Summarise the behavioural differences between subcritical an

Summarise the behavioural differences between subcritical and supercritical flow. How do these two types of flow behave differently?

Solution

Subcritical flow is deep, slow flow with a low energy state and has a Froude number less than one (F<1).

Critical flow occurs when the Froude number equals one (F=1); there is a perfect balance between the gravitational and inertial forces.
Supercritical flow is shallow, fast flow with a high energy state and has a Froude number greater than one (F>1). The energy state of the river affects the potential for entrainment and erosion of the channel boundary as well as affecting the extent to which downstream disturbances to forward flow from boulders or built structures can be transmitted upstream.

When flow conditions are subcritical, disturbances in the flow can generate surface waves, and because the downstream flow is so slow, those surface waves can travel in an upstream direction.

When flow conditions are supercritical, surface waves generated by downstream disturbances cannot travel upstream; the forward flow of water carries the waves downstream.