Weirs, like flumes, work by accelerating slow, sub-critical flow (Fr<1) to faster moving, super-critical flow (Fr>1).  The acceleration develops a know relationship between level and flow upstream of the weir itself.

But what happens when the flow approaching the weir isn’t sub-critical, but instead is critical or super-critical.  Will the weir still operate as usual?

The Iowa Institute of Hydraulic Research answers that question in their video entitled Fluid Motion in a Gravitational Field.

Flow Visualized

Under sub-critical, free-flow conditions with the nappe fully aerated, the flow over a thin plate weir looks like the below (Fr = 0.5).

However, as the velocity of the approaching water increases (Fr > 1), the flow overshoots the weir. 

As the velocity increases the overshoot becomes even greater.

Note that the water level at sub-critical flow is substantially higher than that above unity (Fr = 1).  

While this example is dramatic for effect, the implications are clear:  excessive approach velocities approach thing plate weir must be controlled in order for the weir to function properly.

The video can be view in its entirety below:

 

 

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Source:  Iowa Institute of Hydraulic Research

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