Effect of Differential Roughness on Flow Characteristics in a Compound Open Channel

Abstract

The present thesis is an outcome of a rigorous research effort undertaken by the author with a principal objective to observe the effect of differential roughness between the floodplains and the main channel on the flow characteristics of a compound channel having larger width ratio. Generally the natural channels comprise a wider and rougher floodplain than the main channel (i.e. non-uniform boundary). In this case, much of the hydraulic resistance may be attributed to surface roughness and flow characteristics accounting for other forces. The resistance to flow along the channel boundary is manifested in the form of distribution of boundary shear along the periphery. From critical review of existing literature, it can be concluded that the boundary shear stress distribution is not uniform over the wetted perimeter of uniformly / non-uniformly roughened channel section. Still boundary shear distribution estimation in compound channel having width ratio greater than 10 along with roughness variation is rare to come across. Therefore, investigations which deals with the effect of wide floodplains and the differential roughness on evaluation of boundary shear distribution stress carried by the channel boundary is essential. In the present work, a series of experiments, for boundary shear stress on the wetted perimeter and stream-wise velocity at different points of the channel cross section grid, were carried out by using a pitot tube, for a compound channel having high width ratio of 15.75 with varying differential roughness (the ratio of base n value of floodplain surface roughness to that of main channel). The estimation of boundary shear stress was carried out by using well known Preston-tube technique preceding the estimation of boundary shear force distribution. Some commonly used equations of boundary shear
distribution across the boundary of a compound channel were analyzed and tested with the experimental data. Then, a modified general expression to predict the boundary shear
distribution in compound channels having non-homogeneous roughness in the subsections was derived, which provided significant improvements in the results. The modified boundary shear distribution approach was tested for its validity using the fresh laboratory data collected for this purpose, as well as using the FCF data, Birmingham, UK, for a comparison. Analyses were also done to investigate the effect of differential roughness on depth-average velocity, flow distribution, and zonal and overall Manning’s n variation to give substantial idea of a river system during overbank flow condition.