8/3/2019 Weirs and Flumes

1/3

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

Weirs can be used for both high flows with the discharge measuredby the water stage in the pool behind the weir or for volumetricflows in extremely low flow conditions that are too small to measureby current meter. One disadvantage of using weirs is that in

sediment laden streams the weir will allow sediment depositions inthe pool above the weir.

Many formulas and shapes and sizes of weirs are used to computethe discharge rate. Some commonly used weirs will be describedhere. Materials are typically aluminum or stainless steel.

8/3/2019 Weirs and Flumes

2/3

Flumes are primary devices that constrict an openchannel flow for measurement. Once the flow is backed up behind

the constriction there is a defined relationship between the depth upstream and the flow through the constriction. This relationship caneither be determined from an equation or a table.

The flume may be operated as a free-flow, single-headmeasuring device, or operated under submerged-flow conditionswhere two heads are measured. The head in the converging sectionand the head near the downstream end of the throat section are readon staff gages or in stilling wells. Both gages have their datum at theelevation of the floor of the converging section. Free flow occurswhen the ratio of the lower gage reading to the upper gage is lessthan 0.6. The discharge under this condition depends only on thelength of crest (width of throat section) and depth of water at theupper gage. Submerged flow occurs when the ratio of the lower gage

8/3/2019 Weirs and Flumes

3/3

reading to the upper gage reading exceeds 0.6. When this occurs, areduction adjustment to the free-flow rating of the flume is needed.Since the flow velocity is high it is considered self-cleaning insediment laden streams and deposition of sediments is practically

eliminated. A flume that is properly constructed has an accuracy of2-3 percent under free-flow conditions, but is less accurate duringsubmerged flow.

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

Compiled by MagicWand

Thanks to Rickly Hydrological Company