Intake Structures (surface water sources)

INTAKE STRUCTURES

Intakes are the structures built in surface water sources (such as rivers, lakes, reservoirs etc.) for the withdrawal of water and discharge into conduits of the water supply schemes.

Different parts of an intake are:

(i) Entry ports or Inlets or Penstocks: Ports are provided at different elevations to ensure water flow during all seasons. i.e. to take care of fluctuation during summer. The lowest entry port is placed below the lowest water level (LWL) of the river so that water is available during dry season also.

(ii) Screens: The entry ports are protected with screens to prevent entry of any debris or floating materials into the intakes.

(iii) Intake well: It is built of masonry or concrete which may be rectangular or circular in shape. Water drawn from the sources through entry ports is stored in these wells which is then discharged through conduits for water supply.

(iv) Conduits : They are the pipelines through which water is conveyed from the intake well to the nearby treatment plant or water supply system.

(v) Gate valves and control room : The water flow is regulated by gate valves provided on top at the control tower.

(vi) Foot bridge: A foot bridge is provided on top of the intake tower for access.

 

Factors governing the location of site for intakes:

The site chosen for intakes should be preferably

(i) Near the treatment plant

(To reduce the cost of piplines)

(ii) At the purer zone of the surface source

(To reduce the water treatment cost)

(iii) At the upstream side of source

(iv) Not near any waste water or sewage disposal points.

(v) Provide greater withdrawal of water including expansions in future.

(vi) Provide water even during dry seasons

(vii) Not near any navigation channel

(viii) Should not get flooded.

 

TYPES OF INTAKES

1. River Intakes

• Located on the upstream side of rivers, where pollution is minimum.

• Located inside the river or on river bank such that sufficient water depth is available to meet the demand during dry seasons also.

• Sometimes, weirs are constructed across the rivers to increase the water level.

Classification of river intakes

(i) Twin well type

(ii) Single well type

(i) Twin well type of River Intake Structure

This is the common type of river intake constructed on non-alluvial rivers

The structure consists of

(i) An inlet well

(ii) An inlet pipe

(iii) A jack well

Inlet well or intake or collector well is circular built of concrete or masonry located on river bed such that water is available during low flows. A foot bridge is provided for access from the river bank. River water enters the inlet well through openings or ports provided at various levels and protected by screens to exclude entry of floating debris. The flow into the entry ports is regulated from the control tower using gate valves. The intake well is connected to jack well (sump well) constructed on the river bank, by a RCC intake pipe. The bottom portion of intake well below the level of the intake pipe is used for the accumulation of silt and sediment. The deposited silt is periodically removed either manually or mechanically and it is ensured that the silt does not enter the intake conduits. The pipe is provided at a gentle slope and the flow through the pipe is under gravity at atmospheric pressure. The pipe diameter is chosen based on the discharge capacity. Water entering the jack well is lifted by pumps and fed into the main lines of water supply system.

(ii) Single well type of a River Intake

This type of intake is constructed on alluvial rivers where weirs or approach channels are constructed to increase the water level.

In this intake, there is no separate inlet well or inlet pipe(as in Twin Intake). The opening ports with bar screens are provided in the jack well. Itself. Water entering the  jack well is more clear. Sediments, if any gets deposited in the bottom of jackwell which is periodically cleaned manually by stopping the flow.

2. Reservoir Intake

• Located on the upstream side of earthern or masonry dams where maximum depth of water is available.

• It is similar to a river intake, except that the entry ports of river intakes are replaced here with intake conduits.

• The intake conduits which are installed at different levels withdraw the water from reservoir and let it into a common conduit (inside the intake well or tower) which conveys the water to the sluiceway tunnel downstream.

• The flow of water into the intake conduits is regulated by valves operated from control room.

• The access to the intake is provided through a foot bridge.

• The arrangement is similar to a dry intake tower.

• As the intake well remains dry, it facilitates easy operation and inspection.

 Fig.1.15 Reservoir Intake

3. Lake Intake

• Similar to reservoir intakes if the water depth is reasonable

• At shallow waters, Lake intakes are provided as submerged intakes.

