Note for Civil Engineering Materials and Construction - CEMC by Pratik Acharya

Topics ( 6 )

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Diversion Headworks 1. 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. 2. Objective of Diversion Head Works (a) To rise the water level at the head of the canal. (b) To form a storage by constructing dykes (embankments) on both the banks of the river so that water is available throughout the year (c) To control the entry of silt into the canal and to control the deposition of silt at the head of the canal (d) To control the fluctuation of water level in the river during different seasons 3. Selection of Site for Diversion Head Works  At the site, the river should be straight and narrow   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. The river banks should be well defined. 4. 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. CREST POND LEVEL PONDING BY CREST SHUTTER TOTAL PONDING BY RAISED CREST P1 = P P2 = 0 WEIR WITHOUT SHUTTERS CREST LEVEL = POND LEVEL PONDING BY RAISED CREST P2 P1 CREST LEVEL TOTAL PONDING (P) SHUTTER P1 >>>> P2 (a) Fig: Weir without shutters (b) Fig: Weir with shutters 1

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POND LEVEL GATE PONDING BY GATE P2 POND LEVEL TOTAL PONDING BY GATE CREST LEVEL P1 <<<< P2 PONDING BY CREST P1 = 0 P2 = P (BARRAGE) P1 (c) Fig: Barrage with a small raised crest (d) Fig: Barrage without any raised crest 5. 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. 6. Fig: A typical cross-section of a modern concrete weir 1. 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. (a) 2. Types of weirs Masonry weirs with vertical drop (b) Rock-fill weirs with sloping aprons (c) Concrete weirs with sloping glacis 3. 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 2

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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. 4. 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. 5. 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. 3

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1. 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. 2. Fig: A typical cross-section of a barrage Afflux:  The rise in the highest flood level (HFL) upstream of the weir due to construction of the weir across the river is called.  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. 3. Choice between a weir and a barrage The choice between a weir and a barrage is largely governed by cost and convenience in working. A shuttered weir will be relatively cheaper but will lack the effective control possible in the case of a barrage. 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. 4. Difference between Barrage and Weir SL (a) Barrage Weir Low set crest High set crest 4