Zonal and Overall Discharge Prediction Using Momentum Exchange in Smooth and Rough Asymmetric Compound Channel Flows

The apparent shear stress acting on the vertical interface between floodplain(s) and main channel has been shown to be very influential for the estimation of the zonal and overall discharge in compound channels. Experimental results of 21 test runs from different researchers and six datasets for natural river systems are considered in the present analysis wherein apparent shear stress has been empirically incorporated in the calculation of discharge. The range of datasets included different asymmetric channels, which have large variations in width ratios ($B/b$), aspect ratio ($b/h$), and bed slope ($S_o$), where $B$ is the total width of channel at bankfull, $b$ is the main channel width, and $h$ is the bankfull main channel height. In total, 201 data points considered cover small-scale channels to natural river systems, which lays a foundation for validation for seven apparent shear models given in the literature. The momentum exchange models used here are motivated by scaling arguments and allow a simple analytical solution for the zonal discharge in each section. However, it was found that the apparent shear models perform differently based on different depth ratios. None of the models performed well in channels with low depth ratio. Performance of the different models for apparent shear that are based on width ratio and slope were found to give diverse results, which is discussed in detail. Compared with the traditional divided channel method (DCM), this apparent shear-based method can better predict both overall and zonal discharge with a percentage error of only 8.9% for overall discharge.