 The specific energy diagram is specific to the unit discharge for a given flow rate.For any given flow an obstruction such as a sluice gate, a step in the channel bottom, or a constriction might require more energy than the flow originally possesses, and thus a transient condition is set up where the unit discharge is temporarily reduced as the flow backs up and gains energy.As an example of this consider a sluice gate that lowers below the alternate depth of the flow described above (1.4 ft).