Weir

Weirs are used in conjunction with locks, to render a river navigable and to provide even flow for navigation. In this case, the weir is made significantly longer than the width of the river by forming it in a 'U' shape or running it diagonally, instead of the short perpendicular path. Since the weir is the portion where water is overflowing, a long weir allows a lot more water with a small increase in overflow depth. This is done in order to minimize fluctuation in the depth of the river upstream with changes in the flow rate of the river. Doing so avoids unnecessary complication in designing and using the lock or irrigation diversion devices.^[1]

Weirs allow hydrologists and engineers a simple method of measuring the rate of fluid flow in small to medium-sized streams, or in industrial discharge locations. Since the geometry of the top of the weir is known, and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. The calculation relies on the fact that fluid will pass through the critical depth of the flow regime in the vicinity of the crest of the weir. If water is not carried away from the weir, it can make flow measurement complicated or even impossible.

A weir may be used to maintain the vertical profile of a stream or channel, and is then commonly referred to as a grade stabilizer such as the weir in Duffield, Derbyshire.

Because a weir will typically increase the oxygen content of the water as it passes over the crest, a weir can have a detrimental effect on the local ecology of a river system. A weir will artificially reduce the upstream water velocity, which can lead to an increase in siltation. Weirs can also have an effect on local fauna. While a weir is child's play for some fish to jump over, inferior species may be blocked by the weir. Fish ladders provide a way for fish to get between the water levels. Mill ponds provide a watermill with the power it requires, using the difference in water level above and below the weir to provide the necessary energy.

A walkway over the weir is likely to be useful for the removal of floating debris trapped by the weir, or for working staunches and sluices on it as the rate of flow changes. This is sometimes used as a convenient pedestrian crossing point for the river.

Even though the water around weirs can often appear relatively calm, they are dangerous places to boat, swim or wade; the circulation patterns on the downstream side can submerge a person indefinitely. This phenomenon is described in the article on whitewater.

There are different types of weir. It may be a simple metal plate with a V-notch cut into it, or it may be a concrete and steel structure across the bed of a river. A weir which causes a large change of water level behind it, compared to the error inherent in the depth measurement method, will give an accurate indication of the flow rate.

A broad-crested weir is a flat-crested structure, with a long crest compared to the flow thickness (Chanson 1999, 2004, Henderson 1966, Sturm 2001). When the crest is “broad”, the streamlines become parallel to the crest invert and the pressure distribution above the crest is hydrostatic. The hydraulic characteristics of broad-crested weirs were studied during the 19th and 20th centuries. Practical experience showed that the weir overflow is affected by the upstream flow conditions and the weir geometry.

A sharp-crested weir allows the water to fall cleanly away from the weir. Sharp crested weirs are typically 1/4" or thinner metal plates. Sharp crested weirs come in many different shapes such as rectangular, V-notch and Cipolletti weirs.

The sharp crested weirs can be considered into three groups according to the geometry of weir: a) the rectangular weir, b) the V or triangular notch and c) special notches, such as trapezoidal, circular or parabolic weirs. For accurate flow measurement over a wider range of flow rates, a combination weir combines a V-notch weir with a rectangular weir. An example is manufactured by Thel-Mar Company and has flow rates engraved along the side of the weir. This is typically used in pipes ranging from 4" to 15" in diameter.

The V-notch weir is a triangular channel section, used to measure small discharge values. The upper edge of the section is always above the water level, and so the channel is always triangular simplifying calculation of the cross-sectional area. V-notch weirs are preferred for low discharges as the head above the weir crest is more sensitive to changes in flow compared to rectangular weirs.

The concept of the Minimum Energy Loss (MEL) structure was developed by Gordon McKay in 1971.^[2] The first MEL structure was the Redcliffe storm waterway system, also called Humpybong Creek drainage outfall, completed in 1960 in the Redcliffe peninsula (Australia). It consisted of a MEL weir acting as a streamlined drop inlet followed by a 137 m long culvert discharging into the Pacific Ocean. The weir was designed to prevent beach sand being washed in and choking the culvert, as well as to prevent salt intrusion in Humpybong Creek without afflux. The structure is still in use and passed floods greater than the design flow in several instances without flooding (McKay 1970, Chanson 2007).

The concept of the Minimum Energy Loss (MEL) weir was developed to pass large floods with minimum energy loss and afflux, and nearly-constant total head along the waterway. The flow in the approach channel is contracted through a streamlined chute and the channel width is minimum at the chute toe, just before impinging into the downstream natural channel. The inlet and chute are streamlined to avoid significant form losses and the flow may be critical from the inlet lip to the chute toe at design flow. MEL weirs were designed specifically for situations where the river catchment is characterised by torrential rainfalls and by very small bed slope. The first major MEL weir was the Clermont weir (Qld, Australia 1963), if the small control weir at the entrance of Redcliffe culvert is not counted. The largest, Chinchilla weir (Qld, Australia 1973), is listed as a "large dam" by the International Commission on Large Dams.

• Fishing weir
• Drop structure

• Chanson, H. (2004). "The Hydraulics of Open Channel Flow : An Introduction." Butterworth-Heinemann, Oxford, UK, 2nd edition, 630 pages (ISBN 978 0 7506 5978 9).
• Chanson, H. (2007). Hydraulic Performances of Minimum Energy Loss Culverts in Australia, Journal of Performances of Constructed Facilities, ASCE, Vol. 21, No. 4, pp. 264–272 (doi:10.1061/(ASCE)0887-3828(2007)21:4(264)).
• Gonzalez, C.A., and Chanson, H. (2007). Experimental Measurements of Velocity and Pressure Distribution on a Large Broad-Crested Weir, Flow Measurement and Instrumentation, 18 3-4: 107-113 (DOI 10.1016/j.flowmeasinst.2007.05.005).
• Henderson, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA.
• McKay, G.R. (1971). "Design of Minimum Energy Culverts." Research Report, Dept of Civil Eng., Univ. of Queensland, Brisbane, Australia, 29 pages & 7 plates.
• Sturm, T.W. (2001). "Open Channel Hydraulics." McGraw Hill, Boston, USA, Water Resources and Environmental Engineering Series, 493 pages.

• Hydraulics of Minimum Energy Loss (MEL) culverts and bridge waterways (Click "proceed" at the UQ-ITS Advisory webapge)