Flow Resistance Estimation in Mountain Rivers

Examination of the flow resistance of high‐gradient gravel and boulder‐bed rivers, using data collected in British mountain rivers with slopes of 0.4–4%, shows that there are differences in resistance variation between mountain and lowland rivers and that between‐site variations do not necessarily reflect at‐a‐site variations. Comparison of data with the familiar resistance equation relating the Darcy‐Weisbach friction factor to the logarithm of relative submergence shows that the equation tends to overestimate the resistance in uniform flow. The equation also tends to underestimate the rate of change of resistance at a site (as discharge varies) at high gradients. The influences of nonuniform channel profile, sediment size distribution, channel slope and sediment transport are reviewed, but the data do not allow any quantification of these effects. Instead an empirical approach based on the available data is presented, allowing the friction factor to be calculated from the relative submergence with an error of up to ±25% to ±35%. A summary of the field data is included.