ALLUVIAL RESISTANCE IN TRANSITION REGIME

The resistance of alluvial channels depends upon flow and sediment cha racteristics. This paper considers the bed features and alluvial resis tance that occur at, and close to, the transition zone between the upp er and lower flow regimes. New flume and river data are published and the results are analyzed to provide satisfactory predictive techniques covering the lower, transition, and upper regimes for use by river en gineers. The bed features that occur in the transition regime are show n to be washed-out dunes that diminish rapidly in height and steepness with increasing shear stress. The increase in depth with increasing u nit discharge is much reduced in the transition regime due to lower hy draulic resistance. In certain circumstances, depths in the transition region can decrease with increasing unit discharge. A criterion for t he upper limit of the transition regime corresponding to a moving flat bed is proposed by the writers.