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Module 2: Water Quantity and Intake Detail

Lecture 2: Water Quantity Estimation

Lecture 3: Intake, Pumping and Conveyance

Water Quantity Estimation

The quantity of water required for municipal uses for which the water supply scheme has to be designed requires following data:

1. Water consumption rate (Per Capita Demand in litres per day per head)

2. Population to be served.

Quantity= Per capita demand x Population

Water Consumption Rate

It is very difficult to precisely assess the quantity of water demanded by the public, since there are many variable factors affecting water consumption. The various types of water demands, which a city may have, may be broken into following classes:

Water Consumption for Various Purposes:

Types of Consumption Normal Range (lit/capita/day) Average %

1 Domestic Consumption 65-300 160 35

2 Industrial and Commercial Demand 45-450 135 30

3 Public Uses including Fire Demand 20-90 45 10

4 Losses and Waste 45-150 62 25

Fire Fighting Demand:

The per capita fire demand is very less on an average basis but the rate at which the water is required is very large. The rate of fire demand is sometimes traeted as a function of population and is worked out from following empirical formulae:

Authority Formulae (P in thousand) Q for 1 lakh Population)

1 American Insurance Association Q (L/min)=4637 P (1-0.01 P) 41760

2 Kuchling's Formula Q (L/min)=3182 P 31800

3 Freeman's Formula Q (L/min)= 1136.5(P/5+10) 35050

4 Ministry of Urban Development Manual Formula Q (kilo liters/d)=100 P for P>50000 31623

Factors affecting per capita demand:

a. Size of the city: Per capita demand for big cities is generally large as compared to that for smaller towns as big cities have sewered houses.

b. Presence of industries.

c. Climatic conditions.

d. Habits of people and their economic status.

e. Quality of water: If water is aesthetically $ medically safe, the consumption will increase as people will not resort to private wells, etc.

f. Pressure in the distribution system.

g. Efficiency of water works administration: Leaks in water mains and services; and unauthorised use of water can be kept to a minimum by surveys.

h. Cost of water.

i. Policy of metering and charging method: Water tax is charged in two different ways: on the basis of meter reading and on the basis of certain fixed monthly rate.

Fluctuations in Rate of Demand

Average Daily Per Capita Demand

= Quantity Required in 12 Months/ (365 x Population)

If this average demand is supplied at all the times, it will not be sufficient to meet the fluctuations.

• Seasonal variation: The demand peaks during summer. Firebreak outs are generally more in summer, increasing demand. So, there is seasonal variation .

• Daily variation depends on the activity. People draw out more water on Sundays and Festival days, thus increasing demand on these days.

• Hourly variations are very important as they have a wide range. During active household working hours i.e. from six to ten in the morning and four to eight in the evening, the bulk of the daily requirement is taken. During other hours the requirement is negligible. Moreover, if a fire breaks out, a huge quantity of water is required to be supplied during short duration, necessitating the need for a maximum rate of hourly supply.

So, an adequate quantity of water must be available to meet the peak demand. To meet all the fluctuations, the supply pipes, service reservoirs and distribution pipes must be properly proportioned. The water is supplied by pumping directly and the pumps and distribution system must be designed to meet the peak demand. The effect of monthly variation influences the design of storage reservoirs and the hourly variations influences the design of pumps and service reservoirs. As the population decreases, the fluctuation rate increases.

Maximum daily demand = 1.8 x average daily demand

Maximum hourly demand of maximum day i.e. Peak demand

= 1.5 x average hourly demand

= 1.5 x Maximum daily demand/24

= 1.5 x (1.8 x average daily demand)/24

= 2.7 x average daily demand/24

= 2.7 x annual average hourly demand

Design Periods & Population Forecast

This quantity should be worked out with due provision for the estimated requirements of the future . The future period for which a provision is made in the water supply scheme is known as the design period.

Design period is estimated based on the following:

• Useful life of the component, considering obsolescence, wear, tear, etc.

• Expandability aspect.

• Anticipated rate of growth of population, including industrial, commercial developments & migration-immigration.

• Available resources.

• Performance of the system during initial period.

Population Forecasting Methods

The various methods adopted for estimating future populations are given below. The particular method to be adopted for a particular case or for a particular city depends largely on the factors discussed in the methods, and the selection is left to the discrection and intelligence of the designer.

1. Arithmetic Increase Method

2. Geometric Increase Method

3. Incremental Increase Method

4. Decreasing Rate of Growth Method

5. Simple Graphical Method

6. Comparative Graphical Method

7. Ratio Method

8. Logistic Curve Method

Arithmetic Increase Method

This method is based on the assumption that the population increases at a constant rate; i.e. dP/dt=constant=k; Pt= P0+kt. This method is most applicable to large and established cities.

