8. Exit Flow Conditions

Exit flow conditions can cause as much flow measurement error as approach flow problems. However, these conditions are not encountered as often in practice. In general, ensuring that backwater does not submerge or drown out a device designed for free flow is sufficient. Occasionally, a flume is set too low, and backwater submerges the throat excessively, which can introduce extremely large errors in discharge measurement. The only remedy is to raise the flume, unless some local obstruction downstream can be removed to reduce the backwater. Sharp-crested weirs should discharge freely rather than submerged, although a slight submergence (the backwater may rise above the crest up to 10 percent of the head) reduces the discharge a negligible amount (less than 1 percent). However, a weir operated near submergence may not affect the discharge as much as the possible lack of nappe ventilation resulting from high downstream depth or intermittent waves lapping the underside of the nappe.

The underside of weir nappes should be ventilated sufficiently to provide near atmospheric pressure beneath the nappe, between the under-nappe surface, and the downstream face of the weir. The height of pull-up behind the nappe depends upon the drop, discharge, and crest length. The height that the water raises behind the nappe is a measure of the discharge error. For example, if the measuring head on a 3-ft suppressed weir is 1 ft and the water behind the nappe pulls up 0.3 ft, the error of discharge measurement would be about +6.5 percent. If the water was only pulled up 0.1 ft, the error for the same weir and measuring head would be +2.5 percent.

If the head upstream from the weir is pulled down a significant amount, then the weir is not sufficiently ventilated. An easy test for sufficient ventilation is to part the nappe downstream from the blade for a moment with a hand or a shovel to allow a full supply of air to enter beneath the nappe. After removing the hand or shovel, the nappe should not gradually become depressed (over a period of several or more minutes) toward the weir blade. If the upper nappe profile remains the same as it was while fully ventilated, the weir has sufficient ventilation.

If the nappe clings to the downstream side of the weir and does not spring clear, the weir may discharge up to 25 percent more water than the head reading indicates. This problem is generally a low flow problem with heads near and less than 0.2 ft and occurs more frequently with V-notch weirs. Good practice would involve checking the nappe before and after readings.

Gates calibrated only for free discharge at partial openings should not be submerged, nor should eddies interfere with the jet of water issuing from the gate. Gaging stations should be kept free of deposited sediment bars or other side-projecting obstructions to prevent backflow or eddies from interfering with the uniform flow conditions that should exist in the cross section being measured.