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Giles Waley gravatar image
RedR

Assuming your weir behaves close to one of a few classic designs then it’s fairly straightforward to get a close-enough approximately of flow. Bear in mind that these “classics” are those specifically designed for flow measurement, so may well not be perfect for other types. For flat weirs (ie where the crest of the weir is horizontal), check out the following estimations:

(i) Broad crested weir (ie one that has a flat surface to the weir crest): Flow [m3s-1] = 2.1 x crest width [m] x (water depth over crest [m] ^ 1.5) (ii) Sharp crested weir (ie normally a metal plate): Flow [m3s-1] = 1.75 x crest width [m] x (water depth over crest [m] ^ 1.5)

So if I understand your blurb correctly and we assume you have a broad crested weir, then flow at that moment = 0.9m3s-1 or 900 litres per second.

If you have an 80m head, and you take 20% of the water that you saw available that day, at an efficiency of 60%, multiply by gravity, then available power capacity = 85kW.

60% is quite low, so you will probably find that 70% is more appropriate unless you’re planned scheme is very ‘agricultural’. Modern schemes in the UK can easily achieve efficiencies of over 80%.

20% of available water is also very low. Was the day you took the measurements part of a very wet day? If not then you could easily up this depending on how it compared to an average day and how much water you need to leave behind for river ecology and any downstream uses.