Experimental Study on Resistance in Gravel Bed Channels

Abstract

River beds are frequently experienced by gravel beds the analysis of which is very interesting and challenging to the river engineers and researchers.Gravel bed can be categorized it into three bedload condition,i.e. no load, moderate and intense.The present study investigates the roughness characteristics of gravel bed open channel flows under both no load and intense load conditions over various discharges and flow depths.Experimental investigation has been carried out in an open channel flow with Gravel bed surface of grain size of D50 values 13.5 mm and 6.5 mm for both no load and intense load conditions respectively. Measurement for the velocity and the boundary shear stress of the gravel bed condition has been determined. The longitudinal slope of the channel is 0.25%. Hence,for the case of gravel size 13.5mm, gravel is not transported,thus a case of no load condition occurs.Whereas in the case of 6.5mm gravel size,the gravel is transported under the condition of intense load such that in the particular case the bed load transport rate has been determined.Variation of friction factor for both the roughness conditions for different flow depths has been estimated.The intensity of the bed load has also been calculated with the help of sediment transport rate. It was observed that bed load was falling with increasing grain sizes under both no load and intense load conditions. Hence,an improved model for prediction of friction factor has been devised.This model has been validated with the data set of other investigators,showing satisfactory values pertaining to the actual values of Darcy‘s f.The developed model has also been compared with the models of other researchers and is found to provide better results than those given by others.The bed load transport rate for the intense load conditions was also determined from experimentation on gravel beds of 6.5mm gravel size for different flow depths.By the use of the data set of other researchers and present experimental data,a new model to formulate intensity of the bed load transport with respect to Shield‘s parameter has been formulated.The modified model is found to provide satisfactory results as compared to previous works.