sources of water (water supply engineering)

8/13/2019 Sources of Water (Water Supply Engineering)

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CHAPTER IISOURCES OF WATER

2.1 Classification of Sources of Water

Main source of water is precipitation .

2.2 Surface Sources

Surface sources have water on the surface of the earth such as in stream, river, lake, wetland orocean.

2.2.1 Rivers

Natural channel Main source: either natural precipitation or snow-fed Perennial and non-perennial rivers Vast catchment area; hence, amount of water is large Contaminated source

2.2.2 Streams

Natural drainage Less catchment area Source: Melting snow or precipitation Found in hilly, mountain areas Low quantity of water Potable water

Sources of Water

Surface Source

River, Stream,Lake, Pond,Impounded

Reservoir

Sub-Surface/Under

ground/GroundSource

Spring, Well,Infiltration

Gallery,Infiltration Well


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2.2.3 Lakes

Natural depression filled with water Found in mountain and hilly areas Quantity of water depends on: depression, catchment area and soil type Quality varies

2.2.4 Ponds

Natural/Artificial depression found in plain areas Bad quality of water Not used as water supply source Less quantity of water Can be used for animal bathing and irrigation purposes.

2.2.5 Impounded Reservoirs An impounding reservoir is a basin constructed in the valley of a stream or river for thepurpose of holding stream flow so that the stored water may be used when water supply isinsufficient. E.g. Sundarijal Dam

The dam is constructed across the river in such places where minimum area of land issubmerged, where river width is less and the reservoir basin remains cup shaped havingmaximum possible depth of water. Hence, it is defined as an artificial lake created by theconstruction of a dam across the valley containing a watercourse.

Two functions: i) To impound water for beneficial use

ii) To retard flood

The location of impounded reservoir depends upon the quality and quantity of wateravailable, existence of suitable dam site, distance and elevation of reservoir, density anddistribution of population, geological conditions, etc.

The water quality is the same as in streams and rivers.

2.2.6 Numerical on Capacity Determination of Impounded Reservoirs

The flow in the river during the various months of the year (in m 3/s) is as follows:

January 2.97

February 1.99

March 1

April 0

May 0.51

June 1

July 2

August 3

September 4

October 5

November 4

December 2.8


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The river supplies water to a community having a constant demand of 6202 million litres/month.Determine the capacity of impounded reservoir.

I. ANALYTICAL METHOD

( )

Where, n = number of days in the month

MonthsFlow(in

m 3/s)

Inflow(ML)

Demand(ML)

CumulativeInflow(ML)

CumulativeDemand

(ML)

Surplus(ML)

Deficit(ML)

January 2.97 7954.848 6202 7954.848 6202 1752.85February 1.99 4814.208 6202 12769.056 12404 365.056

March 1 2678.4 6202 15447.456 18606 3158.54

April 0 0 6202 15447.456 24808 9360.54May 0.51 1365.984 6202 16813.44 31010 14196.6

June 1 2592 6202 19405.44 37212 17806.6 July 2 5356.8 6202 24762.24 43414 18651.8

August 3 8035.2 6202 32797.44 49616 16818.6September 4 10368 6202 43165.44 55818 12652.6

October 5 13392 6202 56557.44 62020 5462.56November 4 10368 6202 66925.44 68222 1296.56December 2.8 7499.52 6202 74424.96 74424 0.96

Total 74424.96 74424

II. GRAPHICAL METHOD The largest possible positive difference (perpendicular distance between the two

graphs) gives the value of maximum surplus. The largest possible negative difference (cumulative demand more) gives the value

of maximum deficit. The difference between the ends of the curves gives the value of the required

capacity of impounded reservoir.


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2.3 Ground Sources

When water seeps into the ground, it moves downward due to gravity through the pore spacesbetween soil particles and cracks in rocks. Eventually, the water reaches a depth where the soiland rock are saturated with water. Water which is found in the saturated part of the groundunderneath the land surface is called ground water.

2.3.1 Confined and Unconfined Aquifers

0

10000

20000

30000

40000

50000

60000

70000

80000

0 2 4 6 8 10 12 I n f l o w a n

d D e m a n

d ( C u m u l a t

i v e ) i n M L

Months

Determination of Capacity of Impounded Reservoir

Cumulative Demand (ML)

Cumulative Inflow (ML)


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2.3.2 Springs

A spring is the natural outflow of ground water appearing at the earths surface as a current ofstream of flowing water under the suitable geological conditions. Most favourable conditionsfor spring formation occur in Nepal and may be suitable for water supply schemes in villageareas in hilly region of Nepal.

