Velocity distribution and shear velocity behaviour of decelerating flows over a gravel bed

The behaviour of the bed shear stress in the presence of a decelerating flow over a gravel bed has been examined. The collected observations revealed that the velocity distribution in the outer region of the boundary layer may be described by a parabolic law. The results obtained by parabolic law are comparable to the bed shear stress estimated via the St-Venant method. At a specific cross section, shear velocities estimated by the parabolic and the St-Venant methods divert considerably from the estimation by zero pressure gradient method. For constant bottom slope and relative roughness, the estimated bed shear stresses based on the parabolic and the St-Venant methods are proportional to the flow discharge, whereas this tendency is not accounted for by the zero pressure gradient model.