A weir is a low dam or overflow structure built across an open channel. It has a specific size and shape with a unique free-flow, head-discharge relationship. The edge or surface over which the water flows is called the crest. Discharge rates are determined by measuring the vertical distance from the crest to the water surface in the pool upstream from the crest.

Weirs can be used for both high flows with the discharge measured by the water stage in the pool behind the weir or for volumetric flows in extremely low flow conditions that are too small to measure by current meter. One disadvantage of using weirs is that in sediment laden streams the weir will allow sediment depositions in the pool above the weir.

Many formulas and shapes and sizes of weirs are used to compute the discharge rate. Some commonly used weirs will be described here. Materials are typically aluminum or stainless steel.

Flumes are primary devices that constrict an open channel flow for measurement. Once the flow is backed up behind the constriction there is a defined relationship between the depth up stream and the flow through the constriction. This relationship can either be determined from an equation or a table.

The flume may be operated as a free-flow, single-head measuring device, or operated under submerged-flow conditions where two heads are measured. The head in the converging section and the head near the downstream end of the throat section are read on staff gages or in stilling wells. Both gages have their datum at the elevation of the floor of the converging section. Free flow occurs when the ratio of the lower gage reading to the upper gage is less than 0.6. The discharge under this condition depends only on the length of crest (width of throat section) and depth of water at the upper gage. Submerged flow occurs when the ratio of the lower gage reading to the upper gage reading exceeds 0.6. When this occurs, a reduction adjustment to the free-flow rating of the flume is needed. Since the flow velocity is high it is considered self-cleaning in sediment laden streams and deposition of sediments is practically eliminated. A flume that is properly constructed has an accuracy of 2-3 percent under free-flow conditions, but is less accurate during submerged flow.

The level of flow can be measured by a staff gage, float in an adjacent stilling well, a submersible pressure transducer (SPT) or by an ultrasonic sensor.