Field Predictability of Flume and Weir Operating Conditions

Laboratory studies are conducted on a rectangular broad‐crested weir to determine the effects of entrance and exit conditions on the discharge and required energy loss. In doing so, a method for visually determining whether downstream conditions affect the flume calibration is developed. The current mathematical model of these flumes satisfactorily predicts both discharge and required head losses. Approach‐ramp slopes of 3:1 and 2:1 are shown to be acceptable for accurate discharge prediction, although a slight rounding of the approach ramp corners will improve predictions. At high ratios of energy head to throat length and with a sudden downstream transition, the model slightly underpredicts the required energy loss. As long as the water‐surface profile is dropping continuously from the approach ramp to the end of the throat, the downstream water level is not influencing discharge prediction. Field observations and measurements support these laboratory results, which should apply to most long‐throated flumes, since a rectangular broad‐crested weir represents the worst‐case conditions for head losses.