Unit:4 Definition of Intake works of water supply note(Second year second parts)

4.1 Definition of Intake works

An intake is a device or structure placed in a water source:

•To permit the withdrawal of water from the source, and 

• discharge it into an intake conduit through which it flows to the treatment plant; or discharge it into an intake well from where it is pumped to the treatment plant necessity:

• to collect the water from the surface source and then discharge water so collected by means of pumps or directly to the treatment plant

 * Components of Intake

 i. An intake conduit with a screen at its inlet end and a valve to control the flow of water 

 ii. A structure permitting the withdrawal of water from the source, and housing facility supporting the intake conduit, valve operating devices, pumps etc. 

• The structure may be: 

• Stone masonry, brick masonry, RCC or concrete blocks

4.2 Site Selection of Intakes

The following points should be kept in mind while selecting a site for intakes.

➢As far as possible, the site should be near the treatment plant so that the cost of conveying water will be less.

➢Intake site should be selected where water quality is good so that treatment cost becomes less.

➢Intake site should not be heavy current of water, which may damage the intake structure.

➢The intake must be located at a place from where it can draw water even during the driest period of the year.

➢The intake should never be located near the navigation channels due to the possibility of pollution caused by the discharge of refuge from ships and boats.

➢Location should be such as that future expansion is possible.

➢The intake site should be easily accessible during floods and should not get flooded.

➢As far as possible the intake should not be located in the vicinity of the point of sewage disposal for avoiding the  pollution of water.  

➢Site should be stable and safe from wind currents, landslides etc.

➢In the case of a meandering river, the intake should not be located on the curves. However if the intake is to be located on the curve, it should be located on the concave bank.

➢  Intake site should be well connected by good approach of road.

4.3 Classification of Intakes

•Submerged Intake: Submerged intakes are those intakes that are constructed entirely under water and is commonly used to obtain water from lakes.

•Exposed Intake: Exposed intakes are in the form of  well or tower constructed near the bank of the river, or in some cases even away from the bank of the river. It is common due to ease of its operation.

•Wet Intake: In wet intake the water level of intake tower is practically the same as the water level of sources of supply. It is also known as jack well or sump well.

•Dry Intake: In dry intake there is no water in the water tower. Water enters through the port directly into the conveying pipes. The dry tower is simply used for the operation of valves. Unit 1 Introduction of water supply note(Second year second parts)

* Characteristics of Intakes

     4.3.1 River Intake

✓  A river intake is always located to the upstream side of the city to protect water from contamination from sewerage and solid waste and supply water under force of gravity.

✓  If the water level in the river is low, a weir may be constructed across it to raise the water level and divert it into intake from river. It is constructed along the bank of the river at such place from where required quantity of water can be obtained even in the dry period. As shown in below figure, it consists of masonry or RCC tower. River water enters in the lower portion of the intake through penstocks or entry ports located at different levels to permit withdrawal or water even when water level in river drops. The penstocks are fitted with screens to check floating solids. The penstocks are provided with valves to control the entry of water  through them. The valves are controlled from control room; the penstocks discharge water into intake tower or sump well. The water from the sump well of this intake is then with drawn by suction pipes or pumps.

b. Reservoir Intake  

        

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.

* Earth dam:

• Made of the earth (or soil) and gravel built up by compacting successive layers of soil.
• Can be constructed to a moderate height to which it depends on foundation material.
• The shear strength of soil plays the vital role in resisting the forces.
• It is cheaper in construction.

* Gravity dam:

•  Made up of masonry or solid concrete.

• The weight of the water is actually enough to resist the horizontal thrust of water pushing against it.

• The weight of the gravity dam makes it more stable.

• It can be constructed to any height.

• Once built, it needs less maintenance.

* Reservoir intake for earth dam

•The intake pipes are at different levels with the common vertical pipe.
•The intake pipe has the fine screen to permit entry of clear water.
•Pipes at different level maintain level of reservoir.
•Intake pipes are provided with valves to control the flow of water.
•The control room is at the top to operate valves.
•Common vertical pipe enters the conduit which carries water to the treatment plant.
• The dry intake tower has pipes fitted inside.

* Reservoir intake for gravity dam

•In the case of the reservoir, the type of intake to be provided depends on the type of dam constructed to create the reservoir.
•The reservoir intake for gravity dam consists of an intake created by constructing gravity dam.

