V-Notch Weir, 45 Degree Incline Angle

Triangular or V-notched weirs measure low discharges more accurately than horizontal weirs. The V-notch is most commonly a 90° opening with the sides of the notch inclined 45° with the vertical. Since the V-notch weir has no crest length, much smaller flows are represented by a given head than for a rectangular weir. For example, at a head of 0.2 feet, the discharge through a 1 foot rectangular weir with end contraction would be about six times that through a 90° V-notch. V-notch weirs should always be used when frequent low flows are included in the overall range to be measured.

Other angles used for V-notch weirs are 22 1/2°, 30°, 45°, 60° and 120°. The V-notch weir is best used for flows under 10 CFS. Also the minimum head should be at least 0.2 foot to prevent the nappe from clinging to the crest.