Intakes: Design, Types and Selection | Water Collection | Water Engineering

In this article we will discuss about:- 1. Design of Intakes 2. Types of Intakes 3. Selection.

Design of Intakes:

An intake should be designed and constructed on the basis of the following points:

(i) Sufficient factor of safety should be taken so that intake work can resist external forces caused by heavy waves and currents, impact of floating and submerged bodies and ice pressures etc.

(ii) Intake should have sufficient self-weight, so that it may float by the up-thrust of water and washed away by the current. To prevent floating of intake structure massive masonry work should be done and broken stones should be tilled in the bottom.

(iii) If intake work is constructed in navigation channels, it should be protected by clusters of piles all around from the blows of the moving ships and steamers.

(iv) The foundations of intakes should be taken sufficient deep so that they may not be undermined and current may overturn the structure.

(v) To avoid the entrance of large and medium objects and fishes, screen should be provided on the inlets, sides.

(vi) The inlets of intakes should be of sufficient size and allow required quantity of water to enter.

(vii) The positions of inlets should be such that they can admit water in all seasons near the surface of water where quality of water is good. Number of inlets should be more so that if anyone is blocked, the water can be drawn from others. The inlets should be completely submersible so that air may not enter the suction pipe.

Types of Intakes:

Intakes are used to collect water for water works from various sources. The sources may be lakes, rivers, reservoirs or canals. The intake work for each type of source is designed separately according to its requirements and situations.

Depending on the source of water the intake works are classified as follows:

(a) Lake Intake,

(b) River Intake,

(c) Reservoir Intake, and

(d) Canal Intake.

(a) Lake Intake:

For obtaining water from lakes mostly submersible intakes are used. Fig. 7.1 shows a typical submersible type of intake used for collecting water from the lakes. 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, 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.

These intakes have so many advantages such as no obstruction to the navigation, no danger from floating bodies and no trouble due to ice. If one pipe is not sufficient two or more pipes may be laid to get the required quantity of water.

Fig. 7.2 shows another type of submersible intake. These intakes should be placed in the streams or lakes at a place where they may not get buried under the silt in due course of time. These intakes are cheap in construction, therefore are widely used for small water supply schemes which collect their water from streams or lakes with little change in water surface throughout the year.

As these intakes draw small quantity of water, these are not used on big water supply schemes or on rivers or reservoirs. The main reason being that they are not easily approachable for maintenance work.

(b) River Intake:

Water from the rivers is always drawn from the upstream side, because it is free from the contamination caused by the disposal of sewage in it. Figure 7.3 shows atypical intake used to draw water from the rivers. It is circular masonry tower of 4 to 7 metres 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.

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.

The water from the sump-well of the intake to upper portion of the intake as shown in Fig. 7.3. Number of penstock openings is 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.

Fig. 7.4 shows another type intake well which is generally constructed inside the river at suitable place. This is also known as wet intake and essentially consists of a concrete circular shell filled with water up to the water level inside the river. If the elevation of the water treatment plants is lower, the water will directly flow under gravitational force through withdrawal conduit.

Openings for the entrance of water is provided on the outer concrete shell as well as on the inside shell as shown in Fig. 7.4. In case the elevation of the water works is more than the elevation of top of water in the river, the water is taken to the bank of river through the withdrawal conduit in the sump well, from where it is pumped to the water works.

Some rivers have too much variation in their discharge of monsoon and dry season. If in dry weather the water level falls below the lowest penstock of the intake well, a weir is constructed across the width of the river to raise the water level and maintaining some storage of water for dry period.

In the case of shallow and broad rivers an approach channel is constructed, so that sufficient quantity of water may reach the intake even in dry period.

Figure 7.5 illustrates another typical type of intake well, which can equally be used for collecting water from river or reservoir. It is commonly known as dry intake tower. The main differences between dry and wet intakes are that, in wet intake tower the water enters first in the outer shell and then it enters in the inner shell but in case of dry intake the water directly enters the withdrawal conduit as shown in Fig. 7.5. The entry of water through the ports is controlled by the cylinder gates operated from the top, by means of wheels.

As when there is no water inside the withdrawal conduit, the dry intake will be subjected to greater buoyancy force. Hence, the structure of this intake should be more massive than the wet intake. The water from the desired depth of the river of reservoir can be collected by opening the desired port.

In case of emergency and temporary works, movable intakes can be used. In this type of intake pumping plant is installed in a carriage or trolley and the suction pipe having strainer pipe at the end is lowered in the water. The water is directly pumped from the river and sent for the treatment and distribution.

(c) Reservoir Intake:

There is large variation in discharge of all the rivers during monsoon and summer. 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.

Figure 7.7 shows a reservoir intake which is mostly used to draw the water from earthen dam reservoir. 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 level, 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 intake pipes 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. The valve tower is connected to the top of the dam by means of a foot bridge gang-way for reaching it.

In the case of earthen dams intake towers are separately constructed but in R.C.C. of masonry dams it is constructed inside the dam itself and only porters or intake-pipe are provided at various levels as shown in Fig. 7.8.

(d) Canal Intake:

As the water level in the canals remains more or less constant, there is no necessity of providing porters at various levels. Canal intake is a very simple structure constructed on the bank. Figure 7.9 essentially shows a canal intake. It essentially consists of a pipe placed in a brick masonry chamber constructed partly in the canal bank. On one side of the chamber as opening is provided with coarse screen for the entrance of water.

The end of the pipe in side chamber is provided with a bell-mouth fitted with a hemispherical fine screen as shown in Fig. 7.9. The out-let 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.

Selection of Intakes:

The main function of the intakes works is to collect the water from the surface source (within limitations of the water levels) and then discharge water so collected, by means of pumps or directly to the water treatment plants.

Intakes are structures which essentially consist of opening, grating or strainer through which the raw water from river, canal, or reservoir enters 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 plant.

The following points should be kept in mind while selecting a site for intake works:

(i) The best quality of water should be available at the site so that it can be easily and economically purified in less time by giving less load on the treatment plants.

(ii) At the site, there should not be heavy current of water which might endanger the safety of the intake works.

(iii) The site should be such that intake can draw sufficient quantity of water even in the worst condition, when the discharge of the source is minimum.

(iv) The site of intake should be easily approachable without any obstruction.

(v) The site should be such that intake work can draw more quantity of water if required in the future i.e., there should be sufficient scope for future.

(vi) The site should not be located in navigation channels, because such water is polluted and contains toilet and other discharges from the ships.

(vii) As far as possible the selection of the site should be near the treatment works, it will reduce the conveyance cost from the source to the water-works.

(viii) As far as possible the intake should not be located in the vicinity of the point of sewage disposal. If at all it becomes necessary due to unavoidable reasons to locate intakes in the close proximity of the sewage disposal, a weir should be constructed upstream of the disposal point, and the intake should be located in the upstream side of the weir.

(ix) At the site sufficient quantity should be available for the future expansion of the water-works.

(x) In case of rivers which meander or change their water course or the discharge reduces considerably, a diversion weir of a barrage should be constructed, which will ensure sufficient quantity of water around the intake.

Even after taking all precautions and locating the best site of intake, the quality of water will be affected by temperature, seasonal turnover, current due to wind and so many other causes.