Environmental Engg. 3(2+1)

Lesson-5 Intakes for water supply

Site for location of intake

While selecting a site for location of intakes, the following points should be taken into account:

• Intake work should provide purer water so that its treatment may be less exhaustive

• Heavy water currents should not strike the intake directly

• Intake should be located at such a situation where sufficient quantity of water remains available under all the circumstances

• Site should be well connected by good type or roads

• Site should be such that intake should be in a position to provide more water, if requied to do so. 

• Site should not be located in navigation channels, the reason being water in such channels are generally polluted.

• During floods, the intake should not be submerged by the flooding waters.

• As far as possible, the site should be located on the upstream side of the town / city.

• The intake should be so located that good foundation conditions are prevalent and the possibility of scouring is minimal.

• The site should be selected in such a manner that there is ample scope for further expansion.

 Design of intake

An intake should be designed keeping in mind the following considerations:

• Intake should be sufficiently heavy so that it may not start floating due to upthrust of water.

• All the forces which are expected to work on intake should be carefully analysed and intake should be designed to withstand all these forces.

• The foundation of the intake should be taken sufficiently deep to avoid overturning.

• Strainers in the form of wire mesh should be provided on all the intake inlets to avoid entry of large floating objects.

• Intake should be of such size and so located that sufficient quantity of water can be obtained from the intake in all circumstances.

Types of intakes

Submerged intake

Submerged intake is the one which is constructed entirely under water.  Such an intake is commonly used to obtain supply from a lake.  An exposed intake is in the form of a well or tower constructed near the bank of a river, or in some cases even away from the river banks.  Exposed intakes are more common due to ease in its operation.  A wet intake is that type of intake tower in which the water level is practically the same as the water level of the sources of supply.  Such an intake is sometimes known as jack well and is most commonly used.  In the case of dry intake, however, there is no water in the water tower.  Water enters through entry point directly into the conveying pipes.  The dry tower is simply used for the operation of valves etc.

River intake

A river intake is located to the upstream of the city so that pollution is minimized.  They are either located sufficiently inside the river so that demands of water are met with in all the seasons of the year, or they may be located near the river bank where a sufficient depth of water is available.  Sometimes, an approach channel is constructed and water is led to the intake tower.  If the water level in the river is low, a weir may be constructed across it to raise the water level and divert it to the intake tower.

Reservoir intake

When the flow in the river is not guaranteed throughout the year a dam is constructed across it to store water in the reservoir so formed.  The reservoir intakes are practically similar to the river intake, except that these are located near the upstream face of the dam where maximum depth of water is available.

Lake Intake

Lake intakes are similar to reservoir intakes if the depth of the water near the banks is reasonable.  If however, the depth of the water near the banks is shallow, and greater depth is available only at its centre, a submerged intake is provided.

Canal intake

Sometimes, the source of water supply to a small town may be an irrigation canal passing near the town.  The canal intake essentially consists of concrete or masonry intake chamber of rectangular shape, admitting water through a coarse screen.  A fine screen is provided over the bell mouth entry of the outlet pipe.  The bell mouth entry is located below the expected low water level in the canal.  Water may flow from outlet pipe under gravity if the filter house is situated at a lower elevation.  Otherwise, the outlet pipe may serve as suction pipe, and the pump house may be located on or near the canal bank.  The intake chamber is so constructed that is does not offer any appreciable resistance to normal flow in the canal.  Otherwise, the intake chamber is located inside the canal bank.  Near the location of the intake work, canal is lined.