WHAT IS WEIR? | FUNCTIONS OF A WEIR | TYPES OF WEIRS | LCETED -lceted LCETED INSTITUTE FOR CIVIL ENGINEERS

## Jun 25, 2021

WHAT IS WEIR?

A weir, also known as a low-head dam is a small overﬂow-type dam commonly used to raise the level of a river or stream. Weirs have traditionally been used to create mill ponds in such places. Water ﬂows over the top of a weir, although some weirs have sluice gates, which release water at a level below the top of the weir. The crest of an overﬂow spillway on a large dam is often called a weir.

FUNCTIONS OF A 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 overﬂows, 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.

A weir allows 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.

A weir will typically increase the oxygen content of the water as it passes over the crest, and hence it 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. The weir may pose a barrier to migrating ﬁsh. Fish ladders provide a way for ﬁsh to get between the water levels. Mill ponds provide a water mill 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.

TYPES OF WEIRS

There are diﬀerent types of weirs. 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 that 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.

1.     Sharp crested weir

3.     Crump weir (named after the designer)

4.     Needle dam

5.     Proportional weir

6.     Combination weir

7.     MF weir

8.     V-notch weir

9.     Rectangular weir

10.    Cipolletti (trapezoidal) weir

11.    Labyrinth weir

TYPES OF SHARP-CRESTED WEIR RECTANGULAR WEIR NOTCH

A symmetrically located rectangular notch in a vertical thin (metallic) plate placed perpendicular to the sides and bottom of a straight channel is defined as a rectangular sharp crest weir.

SUBDIVISIONS OF RECTANGULAR WEIR NOTCH

SUPPRESSED RECTANGULAR WEIR

Suppressed rectangular weir, for which the weir extends across the entire channel so that the length of the weir, L, is equal to the width of the channel.

The discharge on the suppressed rectangular line can be calculated as follows:

Q = 1.84 B H3/2

where

Q is the water flow rate in m3 /sec,

B is the length of the weir (and the channel width) in m,

H is the head over the weir in m.

CONTRACTED RECTANGULAR WEIR

A contracted rectangular weir is a weir in which the weir extends over only part of the channel, so that the length of the weir, L, is different from the width of the channel.

The flow rate on the contracted rectangular notch can be calculated as follows:

Q = 1.84(L – 0.2H)H3/2

where

Q is the water flow rate in m3 /sec,

L is the length of the weir in m, and

H is the head over the weir in m.

B is the width of the channel in m, and

Hmax is the maximum expected head over the weir in m.

APPLICATION

Data from flow rate calculations on a rectangular weir can be used in a number of ways. Flood control and public water management policies and practices are often designed around such data. Flow data can be used to determine whether a hydropower project is feasible or profitable. Water flow data can also be useful for environmental impact studies, especially in determining how weir or other structures affect the ecosystem of a stream or river. Irrigation and other water use projects also benefit from this type of data

TRIANGLE OR V-NOTCH WEIR

The V-shaped notch is a vertical thin plate that is placed perpendicular to the shoulders and the V-notch at the bottom of the straight channel is defined as a sharp ridge. The line dividing the line angle should be vertical and at the same distance from both sides of the channel. The V-Notch sharp-crested wire is one of the most accurate discharges measuring devices suitable for a wide range of currents. In international literature, V-Notch Sharp-Crested-Weir is often referred to as ‘Thomson Weir’.

Triangular or V-notch weirs measure low flows more accurately than horizontal weirs. The V-notch is most often a 90 ° opening with the sides of the notch tilted 45 ° from the vertical. Since the V-notch weir does not have a crest length, much lower flows are represented by a given drop height than for a rectangular weir. For a triangular or V-notch weir, the discharge can be expressed as follows:

q = 8/15 cd (2 g)1/2 tan(θ/2) h5/2

where

θ = v-notch angle

cd= discharge constant for the weir - must be determined

g = 9.81 (m/s2 ) – gravity

Application

The V-Notch Weir system uses the water gravity discharge principle on a triangular or rectangular notched wire plate.

Common applications include: Long-term monitoring of dam dams Drainage systems in dams and tunnels • Springs‌ and artesian wells

TRAPEZOIDAL SHARP-EDGE WEIR

The Cipolletti or Trapezoidal Sharp-edge Weir resembles a rectangular weir with a retracted end except that the sides are slanted outward with a ramp of horizontal 1 to vertical 4. This slope essentially causes the discharge to occur as if there was no end contraction. The advantage of this weir is that no correction for distal contraction is required. The downside is that the measurement accuracy is inherently lower than what can be achieved with a rectangular restraint or V-notch weir. Cipolletti Weir is commonly used in irrigation systems. The generally accepted formula for calculating emissions through Cipolletti weirs is:

Q = 3.367 L h1 3/2

Where,

L = length of weir crest in ft

h1 = head on weir crest in ft

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