Table of Contents
Intake is a device or a structure installed in the water source to permit the withdrawal of water and discharge it into intake conduct then to the treatment plant.
It consists of openings, grating or strainers, valves, operating devices, pumps, a structure or housing to support intake conduct, intake conduct, etc.
The main function of The intake works is to collect water from the water source and then discharge water so collected, by means of pumps or directly to the treatment plant.
1. Site Selection of an Intake Structure
The site selection of an intake structure is as follows:
1. Location of the intake site should be as far as possible near the treatment plant so that cost of conveying would be less.
2. Water quality available in the intake sites should be high which will reduce treatment costs. There should not be any disposal point of wastewater upstream of intake.
3. Intake site should not locate near the navigation channel due to the chances of pollution of water.
4. intake site should be located so as to ensure s L-condition. Intake to fetch water from the deeper portion of the river and penstock may be kept two or more to take after in dry season.
5. Intake site should be located such that sufficient future extensions and additions.
6. The intake site should be easily accessible even during the flood.
7. Intake site should not locate in a meandering. It should locate the concave or outer bank so that water is available at all times.
8. intake site should be located geologically stable and free from possibilities of erosion, silting, scouring, and heavy current.
9. The intake site should be well connected by a good approach road.
10 In the selection of the intake site the natural cause such as seasons variations, winds, currents, climate, etc should be studied to ensure the sustainability of intake works.
2. Prevention Needed for Designing of Intake Structure
General preventive measures that should be considered during the design of intake structures
Proper design of intake is required for efficient and effective work Following are the factors which are to be considered in the design a: intakes;
a) Factor of safety
c) Protection of sites
d) Screens and strainers
e) Self-weight Size and number of inlets
a. The factor of safety
The intake structure should be designed with sufficient factors of safety so that it can effectively resist external forces due to heavy waves and currents, ice Pressure impact of floating objects, etc.
The design depth of the foundation of intake should be sufficient so that intake could be prevented from possible damage by the current of water.
c. Protection of sides:
During the flood, boulders may enter to intake and maybe damaged so its sides should be protected by a cluster of piles.
d. Screens and strainers:
To avoid the entry of floating matter and fish in the intake channel screens and strainers are provided. If screenings allow in the conduit that may clog or damage the pumps, valves, etc, and interfere during treatment works.
The intake should be of adequate self-weight so that the chances of its floating or washing by the upthrust of water may be minimized. It is essential to construct the intake structures with masonry work and broken stones should be filled in the bottom to grant additional safety.
f. Size and numbers of inlets:
Water pool levels may vary from season to season so adequate size and numbers of inlets should be provided to draw water in the dry season and during the flood.
3. Classification of Intake Structure
For the various surface source of water, different types of intakes are used. Various types of intakes are commonly used.
a. Submerged intake
Intake constructed entirely under the pool is submerged intake.
b. Exposed intake
When an intake is constructed with showing housing or intake tower above the pool is exposed intake.
c. Wet Intake
In wet intake water is allowed in the intake tower or control room at the gate (valve) closed condition, the entry port is inside the housing.
d. Dry Intake
In dry intake, if gets (valves) are closed there is no water inside the intake tower, or the entry port is directly passed into the convey pipes but operation valves are used.
3.1. River Intakes
An intake tower constructed at the bank of the river to acquire water is river intake. It consists of masonry or RCC, an intake tower (housing) which is provided with several inlets (3 commons) called a penstock.
These penstocks are positioned at different levels to permit the river water for minimum flow, average flow, and maximum flow, sometimes only two penstocks are provided. In the entry, a port screen is provided to prevent the entry of debris.
To control or regulate the flow valves are provided in the penstock which can be operated from the control room. In the control, a room pump is installed at the top.
The intake tower is filled with water during gate or valve closed condition it is a wet intake. In dry intake, there is no water intake tower during gate or valve closed condition.
A typical wet river intake so that the water is always filled in the sump well of the intake tower as wet for all the time. Wet intake can be modified to dry by connecting penstocks to the suction pipe of the pump directly and hence water will not be allowed to sump well.
