Bar Form Resistance in Gravel‐Bed Rivers

Abstract

Resistance to flow in unvegetated gravel‐bed rivers is basically dependent on the skin resistance, due to surface bed material, and bar form resistance, due to accelerations and decelerations in the flow between pools and riffles. Although flow is generally nonuniform, at riffle sections flow is locally uniform. It is these sections which principally control the velocity‐depth characteristics of the reach since hydraulic conditions in the pool result from the backwater effect of the riffle. Isolating the effect of bar forms on flow resistance is achieved by first establishing the roughness height of the surface bed material on the riffle. Overall resistance to flow, in terms of the total roughness height due to bed forms and grains for equivalent uniform flow, is determined from the riffle and reach average flow geometry and the grain roughness height. Differences between total and grain roughness heights define the bar form effect. Field data from 62 sites in the United Kingdom illustrate that at bankfull flow bar roughness heights are generally in the range 0‐0.5 m, although values as high as 1.5 m are also observed.