Vertical distribution of shear stress in unsteady open-channel flow

The shear stress on horizontal planes and its vertical distribution in unsteady open-channel flow-exemplified by different hydrographs in a gravel-bed flume-are calculated from the time records of the point velocities, using the equation of motion (equation (1)). It is shown that during a flood hydrograph: shear stress near the water surface arrives at its maximum value later than that near the bottom (Fig. 3); at the same water height, the shear stress in the rising branch can be considerably larger than the one in the falling branch (Fig. 4(b)); the vertical shear-stress profiles for the rising branch are concave, implying accelerating flows, and for the falling branch are convex, implying decelerating flows (Fig. 5). These observations are theoretically explained (equation (9) and Fig. 6).