The frictional resistance characteristics of a bedrock‐influenced river channel

Much of the published literature relating to reach-scale total flow resistance concentrates on alluvial rivers and little mention is made of the more extreme resistance found in bedrock-influenced river systems. This paper presents the results of a detailed investigation of total flow resistance variation for five channel types on the Sabie River, South Africa (cohesive mixed anastomosed, uncohesive mixed anastomosed, mixed pool–rapid, alluvial braided and alluvial single thread) over a range of discharges. Manning's ‘n’ and Darcy-Weisbach f resistance coefficients are calculated using Barnes' methodology (H. H. Barnes, US Geological Survey Water Supply Paper 1849, 1967). These data are analysed at the scales of channel type and morphologic unit and the resultant resistance values are compared with the literature. Generally the low-flow friction values estimated for the bedrock-influenced channel types are higher than any reported. Flow resistance quantification of the alluvial channel type study sites on the Sabie River produced values slightly higher than earlier studies of similar morphologies. In situations where significant areas of irregular bedrock are exposed (cohesive mixed anastomosed and mixed pool–rapid channel types) the high flow resistance coefficients at low discharges probably reflect energy dissipation due to hydraulic jumps and internal distortions; however, these values fall as discharge increases to values comparable to those recorded for step–pool rivers. It is suggested that the slightly elevated resistance coefficients recorded in the alluvial channel types are the result of the vegetation roughness component, operating when the macro-channel bar features are inundated. Copyright © 2004 John Wiley & Sons, Ltd.