Weir

A weir is basically a barrier in the flow path of an open channel flow, it causes an increase in water depth as water flows over the weir.

A normal, the higher the flow rate, the greater the increase in flow depth and the height of the water above the top of the weir is the measure that is usually used to correlate with the flow rate.

There is a solid barrier placed across the river, its main function is to raise the water level so that water can be diverted by the canal to the crop field due to the difference of head.

Types of Weir:

1.Vertical Drop Weir:

A crest gate could also be provided to store extra water through the flood period.

On the upstream and downstream ends of the impervious floor reduce off, piles are provided, the launching apron is provided both above and below the floor to protect against scouring action.

A graded filter is immediately provided at the lower end of the impermeable floor to reduce the uplift pressure, this type of wear is suitable for every type of foundation.

2.Sloping Weir of Concrete:

This type is suitable for soft sandy foundations used where the difference between the weir crest and downstream river bed is not exceeded by 3 meters.

A hydraulic jump is formed when the water passes over the sloping glaciers, these types of weirs are of recent origin.

3.Dry Stone Slopping Weir:

It is a dry stone or rock fill weir that consists of a body wall and dry stones are placed upward and downward along the middle core wall in the form of glacis.

4.Broad crested weir:

Broad crested wear is reinforced structures that can usually be constructed by reinforced concrete which may typically be across the width of the channel.

They are used to measure the discharge of rivers hence they have comparatively sharp sharps.

Additionally, the advantage of a wider crested weir having a critical depth meter, is that it operates more effectively with a higher drifted water level than sharp-crested weirs.

5.Sharp crested weir:

A weir with a sharp upstream corner or edge like a waterfall from a crest is a sharp crested wear.

Sharp-crested weirs are classified according to the shape of the rectangular opening, such as rectangular weirs, triangular or V-notched weirs, trapezoidal weirs and parabolic weirs.

Weirs that are not sharp-crested are classified according to the size of their cross-sections such as broad-cradled weirs, triangular weirs, and trapezoidal weirs.

6.Labyrinth weir:

It uses a trapezoidal-shaped wear geometry to increase weir length, they are versatile structures and they may be modified for many purposes.

7.Compound weir:

For accurate flow measurements over a variety of flow rates combine two or extra varieties, sometimes V-notch with a rectangular weir.

These weirs are not intended to measure more than 35% of the open channel flow capacity of the pipe.

Advantages of Weirs:

1. The bed level of the canal may be higher than the current water level of the river, weir is built across the river to raise the water level.
2. Adjustable shutters are provided at the crest to raise the water level to some required height.
3. The initial cost of wear is low.

Disadvantages of weirs:

1. Typically increases the oxygen content of the water which brings a detrimental effect on the ecology of the local water bodies.
2. Although the water around weirs can usually appear comparatively calm, they are often extraordinarily harmful places to boat, swimmers, etc.
3. As the circulation patterns on the downstream side usually often known as a hydraulic jump can submerge an individual indefinitely.

Also read: Spillway, River Training Works & Fish Ladder

Difference between Weir & Barrage:

Weir	Barrage
Weir has a low cost.	The barrage has a high cost.
It has less control over flow.	It has relatively high control over flow and water level from the operation of gates.
There is no provision for transport communication across the river.	Typically, a road or rail bridge can be conveniently and economically connected with a barrage wherever necessary.
Silting is more likely to be more on upstream.	Silting can be controlled by judicial handling of gates.
Afflux formed is high due to relatively high wear crusts, due to low crest of the weirs (the ponding being done mostly by gate operation), efflux are low during high floods.	As the gates can be completely elevated even above the high flood level.

Conclusion:

Weir is commonly used to measure or control flow in rivers, drains, irrigation canals, etc.