Rapidly Varied Flow
Critical , Sub-critical and super critical flow
The specific energy change with depth was plotted above for a constant discharge Q, it is also possible to plot a graph with the specific energy fixed and see how Q changes with depth. These two forms are plotted side by side below.
From these graphs we can identify several important features of rapidly varied flow.
For a fixed discharge:
1. The specific energy is a minimum, E sc , at depth y c , This depth is known as critical depth .
2. For all other values of E s there are two possible depths. These are called alternate depths. For
subcritical flow y > yc
supercritical flow y < yc
For a fixed Specific energy
1. The discharge is a maximum at critical depth, yc
2. For all other discharges there are two possible depths of flow for a particular Es
i.e. There is a sub-critical depth and a super-critical depth with the same Es
An equation for critical depth can be obtained by setting the differential of E to zero: