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

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, is known as diversion heads works.

The objective of Diversion Head Works

To raise the water level at the head of the canal.

To form 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 the vicinity of the site.

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

Weir and Barrage

It is a barrier constructed across the river to raise the water level on the upstream side of the obstruction in order to feed the main canal.

The ponding of water can be achieved either only by a raised crest across the river or by a raised crest supplemented by gates or shutters, working over the crest.

Weir and Barrage

Weir 

If the major part of 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.

A typical cross-section of a modern concrete 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 continuously with the divided piers as reinforced structure, such that the weight of the concrete slab together with the weight of divided piers keep the structure safe against the uplift then the structure may be called 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 a vertical drop

(b) Rock-fill weirs with sloping aprons

(c) Concrete weirs with sloping glacis

Masonry weirs with a 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 faces or sloping on both faces or vertical on the downstream faces and sloping in upstream faces.

Masonry weir

Rock-fill weirs with sloping aprons

It consists of a masonry breast wall which is provided with an 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.

Rock-fill weirs

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.

Concrete weir

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.

A typical cross-section of a barrage

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.

The 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

SL	Barrage	Weir
1	Low set crest	High set crest
2	Ponding is done by means of gates	Ponding is done against the raised crest or partly against crest and partly by shutters
3	Gated over entire length	Shutters in part length
4	Gates are of greater height	Shutters are of smaller height, 2 m
5	Gates are raised clear off the high floods to pass floods	Shutters are dropped to pass floods
6	Perfect control on river flow	No control of river in low floods
7	Gates convenient to operate	Operation of shutters is slow, involve labor and time
8	High floods can be passed with minimum afflux	Excessive afflux in high floods
9	Less silting upstream due to low set crest	Raised crest causes silting upstream
10	Longer construction period	Shorter construction period
11	Silt removal is done through under sluices	No means for silt disposal
12	Road and/or rail bridge can be constructed at low cost	Not possible to provide road-rail bridge
13	Costly structure	Relatively cheaper structure

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