ESTIMATION OF RESISTANCE TO FLOW IN GRAVEL-BED STREAMS

Abstract

Resistance characteristics of gravel-bed rivers differ significantly from those of sand-bed rivers; hence estimation of flow resistance in these rivers cannot be achieved through techniques empirically developed for sandLbed rivers. To address the problem, the 'roughness' in gravel-bed rivers Is classified into three scales.' If relative submergence (R/D84) is less• than about 1.2, roughness is large scale. Roughness is small scale if R/D84 exceeds 4 and if it has an intermediate value, the roughness is intermediate scale. A semilogarithmic form of equation based on the boundary layer theory is applicable to small scale roughness. however, available equations explicitly account for boundary resistance only. No success has been achieved in quantifying the effects of important factors such as flow non-uniformity and sediment transport. The boundary layer theory is not applicable in large scale roughness as the velocity profile is influenced by individual roughness elements. The concentration and blocking effect of elements cause a form drag but these effects also remain mathematically unquantified. No theory has explained the flow resistance of intermediate scale roughness. For all these cases, a number of hypotheses, conjectures and suggestions are available. Some of these have been critically analysed in this study and investigations have been carried out to try to get answers to some questions.....