Measuring weirs are overflow structures built across a flow channel to measure discharge. For a given set of weir and channel geometry conditions, a single head value on the device may exist for each discharge under a free-flow, steady state regimen. The existence of such a relationship makes constructing a rating curve of head versus discharge a simple task. Such rating curves are available in the literature for most common configurations (such as rectangular weirs, V-notch weirs, vertical slot weirs, and trapezoidal weirs without the bottom part) (U.S. Department of Interior 1975).

One advantage of weirs is their large relative measurement range. However, weir installation in sewers reduces pipe capacity, may lead to solids accumulation (particularly in combined sewers), may distort flow hydrographs, and may limit operating range because of surcharging or submerging. These constraints will eliminate weirs from consideration for certain locations, but many of the above difficulties can be avoided in open-channel installations at outfalls. For these reasons, weirs should be used only under carefully controlled conditions, such as at detention basin outlets, where suspended solid concentrations are likely to be low.

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