Environmental Engg. 3(2+1)

Lesson-6 Transportation of water

If the source is at higher level than the treatment plant, the water can flow under gravity, automatically.  Similarly after necessary purification of water, it has to be conveyed to the consumers.   Therefore, for conveyance of water some sort of devices or structures is required.  The arrangement may be in the form of open channels, aqueducts, tunnels or pipes. 

Open channels

In any water supply systems, raw water from source to treatment plants may be carried in open channels.  Economical sections of open channels are generally trapezoidal while rectangular sections prove economical when rock cutting is involved.  The channels are to be properly lined to prevent seepage.   Also these kind of channels need to be taken along the gradient and therefore the intitial cost and maintenance cost may be high.  While open channels are not recommended for conveyance of treated water, they may be adopted for conveying raw water.  If these kind of channels are unlined, they have to be run with limited velocity of flow so that it does not effect scouring. 

Aqueducts

The term aqueduct is usually restricted to closed conduits made up of masonry.   These can be used for conveyance of water from source to treatment plant or for distribution.  Aqueducts normaly run half to two-third full at required capacity of supply in most circumstances.  In ancient times, rectangular aqueducts were most commonly used, but these days circular or horse-shoe shaped ones are more common.  Masonry aqueducts unless reinforced with steel, are usually constructed in horse-shoe cross-section.  This cross-section has good hydraulic properties and resists earth pressure well.  It is economical and easy to build.

Tunnels

Tunnels are also like aqueducts.  Tunnels which are not under pressure are usually constructed in horse-shoe shape.  But if they convey water under pressure, circular cross-section is the best.  In pressure tunnels, the depth of cover is generally such that the weight of overlying material overcomes the bursting pressure.  Tunnels are used to convey water into the cities from outside sources.

Pipes

Pipe is a circular closed conduit used to convey water from one point to another, under gravity or under pressure.  Usually pipes follow the profile of the ground surface closely.  If pipes do not run full, they are called to flowing under gravity.  But flow under gravity is possible only if the pipe is given a definite longitudinal slope.  Pipes running full will be said to be running under pressure.  Pipes are mostly made up of materials like cast iron, wrought iron, RCC, asbestos cement, plastic, timber, etc. 

Cast iron pipes

Cast iron pipes are used in majority of water conveyance mains because of centuries of satisfactory experience with it.  Cast iron pipe is resistant to corrosion and accordingly long lived; its life may be over 100 years. 

Advantages

• Cast iron pipes are of moderate cost

• Their jointing is easier

• They are resistant to corrosion

• They have long life

Disadvantages

• They are heavier and hence uneconomical when their diameter is more than 120 cm

• They cannot be used for pressures greater than 7 kg / cm².

• They are fragile

Wrought iron and galvanized iron pipes

Wrought iron pipes are manufactured by rolling flat plates of the wrought iron to the proper diameter and welding the edges.  Such pipes are much lighter than the cast iron pipes and can be more easily cut, threaded and worked.  They look much neater, but are much costlier.  They corrode quickly, and hence are used principally for installation within buildings.  These pipes are usually protected by coating them with a thin film of molten zinc.  Such coated pipes are known as galvanized iron pipes, and they are commonly jointed by screwed and socketed joints.

Steel pipes

Steel pipes of small diameter can be made from the solid, but larger sizes are made by riveting or welding together the edges of suitably-curved plates, the sockets being formed later in a press.  The joints may be either transverse or longitudinal.  Steel pipes cannot be easily made to resist high external pressures.

Cement concrete pipes

Cement concrete pipes may be either plain or reinforced, and are best made by the spinning process.  They may be either precast, or may be cast-in-situ.  The plain cement concrete pipes are used for heads up to 7 m while reinforced cement concrete pipes are normally used for head upto 60 m. 

Advantages

• They are more suitable to resist the external loads and loads due to backfilling.

• The maintenance cost is low.

• The inside surface of pipes can be made smooth, thus reducing the frictional losses.

• The problem of corrosion is not there.

• Pipes can be cast at site and hence the transportation problems are reduced.

• Due to their heavy weight, the problem of floatation is not there when they are empty.

Disadvantages

• Unreinforced pipes are liable to tensile cracks and they cannot withstand high pressure.

• The tendency of leakage is not ruled out as a result of its porosity and shrinkage cracks.

• It is very difficult to repair them.

• Precast pipes are very heavy, and it is difficult to transport them.