Unsteady discharge calibration of a large V-notch weir

Thin-plate weirs are commonly used as measuring devices in flumes and channels, enabling an accurate discharge measurement with simple instruments. The calibration formulae of such devices rely upon some empirical coefficients and there is a need to obtain new accurate physical data to complement the existing evidence. In the present study, the discharge calibration of a large 90° V-notch thin plate weir was performed using an unsteady volume per time technique. The V-notch weir was initially closed by a fast-opening gate. The sudden opening induced an initial phase of the water motion dominated by the free-falling motion of a volume of fluid in the vicinity of the weir, followed by a gradually-varied phase, during which some seiche was observed in the tank. The relationship between water discharge and upstream water elevation was derived from the integral form of the continuity equation. The results yielded a dimensionless discharge coefficient Cd=0.58 close to previous experiments for 90° V-notch weirs. 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, allowing a rapid testing over a broad range of flow rates.

► Unsteady discharge calibration was performed with a 90° V-notch weir.
► The sudden gate opening induced an initial phase dominated by a free-falling motion.
► This was followed by a gradually-varied phase with some seiche in the tank.
► The results yielded a dimensionless discharge coefficient Cd=0.58.
► The unsteady discharge calibration yielded similar results to a more traditional calibration approach.