TABLE OF CONTENTS As discussed in the preceding section, to solve the gradually-varied flow equations we need a boundary condition. To specify this boundary condition, however, we first need to determine, qualitatively, the types of profiles that will occur. This task is not too difficult if we remember some general rules:
Subcritical flow is subject to downstream control
Supercritical flow is subject to upstream control
In the absence of (or far away from) flow controls, flow tends to become normal in prismatic channels
The only possible shapes for the water surface profile occurring in the different zones labeled as M1, S2, etc., are those shown in Figures 4.1, 4.2, and 4.3
Normal flow in a mild channel is subcritical and that in a steep channel is supercritical
Flow is subcritical upstream of a sluice gate and supercritical downstream
When subcritical flow is present in a channel terminating at a free fall, the depth at the free fall will be equal to the critical depth (this is an assumption, and a good one, since the critical depth actually occurs a short distance, about 4 y c, upstream of the free fall)
The change from supercritical to subcritical flow is possible only through a hydraulic jump in a prismatic channel.
For example, let us consider a very long mild channel that terminates at a free fall as in Figure 4.6a. Far away from the free fall the flow...
