Weirs: Types, Functions, and Design Considerations

What Is Weirs?

Weir is characterised as a barrier through which water flows into an open channel. The edge or surface on which the water runs is called the crest. The overflowing water layer is the water table.

If the nappe is discharged into the air, the weir gets a free discharge. If the discharge is partially under water, the weir is either buried or drowned.

A weir is a concrete or masonry bridge that is built over an open canal (such as a river) to alter the water flow characteristics. Weirs are built as an obstacle to the passage of water. This are widely used to calculate the volumetric flow of water, avoid floods, and make rivers navigable.

Types of Weirs

1. Types of Weirs depending on the Opening Shape.
   1. Rectangular weir.
   2. Triangular weir.
   3. Trapezoidal weir.
2. Types of Weirs dependent on the Crest form.
   1. Sharp- weir crested.
   2. Broad crested weir.
   3. Narrow crested weir.
   4. Weir Ogee-shaped.
3. Types of weirs depending on the effect of the sides on the nappe.
   1. Weir with the end of contraction (contracted weir).
   2. Weir without the end of contraction (suppressed weir).

1. Classification Based on Shape of Opening

1.1. Rectangular Weir

It’s the standard form of a weir. The upper edge of the weir can be sharply crested or narrowly crested. It is normally ideal for wider flow channels.

Rectangular weir derives its name from the form of the notch. The discharge through such a weir or notch is connected directly to the depth of water (H) and H is regarded as the head.

One such head is influenced by the state of the crest, the contraction, the strength of the incoming stream as well as the elevation of the water surface downstream of the weir. Rectangular wires can be suppressed, partly contracted or completely contracted.

Flow Over Rectangular Weir

In order to find a flow over a rectangular weir, assume the elementary horizontal strip of water thickness dh and length L at depth h again from water level.

Strip area = L x dh

Theoretical speed of water = √2gh

As a result, discharge by strip

dQ = Cd x area of strip x √2gh

dQ = Cd x L x dh x √2gh

Where,

Cd = Discharge Coefficient

Through integrating of above formula with the limitations 0 to H, we could achieve the complete discharge Q.

Q = ∫_O^H Cd x L x dh x √2gh

In the end, discharge over the weir

Q = 2/3 x C_d x L x √2g x H 3/2

1.2. Triangular Weir

The form of the weir is basically a reverse triangle like V, so it’s often called V-notch weir. This kind of wire is well suited for calculating discharge over small flows with greater precision.

Flow Through Triangular Weir

In this considering the elementary horizontal strip of water width, i.e. distance h from the surface of the water.

Tan [θ/2] = [(AC)/(H-h)]

Therefore,

Area of Strip = dh x Width of strip AB

Area of Strip = dh x 2AC

Area of Strip = 2 (H-h) x Tan θ/2 x dh

Theoretical velocity of water = √2gh

Therefore,

Discharge through strip (dQ) = C_d x area of strip x velocity of water

dQ = C_d x Area of strip x √2gh

dQ = C_d x (2 (H-h) x Tan θ/2 x dh) x √2gh

Through integrating of above formula with the limitations 0 to H, we could achieve the complete discharge Q.

Therefore,

Q =  ∫_O^H C_d x (2 (H-h) x Tan θ/2 x dh) x √2gh

In conclusion, we develop

Q = (8/15) x C_d x Tan θ/2 x √2g x H ^5/2

1.3. Trapezoidal Weir

Trapezoidal weir is often named Cippoletti weir. That’s also trapezoidal in form and alters the rectangular weir with such a slightly greater potential for about the same crest weight.

The sides are bent outward with a slope of 1:4.

Flow Through Trapezoidal Weir

In cippoletti, all sides have an even slope. So, we will segment the trapezoid into rectangles and triangles.

So, Well, Full discharge over a trapezoidal weir

Q = discharge over rectangular weir + discharge through triangular weir

Q = [(2/3)C_d x L x √2g x H ^3/2 ] + [ (8/15) x C_d x Tan θ/2 x √2g x H ^5/2]

Also, Read: Difference Between Plinth Level, Sill Level, and Lintel Level.

2. Classification According to Shape of Crest

2.1. Sharp Crested Weir

The peak of the weir is so sharp that perhaps the water is visible from the crest. The weir plate is beveled edge at the edges of the crest to achieve the required thickness. And the plate should be made of smooth brass, free from corrosion and nicks.

Flow over a sharp-edged weir is similar to a rectangular weir. They are commonly used to assess the water on the farm. They are usually of three forms depending on the form of the notch.

These are

1. Rectangular Weir
2. Cipoletti Weir or Trapezoidal Weir
3. V Notch Weirs or Triangular Weir

2.2. Broad Crested Weir

They are formed only in a rectangular form and are ideal for larger flows. Head damage would be minimal in the event of a large crested weir.

A weir which has a horizontal or almost horizontal crest sufficiently long in the direction of the flow such that the nappe is stabilised and the hydrostatic pressures are completely formed for at least a short distance.

