Intake Structure for Water Supply

Intakes are the structures used for admitting water from the surface sources (i.e., river, reservoir or lake) and conveying it further to the treatment plant. The basic function of the intake structure is to help in safely withdrawing water from the source over predetermined pool levels and then to discharge this water into the withdrawal conduit (normally called intake conduit), through which it flows up to water treatment plant. It essentially consists of opening, grating or strainer through which the raw water from river, canal or reservoir and is carried to a sump well by means of conduits. Water from the sump well is pumped through the rising mains to the treatment plants. Generally, an intake is a masonry or concrete structure with an aim of providing relatively clean water, free from pollution, sand and objectionable floating material. Its main purpose is to provide calm and still water conditions, so that comparatively purer water may be collected from the source. If intake well has to withstand the effects of severe forces which may be due to striking of high water currents, it may be made from reinforced cement concrete. Intake consists of the following.

• Conduit with protective works
• Screens at open ends
• Gates and Valves to regulate flow

Site for Location of Intake

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

• As far as possible, the site should be near the treatment plant so that the cost of conveying water to the city is less.
• The intake must be located in the purer zone of the source to draw best quality water from the source, thereby reducing load on the treatment plant.
• The intake must never be located at the downstream or in the vicinity of the point of disposal of wastewater.
• The site should be such as to permit greater withdrawal of water, if required at a future date.
• The intake must be located at a place from where it can draw water even during the driest period of the year.
• The intake site should remain easily accessible during floods and should not get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.
• Heavy water currents should not strike the intake directly.
• Site should be well connected by good type or roads
• Site should not be located in navigation channels, the reason being water in such channels are generally polluted.
• As far as possible, the site should be located on the upstream side of the watercourse.
• 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 up thrust 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 such as heavy currents, floating materials, submerged bodies, ice pressure etc.
• The foundation of the intake should be taken sufficiently deep to avoid overturning.
• It should have sufficient self-weight so that it does not float by upthrust of water.
• 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

1) 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, 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.

2) Reservoir Intake

There are large variations in discharge of all the rivers during monsoon and winter. The discharge of some rivers in summer remains sufficient to meet up the demand, but some rivers dry up partly or fully and cannot meet the hot weather demand. In such cases reservoirs are constructed by constructing weirs or dams across the rivers. Reservoir intakes is mostly used to draw the water from earthen dam reservoir. 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.

It essentially consists of an intake tower constructed on the slope of the dam at such place from where intake can draw sufficient quantity of water even in the driest period. Intake pipes are fixed at different levels, so as to draw water near the surface in all variations of water level. These all inlet pipes are connected to one vertical pipe inside the intake well. Screens are provided at the mouth of all intakes to prevent the entrance of floating and suspended matter in them. The water which enters the vertical pipe is taken to the other side of the dam by means of an outlet pipe. At the top of the intake tower, sluice valves are provided to control the flow of water.

Fig. 1 Reservoir Intake

3) River Intake

Water from the river is always drawn from the upstream side, because it is free from the contamination caused by the disposal of sewage or industrial waste water disposal in it. It is circular masonry tower of 4 to 7 m in diameter constructed along the bank of the river at such place from where required quantity of water can be obtained even in the dry period. 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. The water enters in the lower portion of the intake known as sump-well from penstocks.

The penstocks are fitted with screens to check the entry of floating solids and are placed on the downstream side so that water free from most of the suspended solids may only enter the jack well. Number of penstock openings are provided in the intake tower to admit water at different levels. The opening and closing of penstock valves is done with the help of wheels provided at the pump house floor. 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.

Fig. 2 River Intake

4) Lake Intake

Lake intakes are similar to reservoir intakes if the depth of the water near the banks is reasonable. For obtaining water from lakes mostly submersible intakes are used. These intakes are constructed in the bed of the lake below the slow water level so as to draw water in dry season also. It essentially consists of a pipe laid in the bed of the river. One end of which is in the middle of the lake is fitted with bell mouth opening covered with a mesh and protected by timber or concrete crib. The water enters in the pipe through the bell mouth opening and flows under gravity to the bank where it is collected in a sump-well and then pumped to the treatment plants for necessary treatment. If one pipe is not sufficient two or more pipes may be laid to get the required quantity of water.

Fig. 3 Lake Intake

Advantages of lake intake

• No obstruction to the navigation.
• No danger from floating bodies.
• No trouble due to ice.

5) Canal Intake

Canal intake is a very simple structure constructed on the bank. 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 outlet pipe carries the water to the other side of the canal bank from where it is taken to the treatment plants. One sluice valve which is operated by a wheel from the top of the masonry chamber is provided to control the flow of water in the pipe. 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.

Fig. 4 Canal Intake