Types of Intakes | Water Collection | Water Engineering

The conditions being different for each of the various surface-water sources, different types of intakes are used for each of these.

The various types of intakes that are commonly used are therefore classified according to the surface-water sources for which these are used as: 1. Canal Intakes 2. Reservoir Intakes 3. River Intakes 4. Lake Intakes.

1. Canal Intakes:

A canal intake consists of a brick masonry intake chamber built partly in the canal bank and partly extending in the canal section. It has a side opening fitted with coarse screen which allows water to enter the chamber but prevents floating debris from entering the chamber. The bottom of the screen is kept about 0.15 m above the canal bed to avoid the entry of bed material into the chamber. The top of the screen is kept either below or at the minimum or low water level in the canal.

The water from the intake chamber flows out through the intake conduit which carries water to the treatment plant. The inlet end of the intake conduit is provided with a bell mouth inlet which is covered with a hemispherical shaped fine screen to permit the entry of relatively clear water in the intake conduit. A valve is provided in the intake conduit to control the entry of water into the conduit.

The valve may be operated from the top. As the water level in a canal remains more or less constant, it is not necessary to provide inlets at different levels. In order to reach upto the bottom of the intake chamber from the manhole at the top a ladder is usually provided in the intake chamber.

The construction of the intake chamber obstructs a portion of the waterway of the canal, which is thus reduced. This results in an increase in the velocity of flow which may cause erosion of bed and banks of the canal in the vicinity of the intake chamber. Thus if the canal is unlined it becomes necessary to provide pitching on the bed and banks of a portion of the canal both on the upstream as well as on the downstream sides near the intake chamber.

The coarse screen is made as a grid of vertical mild steel bars, 15 to 20 mm in diameter, with openings between the bars not less than 25 to 50 mm. The coarse screen is designed for the velocity of flow through the openings in the grid to not to exceed 0.15 m/s and preferably to be much less than this.

The fine screen usually consists of wire net having openings of size 6 mm × 6 mm or less.

The velocity of flow through the bell mouth inlet of the intake conduit is limited to about 0.3 m/s or so. Further the velocity of flow in the intake conduit is generally kept as 1.0 to 1.5 m/s. Thus knowing the discharge the required diameter of the intake conduit may be determined. The head loss in the intake conduit upto the treatment plant can be determined by using Hazen—Williams formula (Eq. 6.10).

2. Reservoir Intakes:

In the case of a reservoir the type of intake to be provided depends on the type of dam constructed to create the reservoir. The two types of dams commonly constructed for creating reservoirs are gravity dams and earth dams, and the types of intakes usually provided in both the cases are as indicated below.

In the case of a reservoir created by constructing a gravity dam water is withdrawn from the reservoir through outlets or sluiceways which are constructed as an integral part of the dam. The outlets are located at different levels to enable the withdrawal of water even when the level of water in the reservoir drops.

In order to control the flow of water through the outlets various devices such as gates and valves are used which are housed in the body of the dam itself. At the entrance to the outlet a structure called trash rack is provided to prevent entry of floating debris into the outlet.

In the case of a reservoir created by constructing an earth dam the intake consists of a masonry or R.C.C. intake tower (or intake well) placed near the upstream toe of the dam. The intake tower is connected to the top of the dam by a foot bridge. The water from the reservoir is withdrawn through intake pipes located at different levels with a common vertical pipe.

The vertical pipe is connected at its bottom to an intake conduit which is taken out through the body of the dam. Each intake pipe is provided with bell mouth inlet which is covered with a hemispherical shaped fine screen to permit the entry of relatively clear water.

The location of intake pipes at different levels ensures supply of water from the reservoir even when the level of water in the reservoir drops. The intake pipes are provided with valves to control the entry of water through them. The valves are operated from the control room at the top.

When the valve of the intake pipe is opened, the water is drawn off from the reservoir through the intake pipe and after flowing through the common vertical pipe enters the intake conduit which carries water to the treatment plant.

Thus in this case the intake tower has no water inside it other than in the intake pipes and hence it is known as dry intake tower. The interior of the tower is thus accessible for inspection and operation. In order to reach upto the bottom of the intake tower from the floor of the control room a ladder is usually provided in the intake tower.

3. River Intakes:

A typical river intake consists of a masonry or R.C.C., intake tower (or intake well or jack well) which is provided with several inlets called penstocks for withdrawing water from the river. The penstocks are located at different levels to permit the withdrawal of water even when the water level in the river drops. The inlet end of each penstock is provided with a screen to prevent the entry of floating debris. The penstocks are provided with valves to control the entry of water through them.

