**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

2. Broad
crested weir (or broad-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 H ^{3/2}**

**where **

**Q
**is
the water flow rate in m^{3} /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)H ^{3/2}**

**where
**

**Q** is
the water flow rate in m^{3} /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

**H _{max}** 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 c _{d} (2 g)^{1/2}
tan(θ/2) h^{5/2}**

**where**

**θ
**=
v-notch angle

**h**=
head of weir

**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 h _{1} ^{3/2}**

**Where,**

L = length of weir crest in
ft

h_{1} = head on
weir crest in ft

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