Divide wall and main function

 The divide wall:

1. The divide wall is a masonry or concrete wall constructed at a right angle to the axis of the weir.

2.The divide wall extends on the upstream side beyond the beginning of the canal head regulator; and on the downstream side, it extends up to the end of the loose protection of the under-sluices.

3.The divide wall is a long wall constructed at right angles in the weir or barrage, it may be constructed with stone masonry or cement concrete. On the upstream side, the wall is extended just to cover the canal head regulator and on the downstream side, it is extended up to the launching aprons.

The main functions of the divide walls:

1. It separates the ‘under-sluices’ with lower crest level from the ‘weir proper’ with higher crest level.

2. It helps in providing a comparatively less turbulent pocket near the canal head regulator, resulting in deposition of silt in this pocket and, thus, to help in the entry of silt-free water into the canal.

3. It helps to keep cross-current, if any, away from the weir.

Fish Ladder 

It is a device by which the flow energy can be dissipated in such a manner as to provide smooth flow at sufficiently low velocity, not exceeding 3 to 3.5 m/s.

A narrow opening including suitable baffles or staggering devices in it is provided adjacent to the divider wall.

The fish ladder is provided just by the side of the divide wall for the free movement of fishes. Rivers are an important source of fishes.

There are various types of fish in the river. The nature of the fish varies from type to type. But in general, the tendency of fish is to move from upstream to downstream in winters and from downstream to upstream in monsoons. This movement is essential for their survival. Due to the construction of the weir or barrage, this movement gets obstructed and is detrimental to the fishes.

In the fish ladder, the fable walls are constructed in a zigzag manner so that the velocity of flow within the ladder does not exceed 3 m/sec

The width, length, and height of the fish ladder depend on the nature of the river and the type of the weir or barrage.

fish ladder,

Canal Head Regulator or Head sluices

A structure which is constructed at the head of the canal to regulate flow of water is known as canal head regulator. It consists of a number of piers which divide the total width of the canal into a number of spans which are known as bays. The piers consist of number tiers on which the adjustable gates are placed.

The gates are operated form the top by suitable mechanical device. A platform is provided on the top of the piers for the facility of operating the gates. Again some piers are constructed on the down stream side of the canal head to support the roadway.

Functions of Canal Head Regulator:

o It regulates the supply of water entering the canal

o It controls the entry of silt in the canal

o It prevents the river-floods from entering the canal

Alignment of a canal head regulator

The water from the under-sluice pocket is made to enter the regulator bays, so as to pass the full supply discharge into the canal. The maximum height of these gated openings, called head sluices will be equal to the difference of Pond Level and Crest Level of the regulator. 

The entry of silt into the canal is controlled by keeping the crest of the head regulator by about 1.2 to 1.5 meters higher than the crest of the under-sluices.

 If a silt-excluder is provided, the regulator crest is further raised by about 0.6 to 0.7 meter.

 Silt gets deposited in the pocket, and only the clear water enters the regulator bays.

The deposited silt can be easily scoured out periodically, and removed through the under-sluice openings.

Fig: A typical section through a Canal Head Regulator (CHR)

River Training Works

River training works are required near the weir site in order to ensure a smooth and an axial flow of water, and thus, to prevent the river from outflanking the works due to a change in its course.The river training works required on a canal headwork are:

(a) Guide banks

(b) Marginal bunds

(c) Spurs or groynes

(a) Guide Bank

When a barrage is constructed across a river that flows through the alluvial soil, the guide banks must be constructed on both the approaches to protect the structure from erosion. Guide bank serves the following purposes:

 It protects the barrage from the effect of scouring and erosion.

 It provides a straight approach towards the barrage.

 It controls the tendency of changing the course of the river.

 It controls the velocity of flow near the structure.

(b) Marginal Bunds

The marginal bunds are earthen embankments that are constructed parallel to the river bank on one or both the banks according to the condition. The top width is generally 3 m to 4 m. The side slope on the riverside is generally 1.5: 1 and that on the countryside is 2:1. The marginal bunds serve the following purposes: 

 It prevents the flood water or storage water from entering the surrounding area which may be submerged or may be waterlogged.

 It retains the flood water or storage water within a specified section

 It protects the towns and villages from devastation during heavy flood.

It protects valuable agricultural lands.

(c) Spurs or groynes

(i) Spurs

These are temporary structures permeable in nature provided on the curve of a river to protect the river bank from erosion. These are projected from the river bank towards the bed making angles 60o to 75o with the bank of the river. The length of the spurs depends on the width of the river and the sharpness of the curve. The function of the spurs is to break the velocity of flow and to form a water pocket on the upstream side where the sediments get deposited. Thus the reclamation of land on the river bank can be achieved. The spurs may be of the following types:

(a) Bamboo Spur

(b) Timber Spur

(c) Boulder Spur

(a) Bamboo Spur: In this type of spur, a box like a compartment is prepared by driving bamboo piles at 15 cm centre to centre. The piles are secured by bamboo bracings. The hollow space is filled up with sand bags. It is permeable in nature and water can seep through its body. This type of spur is suitable for small rivers. This is purely temporary and requires repair work every year. The length of bamboo piles depends on bed condition.

