Flow Analysis of Compound Channels with Rough Floodplains

Abstract

This research aims at flow in compound channels using physical modelling, are important for understanding flow and its behaviour. Experiments in straight compound channels, with differential roughness in main channel and flood plain beds are conducted to analyse two-phase flow at high river stage. Stage- discharge, boundary shear stress, velocity distributions and secondary currents are all measured, and the results are presented graphically and with tabulated summaries. Although it is known that Manning's roughness coefficient varies with river stage without accounting the effect of momentum transfer at the interface of main channel and adjoining flood plains, information on this subject is scarce, forcing practitioners to rely on empirical equations or their judgment to calculate the differences. The aim of this study is to look into the trends of Manning's roughness coefficient variance over width ratio, relative flow depth, and roughness ratio. Along the channel's wetted perimeter, this experimental channel allowed for a thorough investigation of composite roughness aspects. Physically, the composite/compound roughness on the wall as well as the shape of the channel modifies the velocity distribution across the cross section, and hence alters the resistance coefficient. The existing equations are typically intended for simple channels. When these formulas are proposed for compound and composite channels based on different assumptions about the relationships of the discharges, velocities, forces, or shear stresses between the component subsections and total cross section, it provide a significant error in estimating the composite roughness of compound channels. An analytical method (NCRM) is proposed to predict the composite roughness as a function of geometric, roughness, hydraulic parameters and the momentum transfer between main channel and floodplain interfaces. The momentum transfer magnitude is quantified in terms of interface length of main channel and flood plain (Imc or Ifp). The efficacy of the methods is checked against a collection of wide range of data sets. It also validated with natural river data sets. Predicting the stage-discharge curve of a compound channel is difficult due to additional flow resistance caused by momentum exchange between subsections, and it is much more difficult when the main channel and floodplains have a differential roughened boundary. There are few approaches that rely on calculation of the momentum transfer coefficients of compound channels to determine total flow, despite the fact that there are multiple models for predicting discharge. The study introduces an optimized procedure with an innovative methodology for testing the momentum transfer coefficients of compound channels with differentially roughened boundary in subsections using a sequence of experiments. The momentum transfer coefficients at the vertical and horizontal interfaces (χvl, χvr, and χh) of a compound channel can be accurately predicted using the suggested technique. Calibration performance reveals that the proposed model is capable of predicting the stage-discharge results, which are well compared to other existing approaches. The error statistic is established, confirming that the present method is providing better discharge results as compared to the other approaches. The method is successfully applied to the FCF, UK Wallingford, and the river data sets of compound channels of different roughness, flow, and geometric conditions. Flow discharge calculation in rivers is one of the parameter in flood management. In this study the hydraulic characteristic of open channel is analysed. The methods for evaluating the critical depth in a compound channel are also reviewed and assessed against experimental data. In this regard distribution of Momentum Coefficient (β), Energy Coefficient (αk), Depth- Froude’s Number, Specific energy curve are evaluated. The kinetic energy coefficient is estimated by the divided channel method with diagonal interface (DCM-D). Using the kinetic energy coefficient the Froude number is corrected. The critical depth and specific energy curve are also verified with the corrected value of Froude number. The method is also evaluating the lateral flow regime in a compound channel and assessed against experimental data.