I have thin weir, really just a sheet of metal with a smooth and straight upper edge. I want to know how much water will flow over this weir, when the water level on the higher side is a few cm above the edge.  

I'm interested in the special case when the water on the lower side i below the level of the weir:  

[![enter image description here](https://i.stack.imgur.com/mceEg.png)](https://i.stack.imgur.com/mceEg.png)

* What formula describes this flow?
* What are important caveats?