Design Of Hydraulic Structures

Diversion Head Works

The works, which are constructed at the head of the canal, in order to divert the river water towards the

canal, so as to ensure a regulated continuous supply of silt-free water with a certain minimum head into the canal, are known as diversion heads works.

Objective of Diversion Head Works

 To rise the water level at the head of the canal.

 To form a storage by constructing dykes (embankments) on both the banks of the river so that water is

available throughout the year

 To control the entry of silt into the canal and to control the deposition of silt at the head of the canal

 To control the fluctuation of water level in the river during different seasons

Selection of Site for Diversion Head Works

 At the site, the river should be straight and narrow

 The river banks should be well defined.

 The valuable land should not be submerged when the weir or barrage is constructed.

 The elevation of the site should be much higher than the area to be irrigated.

 The site should be easily accessible by roads or railways.

 The materials of construction should be available in vicinity of the site.

 The site should not be far away from the command area of the project, to avoid transmission loss.

Weir

 If the major part or the entire ponding of water is achieved by a raised crest and a smaller part or nil part of it is achieved by the shutters, then this barrier is known as a weir.

Gravity and Non-gravity weirs:

When the weight of the weir (i.e. its body and floor) balances the uplift pressure caused by the head of the water seeping below the weir, it is called a gravity weir.

On the other hand, if the weir floor is designed continuous with the divide piers as reinforced structure, such that the weight of concrete slab together with the weight of divide piers keep the structure safe against the uplift then the structure may be called as a non-gravity weir.

o In the latter case, RCC is to be used in place of brick piers

o Considerable savings may be obtained, as the weight of the floor can be much less than what is

required in gravity weir.

Types of weirs

(a) Masonry weirs with vertical drop

(b) Rock-fill weirs with sloping aprons

Masonry weirs with vertical drop

Masonry weir wall is constructed over the impervious floor. Cut-off walls are provided at both ends

of the floor. Sheet piles are provided below the cut off walls. The crest shutters are provided to raise the water level, if required. The shutters are dropped down during flood. The masonry weir wall may be vertical on both face or sloping on both face or vertical on downstream face and sloping in upstream face.

Rock-fill weirs with sloping aprons

It consists of masonry breast wall which is provided with adjustable crest shutter. The upstream

rock-fill portion is constructed with boulders forming a slope of 1 in 4. The boulders are grouted with

cement mortar. The downstream sloping apron consists of core walls. The intermediate spaces between the core walls are filled up with boulders maintaining a slope of 1 in 20. The boulders are grouted properly with cement mortar.

Concrete weir

Now-a-days, the weir is constructed with reinforced cement concrete. The impervious floor and the

weir are made monolithic. The cut off walls are provided at the upstream and downstream end of the floor and at the toe of the weir. Sheet piles are provided below the cut-off walls. The crest shutters are also provided which hare dropped down during the flood.

Barrage

 If most of the ponding is done by gates and a smaller or nil part of it is done by the raised crest, then the barrier is known as a barrage or a river regulator.

Afflux:

o The rise in the highest flood level (HFL) upstream of the weir due to construction of the weir

across the river is called.

o In case of weir, the afflux caused during high floods is quite high. But in case of a barrage, the

gates can be opened during high floods and the afflux will be nil or minimum.

Choice between a weir and a barrage

The choice between a weir and a barrage is largely governed by cost and convenience in working.

o A shuttered weir will be relatively cheaper but will lack the effective control possible in the

case of a barrage.

o A barrage type construction can be easily supplemented with a roadway across the river at a

small additional cost. Barrages are almost invariably constructed now-a-days on all important

rivers.

Difference between Barrage and Weir

Layout of a Diversion Head Works and its components

A typical layout of a canal head-works is shown in figure below. Such a head-works consists of:

Weir proper

Under-sluices

Divide wall

River Training works

Fish Ladder

Canal Head Regulator

River Training Works e.g. Guide bank, Marginal bunds, spur and groyne etc.

Shutters and Gates Silt

Regulation Works

Weir Proper:

It is a barrier constructed across the river. It aims to raise the water level in order to feed the canal.

Under-sluices:

The under sluices are the openings provided at the base of the weir or barrage. These openings are

provided with adjustable gates. Normally, the gates are kept closed. The crest of the under-under sluice portion of the weir is kept at a lower level (1 1.5 m) than the crest of the normal portion of the weir. The suspended silt goes on depositing in front of the canal head regulator. When the silt deposition becomes appreciable the gates are opened and the deposited silt is loosened with an agitator mounting on a boat.

The muddy water flows towards the downstream through the scouring sluices. The gates are then closed.

But, at the period of flood, the gates are kept opened.

The main functions of under-sluices are:

o To maintain a well defined deep channel approaching the canal head regulator.

o To ensure easy diversion of water into the canal through the canal head regulator even during low

flow.

o To control the entry of silt into the canal

o To help scouring and of the silt deposited over the under-sluice floor and removing towards the

downstream side.

o To help passing the low floods without dropping the shutters of the weir.

The divide wall:

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

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

 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 apron.

The main functions of the divide walls:

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

level.

o 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.

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

Fish Ladder

 It is 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 divide wall.

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

construction of 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.

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

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.

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