2.3. Narrow Crested Weir

It is identical to a rectangular weir with a narrow rounded crest at the end. The discharge over shallow crested weirs is close to the discharge over rectangular weirs.

2.4. Weir Ogee Shaped

Normally, ogee-shaped weirs are established for the spillway of the storage dam. The crest of the ogee weir rises slightly and sinks into a parabolic shape. Flow over ogee weir is almost close to flow over rectangular weirs.

3. Classification Based on End Contractions

3.1. Contracted Weir

The crest is cut in the shape of a notch and is similar to a rectangular weir. Head loss is going to occur in this type.

The sides and the crest of the weir are far from the sides and the bottom of the approach channel. The nappe is fully contracted laterally at the ends and vertically at the crest of the weir. It was often considered an “unsuppressed” weir.

3.2. Suppressed Weir

The crest runs all the way through the river so the head loss is marginal. Rectangular wire, the notch or the opening sides of which coincide with the sides of the approach canal, often rectangular, extending unaltered downstream from the weir.

This is the horizontal flow contraction that is “suppressed.”

Advantages of Weirs

1. Able to measure a diverse variety of flows correctly.
2. Can be compact and adjustable.
3. Easy to build Seems to have a more precise discharge rating than flumes and orifices.

Disadvantages of Weir

1. Relatively large head required, especially in free-flow conditions.
2. The upstream pool must be kept clear of sediment and kept free of weeds and litter. Otherwise, the accuracy of the calculation would be impaired.

Operation & Limitations For Weir

1. Properly designed and mounted wires provide the most precise flow measurement. However, incorrect setting and procedure can result in significant errors in the calculation of discharge. In order to ensure accurate calculation performance, the following steps are mandatory for the use of weirs.
2. The weirs should be set at the bottom of a long pool sufficiently wide and deep with a steady flow at speeds of less than 15cm/sec.
3. Baffles can be placed in a weir pond to minimise velocity.
4. The wall of the weir must be vertical.
5. The middle line of the wire should be parallel to the direction of flow.
6. The crest of the weir ought to be level such that the water flowing into it would be at the same width at both points around the crest.
7. Notch should be a normal form and should have a rigid and straight tip.
8. The weir crest is to be above the bottom of the approach channel.
9. The crest of the weir should be set high enough to allow the water to fall freely below the weir.
10. The depth of the water flow over the rectangular weir should not be less than 5 cm and not more than 2/3 crest distance.
11. The scale or gauge used to measure the head should be about four times the approximate head. Zero of magnitude should be precisely at the same degree as the crest of the weir.

Limitations of Weirs

1. Weirs are not necessarily optimal for flow measurement. A sufficient head is required to operate some kind of weir.
2. They are not correct until the right circumstances are maintained.
3. They need a significant loss of head, which is often not possible in flat gradient channels.
4. Weirs are not ideal for silt transporting water.
5. Weirs are not conveniently paired with a turnout structure.

Location of Weirs

1. A weir must be situated in a balanced section of the river, where even the river is unwilling to alter direction.
2. The weir needs to be installed high enough to meet the command specifications. During high flooding, the river could overwhelm its banks and reverse its course. The position with firm, well-defined banks must therefore be chosen for the construction of the weir.
3. The site must have nice bed requirements, including such rock outcrops, where appropriate.
4. Optionally, weirs must be reduced to a minimum.

Frequently Asked Questions (FAQ) About Weirs

What is a weir?

A weir is a structure built across an open channel, such as a river, to control or measure the flow of water.

What are the main types of weirs?

Weirs are classified based on their shape, including rectangular, triangular (V-notch), and trapezoidal (Cippoletti). They can also be categorized by the shape of their crest and the contraction of flow at the edges.

What are weirs used for?

Weirs serve several purposes, including measuring flow rates, controlling floods, making rivers navigable, and regulating water levels in canals and reservoirs.

How do different types of weirs affect water flow?

The shape of the weir affects how water flows over it, influencing factors such as discharge rate and the accuracy of flow measurement.

What are the advantages of using weirs?

Weirs are precise in measuring flow rates, adjustable, and relatively easy to construct compared to other flow measurement devices like flumes or orifices.

What are the challenges or limitations of using weirs?

Weirs require a sufficient head (difference in water levels) to operate effectively and can be prone to errors if not properly maintained or installed in appropriate conditions.

How should a weir be installed for accurate flow measurement?

Proper installation involves placing the weir at the bottom of a long, wide pool with steady flow, ensuring the crest is level and the flow is not obstructed by debris or sediment.

Where should a weir be located?

Weirs should be located in sections of the river with stable flow patterns and solid bed conditions, ensuring they can function effectively even during high water levels or floods.

What are the design considerations for choosing a weir type?

Design factors include flow conditions, required accuracy of measurement, and the hydraulic characteristics of the water body where the weir will be installed.

How can weirs contribute to water management and conservation?

By accurately measuring flow rates and regulating water levels, weirs help in efficient water resource management, ensuring optimal use and conservation.