Springs are capable of supplying small quantity of water so it cant be used as a source of waterto big towns but a well developed or combinations of the various springs can be used for watersupply especially villages near hills or bases of hills. The quality of water in spring is generallygood and may contain sulphur in certain springs which discharge hot water which can be usedonly for taking dips for the cure of certain skin diseases. It may be less costly because it maynot need treatment plant. Springs may be classified into the following two types:

a. Gravity Springsb. Non Gravity Springs


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1. Gravity Springs

These springs result from water flowing under hydrostatic pressure and they are of thefollowing three types:

i. Depression SpringThese springs are formed due to the overflowing of the water table, where the ground surfaceintersects the water table. The flow from such spring is variable with the rise or fall of watertable and hence in order to meet with such fluctuations, a deep trench may be constructednear such spring. The deeper the trench, the greater is the certainty of continuous flowbecause the saturated ground above the elevation of the trench bottom will act as a storagereservoir to compensate for the fluctuations of the water table.

ii. Surface Spring or Contact Spring

These are created by a permeable water bearing formation overlying a less permeable orimpermeable formation that intersects the ground surface. However, in such springs, becauseof the relatively small amount of underground storage available above the elevation of theoverflow crest, the flow from them is uncertain and likely to cease after a drought. Suchsprings can also be developed by the construction of a cutoff trench or a cutoff wall.

iii. Artesian Spring

These springs result from release of water under pressure from confined aquifers either at anoutcrop of the aquifer or through an opening in the confining bed. The amount of water

available in an artesian spring may be large if the catchment area is large. The flow may beslightly increased by removal of obstructions from the mouth of the spring.

2. Non Gravity Springs

Non gravity springs include volcanic spring (associated with volcanic rocks) and fissure spring (results from fractures extending to the great depths in the earths crust). These are also calledhot springs and contain high minerals as well as sulphur also.

2.3.3 Wells

A well is a hole or shaft, usually vertical and excavated in the ground for bringing groundwaterto the surface. Wells are classified as follows:

1. Open or Dug or Draw or Percolation Well

They are of large diameters (1 to 10 m), low yields and not very deep (2 to 20 m). These areconstructed by digging hence also called dug wells. The walls may be of brick, stone masonryor precast rings and thickness varies from 0.5 to 0.75 m depending upon the depth of the well.It is also further classified as following two types:

i. Shallow Open Well

ii. Deep Open Well


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The quantity and quality depends upon the location and area of coverage. It is constructed bythe cut and covers method and made up with dry brick masonry wall or porous concreteblocks with weep holes and R.C.C. slab roof or an arch roof. Manholes are provided at suitablepoints for inspection. The perforations are covered by the graded gravel to prevent the entry offine particles in the gallery. Series of galleries may be laid in the proper slope and collected atcertain reservoir then it can be used as the water supply after certain treatment.

Infiltration Wells

Shallow wells constructed in series along the banks and sometimes under the bed of rivers tocollect water seeping through the walls of the wells are called infiltration wells. These wells areconstructed of brick masonry with open joints. For purpose of inspection, manhole is providedin the top cover of the well.

The water infiltrates through the walls and bottom of these wells and has to pass through sand

bed and gets purified to some extent. Various infiltration wells are connected by porous pipesand collected to the collecting sump well called Jack from where it can be conveyed for watersupply. The water quality is better in such well because the bed soil acts as a filter and lessertreatment may be required.

2.4 Selection of Water Source

The selection of the sources of water depends upon the following factors:

a. Location It should be near to the consumers area or town as far as possible. They may be either surface or ground sources and the selection of the source depends

upon other factors. If there is no river, stream or reservoir in the area, the ultimatesource is ground source.

Location may be at higher elevation such that required pressure may be obtained and water can be supplied by gravity flow.

b. Quantity of Water It should have sufficient quantity of water to meet the demand for that design period

in the wet and dry seasons also. Two or more sources can be joined for requiredquantity.

If possible, there should be sufficient supply for future extension of project.c. Quality of Water

The water should be safe and free from pathogenic bacteria, germs and pollution andso good that water can be cheaply treated.

The water quality should be such that it has less quantity of impurity, which furtherneeds less treatment.

d. Cost It should be able to supply water of good quality and quantity at the less cost. Gravity system of flow is generally cheaper than pumping. Lesser the impurities, lesser the treatment and cost is reduced. Cost analysis is necessary for various options and suitable one is selected.

e. Sustainable and Safe


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f. Reliableg. Non conflict among water users

(For pictures, refer any standard book. )

sources of water (water supply engineering)

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