In the case of the unstable river bed, the intake tower may be found slightly well. offset from the river bed. In this type of river intake, a pipe from the submerged intake delivers water to the jack well then water is lifted and delivered to the treatment plant through the transmission mains.
Water available in the river may not be sufficient to meet demand in k season in such case a darn or weir across the river is constructed to meet dry demand called an impended reservoir. When the intake tower is constructed in such a case is called reservoir intake.
There are two types of reservoir intake (Earthen and Gravity dams) commonly used.
Earthen darn consists of an intake tower constructed on the upstream toe at the dam from where intake can draw a sufficient quantity of water even in the worst condition. Penstocks are installed at different levels through which water is withdrawn. There is the provision of a hemispherical screen in the entry of these penstocks to prevent the entry of floating matters.
In the penstock valves are provided to control and regulate the flow of water. This intake is a dry intake because there is no water inside the intake tower. For inspection and cleaning inside the housing ladder or footbridge is provided from the control room.
A gravity dam has two alternative forms of intake works as having a single port and multiple ports. An intake with the entry of water is through a single port which has a trash rack structure to check the entry of debris and other floating matters. These are made in the form of semi polygonal grid of iron or steel bars. In order to control flow generally slide gates or sometimes valves are used which may be housed in the body of the dam itself.
An intake well is provided in the main body of the Dam. Water enters the well through inlet ports located at different levels and provided with screened openings.
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3.2. Spring Intake
Spring is a place on the earth’s surface where groundwater emerges naturally. An intake is constructed at the spring source to draw off water called spring intake.
Springs are generally found on hill slopes due to geological formation as impervious layer outcrops.
Generally, spring water does not contain suspended impurities and harmful bacteria. It may be used for a small rural water supply scheme.
Springs are susceptible to contamination by surface water, especially during rainstorms Hence, a U-shaped surface drainage diversion ditch or an earth berm at least 15 meters uphill from the spring to divert any surface runoff away from the spring has to be constructed.
An area has to be fenced at least 30 meters in all directions around the spring box to prevent contamination by livestock and people who are unaware of the spring’s location. To maintain discharge plantation may be done in the periphery of the spring source. Plan and section of spring.
4. Selection of Intake Structure
General requirements for selecting the location of the spring intake in order to get good quality water.
a. It should be as close to the source as possible.
b. It should be above populated or farming (agricultural) areas.
c. It should be above the footpath, cattle watering, and washing places.
d. It should be easy to drain off surface runoff during rain.
e. It should not be easy access to people and livestock.
f. It should not allow water logging near the intake.
5. Important Factor for Construction Intake Structure
Factors that should be taken into account or considered while Constructing the spring intake.
a.To prevent the creation of backup pressure, the collection chamber needs to be constructed away from the source by providing ahead of about 4 to 5 meters of free flow to occur from the intake
b. Heavy intake structure should be avoided to prevent settlement.
c. Stone soiling below the floor should be avoided to prevent leakage.
d. Adequate space in the valve box should be provided so that repair and maintenance work could be performed easily.
e. Union should be provided to avoid complex problems fluxing replacement of gate valve at the time of repair.
f. Over-excavation of an impervious layer at the base of the Outlet of the spring may lose flow so to avoid such a problem special Care is required at the time of excavation.
g. Inlet pipe should be covered with stone soiling and upstream of intake with an impervious material to prevent the entry of suspended particles.
h. It is essential to restrict access to animal inhabitation of intake at least 30 m to avoid contamination.
i. Surface runoff that occurs after rain should be easily drained off so that provision of the drain in the periphery of habitation of intake should be facilitated to prevent pollution.
j. Spring of low yield less than 0.05 lps should not be tapped for gravity flow schemes.
6. Function of Intake Structure
The basic function of intake is;
a. To ensure the required water
b. To check trash and debris entry along with water entering
c. To prevent the entry of the ice
d. To secure entry of water with minimum disturbance
e. To reduce sediment entry
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