V-Notch Weir Overflow: an Unsteady Calibration

Thin-plate V-notch weirs are commonly used as measuring devices in flumes and channels. Herein, the discharge calibration of a large 90o V-notch weir was performed using an unsteady volume per time technique. The weir overflow was initially shut. The sudden opening of the gate was associated with an initial phase of transient water motion associated with a negative wave propagating in the reservoir and the free-falling motion of a mass of fluid in the close proximity of the overflow behind the initial wave. This initial phase was followed by a more gradual overflow motion which was affected by the seiche in the upstream reservoir. The integral form of the continuity equation was used to derive the relationship between flow rate and upstream water depth. The findings showed that the unsteady discharge calibration of the V-notch weir yielded similar results to a more traditional calibration approach based upon steady flow experiments, while allowing the rapid testing of a wide range of discharges.