An experimental investigation to calculate flow resistance in a steep river

Abstract

High gradient streams are characterised by steep slopes, coarse bed material with non-uniform size distribution, flash floods and flow depths of magnitude comparable to the bed material size. Another characteristic of high gradient streams is development of armor layers with cobbles and boulders. In these streams, it is often difficult to measure water discharge or flow velocity directly, particularly at high flows; instead, it is necessary to use the indirect methods in which flow velocity and discharge is calculated using a flow resistance predictor. The problem of predicting flow resistance in high gradient alluvial channels with sufficient accuracy is of great interest to hydraulic engineers and is essential for the effective design of water-resources related projects. In this paper, the flow resistance of a steep river with coarse gravel bed material has been investigated by using 75 accurate measured hydraulic data in a relatively wide flume with different coarse gravel bed materials, flow rates and longitudinal slopes. The main purpose of this article reviews the ability of the resistance empirical equations to predict mean flow velocity in gravel river beds. Also, based on measured data, a new equation to predict the flow resistance in steep river has been proposed. This equation along with the other empirical equations has been used to predict the parameters introducing the flow resistance in steep rivers including Manning roughness coefficient, mean flow velocity and Darcy-Weisbach roughness coefficient. The results were compared with measured and calculated hydraulic data and the best equations to predict the Manning’s roughness coefficient in a steep river with coarse gravel bed material are introduced. Finally in order to show the generality and accuracy of the proposed equation, it has been used for the other field data and the results have been discussed.