Experimental Study of Roughness on Selected Flow Parameters in a Compound Meandering Channel

Abstract

Measurements of velocity distributions, discharge variation, and other flow characteristics have been made under overbank flow conditions of a compound meandering channel with varying roughness intensities in the floodplains, using the large-scale open air facility of Department of Water Resources Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka. The aim of the study was to investigate the effects of floodplain roughness intensities on compound meandering channel flow phenomenon. In this study wooden roughness strips are used in the floodplain bed only with cross sectional area of 1cmx1cm. Spacing of roughness strips have been varied for different set-ups. In this study four roughness conditions are examined and for each roughness condition two runs have been performed for two different depth ratios. Point velocity data have been xiii collected using an ADV (Acoustic Doppler Velocity-meter) at a number of locations across the channel sections. Analysis of velocity profiles show that, significant deflections occur at the bankfull level for overbank flow condition. In all the cases, the transverse velocity components indicate the flow interaction between the floodplains and the main channel. In the cross-over section the interaction is more significant. Depth ratio plays another significant role here, as more interaction of main channel is observed at lower depth ratio. For depth ratio 0.5, the floodplain velocity decreases with the increase of roughness intensity and lower values are obtained for 30 cm roughness spacing; further decrease of roughness spacing cause the value to increase again, so at 15 cm roughness spacing the velocity profile has higher values than 30 cm roughness spacing in most of the cases. For depth ratio 0.35 velocity patterns show nearly opposite characteristics than that of higher depth ratio and it has higher values at 30 cm roughness spacing and further decrease of roughness spacing cause to decrease of velocity profile again. In case of velocity profile of main channel, no definite correlation with the roughness spacing can be identified. In all the cross sections for 0.5 depth ratio, as the roughness intensity increases, the total discharge decreases and the minimum value is obtained at 30 cm roughness strip spacing; further decrease of roughness spacing cause the total discharge to increase again. For depth ratio 0.35, as the roughness intensity increases the total discharge also increases as well and the maximum value is obtained at 30 cm roughness spacing; further decrease of roughness spacing cause the total discharge to decrease again. In case of percentage of main channel discharge, as roughness intensity of the floodplains increase the percentage of main channel discharge increase for all the cases. Investigation of shear stress distribution shows that, the placement of artificial roughness strips in the floodplains cause to increase of floodplain shear stress at a high rate but main channel shear stress does not significantly change with the change of floodplain roughness intensity. Velocity distribution coefficients and roughness factors are also calculated and the variations with the roughness intensity have been investigated. xiv In the present study a set of new formula have been proposed for calculating percentage of discharge in the main channel for overbank flow condition with different roughness strips spacing in the floodplains. Here correlation and regression analysis has been done and some linear regression equations have been proposes for calculating different flow parameters. The equations developed here may be improved by incorporating more data from channels of different geometries and sinuosity and different roughness conditions and these may well need to be incorporated into reliable numerical models for the prediction of discharge and other flow parameters in the meandering rivers of Bangladesh.