface profiles which have been measured in the laboratory by other researchers. To better understand the performance of different methods, the best results obtained by Seyed Mohammadi Haghighi in 2014 based on using one-dimensional (1D) conveyance estimation methods, are also presented in this study. The selected experimental works in this study are those conducted by Bousmar in 2002 on compound channels with convergent floodplains and Bousmar et al. experiments in 2006 on compound channels with divergent floodplains. In this study, the CES software is used to calculate the conveyance, energy and momentum correction factors in any section of the channel. The CES software uses SKM method. The results show that the computed water surface profiles in compound channels with convergent floodplains using SKM method and momentum equation (SKM-Momentum) are in more agreement with the observed data compared with those of SKM method and energy approach (SKM-Energy). However, in general, the water surface profiles computed by using 1D Coherence method of conveyance estimation method and energy equation (Coherence-Energy) show the best performance in compound channels with convergent floodplains. The maximum mean absolute relative errors (MARE) of the computed water surface elevation in compound channels with convergent floodplains are 0.58%, 3.02% and 2.60% for the Coherence-Energy, SKM-Momentum and SKM-Energy methods, respectively. In the case of the compound channels with divergent floodplains, the computed water surface profiles by the SKM-Energy method better follow the observed water surface profiles than those computed by the SKM-Momentum method. However, in general, the water surface profiles calculated by using 1D vertically divided channel method (VDCM) of conveyance estimation method and momentum equation (VDCM-Momentum) show the best performance in compound channels with divergent floodplains. The MAREs of the computed water surface elevation in compound channels with divergent floodplains are 1.54%, 7.52% and 6.90% for the VDCM-Momentum, SKM-Momentum and SKM-Energy methods, respectively