For Thin-Plate Weirs, the correct point of measurement is 3-5 times the maximum head (3-5 Hmax) upstream of the weir crest. Closer than this and the flow begins to draw down as it approaches the weir crest.
The level should NOT be take at the face of the weir as the level there is lower than the level at the correcrt point of measurement.
Image: Montana Water Commissioners Training