• Submerged intakes are constructed as cribs or bell mouths.

• The cribs are made of heavy timber framework with cast iron or mesh grating on top. It protects the intake conduit from damage.

4. Canal Intake

• The intake well is located in the bank of canal.

• The water enters the intake well through a port protected with coarse screen, provided at minimum water level in the canal.

• The water from the chamber is then conveyed through a outlet (withdrawal) conduit to the distribution system.

• The inlet end of the outlet conduit is of bell mouth shape protected with a fine screen.

• An outlet valve is provided to control the flow which is operated from top.

• The flow through the outlet pipe may be under gravity or by pumping.

  Fig.1.16 Canal Intake

5. Submerged Intake

• Such intakes are placed in streams or lakes where there are deep waters and very less sediments.

•  This consists of a simple concrete block or a rock filled timber crib supporting the starting end of the withdrawal pipe. The intake opening is protected by screen.

The intake opening is kept about 2 m above the bottom of lakes to avoid entry of sediments. These intakes are cheap and do not obstruct navigation. They are widely used for small water supply projects.

The disadvantage of these intakes is that they are not accessible for cleaning, repairing etc.

 

INTAKE TOWERS

Types of Intake towers:

 (i) Wet intake tower

(ii) Dry intake tower

 

(i) Wet Intake Towerse

• In wet Intake Towers, water is filled in both the intake well (chamber) and the conduits.

• It consists of a concrete circular shell filled with water and vertical inside shaft connected to the withdrawal pipe.

• Openings (Ports) are provided in the concrete shell as well as the vertical shaft for the entry of water. The flow is regulated by gate valves.

• Water from the withdrawal pipe flows to the nearby water treatment plant either under gravity or by pumping.

Fig. 1.19 Wet Intake Tower(Standing in river or resservoir)

(ii) Dry Intake Towers

• In dry intake towers, water is directly drawn into withdrawal conduits through gated entry ports.

• There will be no water in the tower.

• Heavier construction is required than the wet intake towers.

•  (It facilitates easy operation and maintenance.

 

Types of conduits

Depending upon the flow conditions, the conduits are categorised as

(1) Gravity conduits

(2) Pressure Conduits

 

(1) Gravity conduits / Open channels

• Water flows under gravity at atmospheric pressure

• The hydraulic gradient line will coincide with the water surface and will be parallel to bed of conduits.

• Conduits are provided at gradual slope. They cannot follow the natural surface level.

• The length of conduit is more which increases the cost of construction.

• In order to maintain the HGL, these conduits have to be taken on trestles constructed across Valleys/depressions. It further increases the cost.

• The advantage is that no pumping is required.

Different forms of gravity conduits are:

(i) Canals

(ii) Flumes

(iii) Aqueducts

Canals:

• Open channels constructed by cutting high ground and filling low grounds.

• They are lined or unlined depending upon the slope/velocity/ water quality / losses.

Flume:

• Open channels supported above ground over trestles.

• Used to convey water across valley or depressions or obstructions

• Made of masonry, RCC, metal, wood

• Usually regtangular or circular in cross-section

Aqueducts:

• Closed, rectangular or circular or horse shoe section

• Built of Masonry or RCC.

• Usually covered/closed.

• As they are designed to run half or 3/4th full, water does not flow under pressure

2. Pressure Conduits

• Closed conduits in which air entry is restricted. Water flow is under pressure above the atmospheric pressure. The bed of conduit is independent of Hydraulic gradient line (HGL). The pipes can follow natural surface level (NSL) of ground

• As the length of conduit is less, the construction cost is less compared to gravity conduits.

• As pressure conduits follow the natural surface level, deep excavations are n required (as in the case of gravity conduits which follow HGL) and therefore is economical. But in depressions, pumping may be required to achieve pressu flow which increases the cost.

• Pressure conduits may be in the form of pressure aqueducts/pressure tunne preferably circular in shape.

• As the conduits are closed, there are less chances of water pollution.