The valves are operated from the control room at the top. The penstocks discharge water into the intake tower (or intake well) from where it is pumped to the treatment plant by the pumps installed in the control room at the top. Thus in this case the intake tower is filled with water and hence it is known as wet intake tower. In order to reach upto the bottom of the intake tower from the floor of the control room a ladder is usually provided in the intake tower.

The river intake should be so located that even during the low water level in the river water remains available at the intake in sufficient quantity. Sometimes an approach channel may have to be constructed to lead the water from the upstream side of the river to the intake. If the river has a wide basin then a cross approach channel may be constructed upto the intake to fetch water from the deeper portion of the river to the intake.

Further if there is considerable variation in water levels in the river during summer and other seasons, a weir or barrage may be constructed across the river to raise the water level in the river, thereby ensuring the availability of water at the intake in sufficient quantity at all times.

4. Lake Intakes:

In the case of a natural lake the maximum depth of water occurs at the central portion of the lake. As such in this case the intake is located in the central portion of the lake and instead of a tower type intake a submerged intake is usually provided. It consists of an intake conduit laid below the bed of the lake with its inlet end placed in the middle of the lake projecting above the bed of the lake.

The inlet end of the intake conduit is protected by concrete or timber crib and it is provided with bell mouth entry covered with a screen. The water entering the intake conduit flows under gravity to the bank of the lake and collected in a sump well from where it is pumped to the treatment plant. More than one intake conduit may be laid if one conduit is not enough for obtaining the required quantity of water.

The intakes may also be classified according to the type of the intake structure as:

I. Intake Towers or Tower Type Intakes:

Intake towers are used to draw water from reservoirs, lakes or rivers in which there is either a wide fluctuation in water level or the desire to draw water at the most desirable depth, or both. An intake tower is a well type masonry or concrete structure provided with a number of openings at different levels which permit the withdrawal of water from the source of supply.

The openings (usually known as intake ports) are provided with intake pipes or penstocks with screens at their inlet ends to prevent the entry of floating debris, and valves to control the entry of water through them. The intake tower houses the valve operating devices, pumps, etc., which are installed in a control room usually located in the top portion of the tower.

The intake tower provided in a reservoir created by constructing an earth dam is located at some distance away from the top of the dam. Similarly in lakes and in some rivers the intake towers Eire usually located at some distance away from the shore line.

As such for operating the valves, pumps, etc., access to the tower is provided by means of a foot bridge connecting the tower and the dam or the shore line. Further in order to reach upto the bottom of the intake tower from the floor of the control room a ladder is usually provided.

The intake towers may be classified as:

(i) Dry intake towers

(ii) Wet intake towers

(i) A dry intake tower has no water inside it other than in the intake pipes. Water from the source of supply is admitted through the intake pipes and conveyed to an intake conduit which carry water to the treatment plant. The interior of the tower is thus accessible for inspection and operation.

(ii) A wet intake tower is filled with water upto the level of water in the source of supply. Water from the source of supply is admitted through penstocks which discharge water into the tower from where it is pumped to the treatment plant.

II. Submerged Intakes:

Submerged intakes are constructed entirely under water. These intakes are generally used to draw water from natural lakes and some rivers. A submerged intake consists of an intake conduit laid below the bed of the lake or river with its inlet end placed in the middle of the lake or river projecting above the bed of the lake or river.

The inlet end of the intake conduit is protected by concrete or timber crib and it is provided with bell mouth inlet covered with a screen. The water entering the intake conduit flows under gravity to the bank of the lake or river and collected in a sump well from where it is pumped to the treatment plant.

As compared to a tower type intake a submerged intake costs less. Further a submerged intake does not cause any obstruction to navigation, causes only little obstruction to the flow of the river, has little danger from floating material and has a minimum trouble from ice if formed on the surface.

III. Portable Intakes or Movable Intakes:

Portable intakes or movable intakes are generally used to draw water from rivers or streams with sloping banks and having wide variations in water surface levels. These intakes are also used where shifting shoals, lack of foundation in the stream bed or other conditions prevent the erection of other types of intakes in the river or stream. Moreover, these intakes may also be used in case of emergencies such as need for making water supply arrangements for a mela or festival site, or during war times, etc.

A portable intake consists of a pump mounted on a carriage moving on a track laid at the site of intake. The carriage is moved and placed in such a position that the suction pipe of the pump is held immersed in the river water. At the end of the suction pipe a strainer is provided so that the water withdrawn from the river is relatively free from suspended solids.

The water withdrawn from the river is conveyed through the discharge pipe of the pumping plant. As the water level in the river changes the discharge pipe is disconnected and the carriage along with the pump is moved on the track till the suction pipe is suitably adjusted. The discharge pipe is then reconnected at the appropriate level.