The frictional resistance characteristics of a bedrock‐influenced river channel

Abstract

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

Thresholds of erosion and sediment movement in bedrock channels

“…In general, the storms resulting in the highest discharge were found to have the lowest roughness values. This is similar to what Heritage et al (2004) reported for the Sabie River in South Africa. They suggested that at low discharges the flow resistance coefficients are probably reflecting energy dissipations from hydraulic jumps and internal distortions.…”

“…The roughness values are slightly high, but comparable values have been reported for bedrock channels by other authors (e.g., Dingman, 1984;Heritage et al, 2004). These high values are likely a result of the irregular nature of the bed.…”

“…Bedrock reaches with high water surface slopes or a greater incidence of boulders in the channel also are characterized by high friction factor coefficients. These high values suggest that the flow is severely nonuniform and the resistance coefficients are being used to quantify the effects of energy dissipation from hydraulic jumps and internal distortions resulting from highly irregular planforms, boulders, constrictions and other disturbances (Carling and Tinkler, 1998;Carling et al, 2002;Heritage et al, 2004). This provides a reach-scale focus for investigations of bedrock channel hydraulics, flow routing, and sediment transfer mechanisms, which are of increasing relevance to land use and water resource development (Tinkler and Wohl, 1998a;Carling, 2006).…”

The geomorphological characteristics of the Mekong River in northern Cambodia: A mixed bedrock–alluvial multi-channel network

“…For many of these rivers, the collection of field survey data remains difficult not only due to the scale of the system but also its remoteness and inaccessibility. In particular, detailed geomorphological descriptions of large mixed bedrock-alluvial rivers are limited (van Niekerk et al, 1995;Heritage et al, 2004;Tooth and McCarthy, 2004) and there is no detailed description of channels of this type for a large river within South East Asia. The Mekong River, in the north-east of Cambodia is one such example.…”

Riparian vegetation and the late Holocene development of an anabranching river: Magela Creek, northern Australia

“…(1) fl ow resistance (e.g., Darby, 1999;Kean and Smith, 2004;Heritage et al, 2004); (2) the strength and stability of channel banks, beds, bars, and islands (e.g., Ikeda and Izumi, 1990;Huang and Nanson, 1998;Abernethy and Rutherfurd, 1998;Gurnell et al, 2001;Brooks and Brierley, 2002;Pollen et al, 2004;van de Wiel and Darby, 2004); and (3) channel crosssection and planform adjustments (e.g., Millar, 2000;Gran and Paola, 2001;Murray and Paola, 2003;Tooth and Nanson, 2004;Tooth and McCarthy, 2004;Tal et al, 2004;Tal and Paola, 2007).…”