EXPERIMENTAL STUDY OF FLOW RESISTANCE IN GRAVEL -BED STREAMS

Abstract

In the case of the sand bed rivers, a large number of theoretical as well as the empirical type of resistance relationships are available. Since the resistance, characteristics of gravel bed rivers differ significantly from that of sand bed rivers; estimation of flow resistance in these rivers cannot be achieved through the techniques developed for the sand bed rivers. To address the problem, 'roughness' in gravel bed rivers, has been classified into three scales in terms of relative submergence (R/D84). if R/D84 is less than about 1.2, roughness is large scale. Roughness is small scale if R/D84 is greater than 4 and it is in the intermediate scale, if the value falls within this range. The boundary layer theory initially developed for the pipe flow problems can. be successfully applied in the case of small-scale resistance. However in the case of the large-scale roughness, velocity profile is influenced by individual roughness elements and form drag. is the most important factor in the flow because of the blocking and concentration of the roughness elements. The flow characteristics also do not remain uniform in such cases. No theory has explained the flow resistance of intermediate scale roughness. It is realized that the available equations for the prediction of resistance in gravel bed streams have, limited applicability because of small database used and the non- consideration of entire range of flow.