Response of velocity to a sudden change of bed roughness in sub critical open channel flow

摘要

Large variations in the flow field and bottom stress may be determined by an abrupt change of the channel bed from smooth to rough. The variation in the mean local velocities propagates in the current by a gradual increase in thickness of an internal boundary layer. Instead, the presence of a sudden peak in shear velocity downstream the bed change may be observed, the so-called overshooting. The entity of the peak is related to several factors, including the entity of the roughness change, the size of the bed elements and emergence of the rough part of the bed with respect to the smooth one, the reduction in the current cross section due to water level depression and bed elevation and the entity of the pressure gradient. A large number of experiments appears necessary in order to investigate all these interactions: the writers are currently carrying out such experimental work. The results of a part of these experiments are discussed in this paper. An abrupt change in bed roughness from smooth to rough was brought about by positioning a single layer of stones on the smooth bed of a flume. Emersion of the rough part of the bed with respect to the smooth one, flow rate, water levels were systematically varied. Experiments confirmed that the spatial distribution of bottom shear stresses down the roughness change is related both to flow characteristics and bottom geometry.