Work out weir flow rate instantly with clear inputs, formula shown and shareable results.
A sharp-crested rectangular weir passes Q = 1.84 L H^1.5, so 2.5 m of crest under 0.3 m of head gives 0.756 m3/s. A 90 degree V-notch follows H^2.5, which makes it far more sensitive at low flows and the standard choice for gauging small streams. Head must be measured upstream of the drawdown curve.
Rectangular weir
Q = 1.84 x L x H^1.5
90 degree V-notch
Q = 1.38 x H^2.5
Because the flow width grows with head as well as the depth, so discharge rises faster - which gives better resolution at low flows.
At least four times the head upstream of the crest, beyond the surface drawdown, or the reading will be too low.