Critical flow

Understanding critical flow
The concept of critical flow is not an easy one to impart, but some understanding of it is necessary to understand the full range of flow conditions that may exist in a reach. We start by understanding that water has appreciable internal cohesion. As water accelerates, that is it moves away from water flowing more slowly behind it, it does not initially separate, butrather stretches, as the forces of cohesion, including gravity and viscosity, are being overcome by the increasing movement of the water. This can be seen as water passes over a weir or approaches a constriction in the channel (e.g. a flume). The water falling over the crest of the weir is accelerating and indeed may break up, but as it does so its store of kinetic energy increases until it isreleased in a chaotic explosion of activity. This increase in kinetic energy can be seen by a noticeable dip in the water surface just upstream of the crest of the weir. Prior to this dip the velocity of water is said to be sub-critical; at the dip it is critical and below the dip, it is super-critical.

Stream flow cannot go on accelerating in the natural river environment. At some stage theirneeds to be a resolution of super-critical flow, as deceleration occurs and the water assumes the same velocity as the water in front of it (sub-critical velocity). It does so by means of a hydraulic jump, where the super-critical flow, on reaching the slow moving water at the base of the weir, riffle, set of rapids, or individual rock or log, is swept upwards and falls back on itself, like acrashing wave, but falling in the upstream direction. Here the water decelerates and returns to its sub-critical state, to join the slower, placid, steadily moving downstream flow, unless of course there is yet another fall to be negotiated. Over the course of the hydraulic jump undissipated kinetic energy is explosively converted into turbulence and potential energy which lifts the water level after thejump. Whether a section of channel will yield a change in (critical) flow, from sub-critical to super-critical, can be assessed by calculating the Froude number.

Essentially the Froude number compares the velocity of flow with what is called the critical velocity for the reach - the point at which water appears to stretch out. A value of one is the point of critical velocity, which itself issimply the square root of the product of acceleration due to gravity multiplied by the depth of the reach.

Depth and gravity are the key factors in determining critical flow. Where the Froude number is less than 1 the water flow has no opportunity to accelerate beyond the critical velocity. It is hindered by the bulk of water immediately downstream. The flow is considered sub-critical.Turbulence can and does exist but local velocities are not sufficient to overcome the internal viscous character of the water. At a Froude number greater than 1 the water flow has been able to accelerate (over a hydraulic drop) and has now exceeded the critical. Its energy state is unstable and its flow is considered super-critical. The sudden enforcement of sub-critical conditions on this flow as itre-encounters the inertia of the water downstream, results in a release of internal kinetic energy, some of which overcomes the cohesive forces holding the water molecules together foaming the water and tossing it about as can be seen in rapids



The significance of super critical flow
Super critical flow conditions have both ecological and hydrological implications of great significance tostream management.

For the stream ecosystem the hydraulic jump is a very significant component. Firstly it entrains air into the water column, oxygenating the stream ecosystem. Secondly, it provides a ‘launch pad’ for fish seeking to leap obstructions in their efforts to move upstream. For jumps that have a chute form, water may also swirl upstream either side of the fall of water, again...