Bamboo Spur and timber spur

(b) Timber Spur: In this type, a box like a compartment is prepared by driving timber piles at 15 cm to

30 cm centre to centre. The piles are secured by wooden bracings. The hollow space is filled up by

boulders. This spur is permeable but stable. This is recommended for bi rivers with a high velocity of

flow. The length of the timber piles depend on bed condition.

(c) Boulder Spur: In this type of spur, boulders are enclosed in G.I wire net in circular shape. The

boulder should be heavy, varying from 30 kg to 50 kg and the wire net should be made of 4 mm

diameter G.I wires. It is laid from the river bank towards the bed making an angle of 60o-75o with

the bank. This type of spur is recommended for rivers where the velocity of flow is very high.

Boulder Spur

(ii) Groynes

The function of groynes is similar to that of spur. But these are impervious permanent structures

constructed on the curve of a river to protect the river bank from erosion. They extend from the bank

towards the bed by making an angle of 60° to 75° with the bank. The angle may be towards the upstream or downstream. Sometimes, it is made perpendicular to the river bank. These are constructed with rubble masonry in a trapezoidal section and the surface is finished with stone pitching or concrete blocks.

o The stone pitching or the concrete blocks are set with rich cement mortar.

o The length of the groyne depends on the width and nature of the river.

o The top width varies from 3 m to 4 m. The side slope may be 1½: 1 or 2:1.

o The groynes are provided in series throughout the affected length of the river bank.

o The spacing between the adjacent groynes is generally kept as 2L, where L is the length of the

groyne.

o These are recommended for the river where the permanent solution of erosion control is extremely

necessary.

Groynes

The groynes may be designated as follows:

(a) Attracting Groyne: The groyne which is constructed obliquely to the bank by making an angle of

60 to 75o towards the downstream is known as attracting groyne, here the flow of water is attracted

towards the bank, and the velocity of flow is reduced to such an extent that it can not cause any

erosion to the bank. However, a bank protected of stone pitching is provided for safety.

Attracting groyne and repelling groyne

(b) Repelling Groyne: A groyne that is aligned towards upstream at an angle of 60o to 75o with the

river bank is known as repelling groyne. A still water pocket is formed on the upstream where

silting takes place. Here, bank protection is not necessary, because the flow of water does not

touch the bank and there is no effect of erosion on the bank. But still, boulder pitching should be

provided for safety.

(c) Deflecting Groyne: The groyne which is constructed perpendicular to the river bank is known as

deflecting groyne. Here the flow of water is deflected from the bank by the perpendicular obstruction

i.e. groyne. The flow of water follows an undulating path just outside the head of the groyne. An

eddy current is formed on the upstream side of the groyne. This eddy current will not affect the

river bank. But the bank protection is provided for safety.

Deflecting groyne

Modification of Groyne:

(a) Denehy’s Groyne or T-Headed Groyne: After a long investigation in different sites, Denehy

developed a groyne in the shape of a T. The length of the head is kept as ½, where L is the length of

the groyne. A still water pocket is formed on the upstream side where silting takes place. It is

constructed with rubble masonry in a trapezoidal section. The upstream face is finished with concrete

blocks with cement mortar.

(b) Hockey Head Groyne: Another development is hockey head groyne. Here, the head of the groyne

is curved towards the downstream in the shape of a hockey stick. It behaves like an attractive

groyne. But it allows the water to flow smoothly over the head of the groyne. It is also constructed

with rubble masonry in a trapezoidal section. Here, bank protection by stone pitching is

necessary.

Hockey Head Groyne and Denehy's Groyne

Comparison between spur and groyne

Spur	Groyne
It is a temporary structure.	It is a permanent structure.
It is permeable.	It is impermeable.
It is constructed with the bamboo pile, timber pile	It is constructed with rubble masonry with a sand bag, boulders etc. cement mortar.
It requires repair works.	It does not require any repair work.
It is recommended for small rivers.	It is recommended for large rivers.
It is useful for low or medium velocity of flow	It is suitable for high velocity of flow.

Shutters and Gates:

Functions of shutters and gates are:

o They maintain pond level.

o They raise water level during low flow.

Pond Level

The water level required in the under-sluice pocket upstream of the Canal Head Regulator, so as to feed

the canal with its full supply is known as Pond Level.

The FSL of the canal at the head depends upon the level of the irrigated areas and the slope of the

canal. Pond Level = Canal FSL + 1.0 to 1.2 m

Silt Regulation works

The entry of silt into a canal, which takes off from a head works, can be reduced by constructing certain

special works called silt control works.

These works may be classified into the following two types:

(a) Silt Excluders

(b) Silt Ejectors

(a) Silt Excluders

Silt excluders are those works that are constructed on the bed of the river, upstream of the head

regulator. The clearer water enters the head regulator and silted water enters the silt excluder. In this type

of works, the silt is, therefore,, removed from the water before it enters the canal.

(b) Silt Ejectors

Silt ejectors, also called silt extractors, are those devices which extract the silt from the canal water after

the silted water has traveled a certain distance in the off-take canal. These works are, therefore, constructed

on the bed of the canal, and little distance downstream from the head regulator

plan of silt ejector

 

L section along Canal

Main Canal

READ MORE

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Divide wall and main function
The layout of a Diversion Head Works and its components

Diversion Head Works and their objective? Weir and Barrage their types and difference?

What is Canal Fall?The necessity of Canal Falls and Types of Canal Falls

What are cross drainage works? Types of Cross-Drainage Works

Spillway and Necessity of Spillways