The reservoir intake involves two alternative forms of intake works as in figure below:

1.  Single inlet port:

• The trash rack provided at the inlet check the entry of debris and other floating materials.

•  Water enters from the single port which has trash rack.

•  The slide gate allows water to flow through trash rack to another supply system.

2.  Many inlet ports:

•In this, intake well is provided in the main body of the dam.

•Inlet at various levels enables withdrawal of water even if the level of reservoir water drops.

•Gates and valves are used to control the flow of water.

•Inlet ports have screened openings to prevent debris from entering.

•The shear strength of soil plays the vital role in resisting the forces.

•It is cheaper in construction.

3.3 Spring Intake: spring intake are mostly built intakes in hilly areas in Nepal due to following reasons:

1.There is availability of spring sources in hilly areas at high elevation from where it is possible to flow under gravity.

2.In general, the quality of water found is relatively good in most of springs than stream sources that get turbid water in rainy season.  Therefore, complicated treatments are not necessary in springs.

3.The construction cost of spring intake is relatively low in compare with stream intakes as it needs less protection from heavy floods in monsoon.

4.To design and construct spring intakes the technology used is relatively simple and understand by low level technicians.

5.In general, spring water supply systems are gravity system so no need of pumping and affordable by poor community in rural hills.

* Selection basis for location of spring intakes:

 -The place should be close to the source. 
-The place should be above populated or farming areas.
- The place should be above foot path, cattle watering and washing places.
- Places where surface water run-off during the monsoon can be easily drained off. 
- Where the immediate surrounding above the spring is not easily accessible to people and livestock."  

                 

It consists of two chambers as collection chamber and valve chamber. Collection chamber should be away from the source as far as possible. The base of the collection chamber is made of plain cement concrete to avoid leakage. All walls are made of stone masonry. The heavy structure is avoided to avoid its settlement. In order to reduce the backup pressure, the collection chamber needs to be constructed away from the source. The collection chamber is provided with the wing walls on both sides which divert the water from the source to the collection chamber. The collection chamber acts as sedimentation tank, which removes suspended particles and turbidity. In monsoon, the turbidity of the water is high, so special treatment with sedimentation and filters are needed. As far as possible the treatment work should be avoided to reduce the cost so a pure source needs to be selected. The gravel and packing are done in the water hearing layer upstream to prevent the coarse material entering the Collection chamber. Water-bearing layer is covered with the plastic sheet and clay filling. The outlet pipe fitted with screens is kept at about 10 to 15 cm above the floor to screen out the suspended particles to enter into the transmission main of water supply system. The water contains particle in it which may settle down in the collection chamber as sediments which should be washed out when it reaches 5 cm of the outlet pipe. The washout pipe is carried down the slope to allow the sediments to flow into the nearby drainage system. The overflow pipe is provided in the collection chamber to prevent the backup pressure. The washout valve is operated when washing of the sediments is done in the collection chamber. Mild steel, concrete or stone masonry covers are used to cover the collection and the valve chamber. In the valve chamber, the valves are connected to outlet pipe and washout pipe. During normal operation outlet valve is opened while washout valves are closed but during washing of the sediments from the collection is done it is opened. The vent pipe is provided to outlet pipe to release air pressure. The unions are provided to facilitate the removal of valves during maintenance works. 

* Protection of spring intakes

For safe drinking water free of contamination. 

 For increasing quality and quantity of water content.

For prevention of scarcity of water in near future. 

Proper drainage should be provided around source to divert the run-off water. Surface run-off must not be allowed to flow into the catchment of springs. 

Elements for protection of spring intake are: 

Afforestation: Tress are planted above the spring sources allow the water to seep rather than as surface runoff and increase intake water. 

Surface water drain: Surface water drain should be 8m above and around the spring to drain the surface water run off during monsoon. The ditch should be deep, and can be lined with dry stone masonry. 

Plantation (Bush): Plantation of grasses below the barbed wire fence which also allow the water to seep. Grasses and bushes prevent the surface soil erosion.

Barbed wire fence: There should be no habitant and easy access to animals around springs up to a distance of 30 m to 90 m to avoid contamination. To prevent trespassing of humans and grazing animals and contamination of spring water, barbed wire fencing 5m from spring intake.  

Concrete Covers: The catchment of a spring source can be re with concrete slab and buried for further protection.

 Retaining walls: If erosion is seen to be a major problem then retaining walls of gabion or dry stone masonry are bu stabilize land around the  intake.