The following points highlight the five important modes used for conveyance of water to the city. The modes are: 1. Open Channels 2. Aqueducts 3. Tunnels 4. Flumes 5. Pipes.
Mode # 1. Open Channels:
These are occasionally used to convey the water from the source to the treatment plant. These can be easily and cheaply constructed by cutting in high grounds and banking in low grounds. The channels should be lined properly to prevent the seepage and contamination of water. As water flows only due to gravitational force, a uniform longitudinal slope should be given.
The hydraulic gradient line in channels touches the water level at every place. The velocity of water in channels should not exceed the permissible limit otherwise scouring will start in the bed and water will get dirty. In channels there is always loss of water by seepage and evaporation.
Mode # 2. Aqueducts:
Aqueduct is the name given to the closed conduit constructed with masonry and used for conveying water from source to the treatment plant or point of distribution. Aqueduct may be constructed with bricks, stones or reinforced cement concrete.
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In olden days rectangular aqueducts were used, but now a days horse-shoe or circular section are used. These aqueduct are mostly constructed with cement concrete. The average velocity should be 1.0 m/sec.
Mode # 3. Tunnels:
This is also gravity conduit, in which water flows under gravitational force. But sometimes water flows under pressure and in such cases these are called pressure tunnels. Grade tunnels are mostly constructed in horse-shoe cross-section, but pressure tunnels have circular cross-section.
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In pressure tunnels the depth of water is generally such that the weight of the overlying material will be sufficient to check, the bursting pressure. Tunnels should be water-tight, and there should be no loss of water.
Mode # 4. Flumes:
These are open channels supported above the ground over trestles etc. Flumes are usually used for conveying water across valleys and minor low lying areas or over drains and other obstructions coming in the way. Flumes may be constructed with R.C.C., wood or metal. The common sections are rectangular and circular.
Mode # 5. Pipes:
These are circular conduits, in which water flows under pressure. Now a days pressure pipes are mostly used at every place and they have eliminated the use of channels, aqueducts and tunnels to a large extent. These are made of various materials as cast iron, wrought iron, steel, cement concrete, asbestos, cement, timber etc. In the town pipes are also used for distribution system.
In distribution system pipes of various diameters, having many connections and branches, are used. Water pipe lines follow the profile of the ground and its location which is most economical, causing less pressure in the pipes is chosen. The cost of pipe line depends on the internal pressure to bear and the length of pipe line.
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Therefore as far as possible the hydraulic line is kept closer to the pipe line. In the valley or low points a scour-valve is provided to drain the line and removing accumulated suspended matter. Similarly at high points air-relief valves are provided to remove the accumulated air. To prevent the bursting of pipes due to water hammer, surge tanks or stand pipes are provided at the end of the pipes.
The selection of material for the pipe is done on the following points:
(i) Carrying capacity of the pipe.
(ii) Durability and life of the pipe.
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(iii) Type of water to be conveyed and its corrosive effect on the pipe material.
(iv) Availability of funds,
(v) Maintenance cost, repair etc.
The pipe material which will give the smallest annual cost or capitalized cost will be selected, because it will be most economical.