Unsteady Flow in Open Channels

6.1.1 Introduction. In considering unsteady flow in channels it is necessary to add time as a variable to the other combinations of hydraulic variables considered primarily in the first three chapters. By virtue of its free surface, unsteady channel flow is essentially non-uniform flow and it can therefore be conveniently considered in two categories, gradually varied and rapidly varied unsteady flow. Such a classification is made on the same basis as that used for steady non-uniform flow. The first category will have almost parallel streamlines, gradual changes in depth, vertical accelerations will play a minor part and channel friction forces a large one. The second category will be characterised by very pronounced curvature of the streamlines, rapid changes in depth and sometimes discontinuous water surface profiles, large vertical accelerations and channel friction forces of secondary importance.