Geometric Increase Method

This method is based on the assumption that percentage growth rate is constant i.e. dP/dt=kP; lnP= lnP0+kt. This method must be used with caution, for when applied it may produce too large results for rapidly grown cities in comparatively short time. This would apply to cities with unlimited scope of expansion. As cities grow large, there is a tendency to decrease in the rate of growth

Incremental Increase Method

Growth rate is assumed to be progressively increasing or decreasing, depending upon whether the average of the incremental increases in the past is positive or negative. The population for a future decade is worked out by adding the mean arithmetic increase to the last known population as in the arithmetic increase method, and to this is added the average of incremental increases, once for first decade, twice for second and so on.

Decreasing Rate of Growth Method

In this method, the average decrease in the percentage increase is worked out, and is then subtracted from the latest percentage increase to get the percentage increase of next decade.

Simple Graphical Method

In this method, a graph is plotted from the available data, between time and population. The curve is then smoothly extended up to the desired year. This method gives very approximate results and should be used along with other forecasting methods.

Comparative Graphical Method

In this method, the cities having conditions and characteristics similar to the city whose future population is to be estimated are selected. It is then assumed that the city under consideration will develop, as the selected similar cities have developed in the past.

Ratio Method

In this method, the local population and the country's population for the last four to five decades is obtained from the census records. The ratios of the local population to national population are then worked out for these decades. A graph is then plotted between time and these ratios, and extended upto the design period to extrapolate the ratio corresponding to future design year. This ratio is then multiplied by the expected national population at the end of the design period, so as to obtain the required city's future population.

Drawbacks:

1. Depends on accuracy of national population estimate.

2. Does not consider the abnormal or special conditions which can lead to population shifts from one city to another.

Logistic Curve Method

The three factors responsible for changes in population are :

(i) Births, (ii) Deaths and (iii) Migrations.

Logistic curve method is based on the hypothesis that when these varying influences do not produce extraordinary changes, the population would probably follow the growth curve characteristics of living things within limited space and with limited economic opportunity. The curve is S-shaped and is known as logistic curve.

Intake Structure

The basic function of the intake structure is to help in safely withdrawing water from the source over predetermined pool levels and then to discharge this water into the withdrawal conduit (normally called intake conduit), through which it flows up to water treatment plant.

Factors Governing Location of Intake

1. As far as possible, the site should be near the treatment plant so that the cost of conveying water to the city is less.

2. The intake must be located in the purer zone of the source to draw best quality water from the source, thereby reducing load on the treatment plant.

3. The intake must never be located at the downstream or in the vicinity of the point of disposal of wastewater.

4. The site should be such as to permit greater withdrawal of water, if required at a future date.

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

6. The intake site should remain easily accessible during floods and should noy get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.

Design Considerations

1. sufficient factor of safety against external forces such as heavy currents, floating materials, submerged bodies, ice pressure, etc.

2. should have sufficient self weight so that it does not float by upthrust of water.

Types of Intake

Depending on the source of water, the intake works are classified as follows:

Pumping

A pump is a device which converts mechanical energy into hydraulic energy. It lifts water from a lower to a higher level and delivers it at high pressure. Pumps are employed in water supply projects at various stages for following purposes:

1. To lift raw water from wells.

2. To deliver treated water to the consumer at desired pressure.

3. To supply pressured water for fire hydrants.

4. To boost up pressure in water mains.

5. To fill elevated overhead water tanks.

6. To back-wash filters.

7. To pump chemical solutions, needed for water treatment.

Classification of Pumps

Based on principle of operation, pumps may be classified as follows:

1. Displacement pumps (reciprocating, rotary)

2. Velocity pumps (centrifugal, turbine and jet pumps)

3. Buoyancy pumps (air lift pumps)

4. Impulse pumps (hydraulic rams)

Capacity of Pumps

Work done by the pump,

H.P.=QH/75

where, = specific weight of water kg/m3, Q= discharge of pump, m3/s; and H= total head against which pump has to work.

H= Hs + Hd + Hf + (losses due to exit, entrance, bends, valves, and so on)

where, Hs=suction head, Hd = delivery head, and Hf = friction loss.

Efficiency of pump (E) = QH/Brake H.P.

Total brake horse power required = QH/E

Provide even number of motors say 2,4,... with their total capacity being equal to the total BHP and provide half of the motors required as stand-by.

Conveyance

There are two stages in the transportation of water:

1. Conveyance of water from the source to the treatment plant.

2. Conveyance of treated water from treatment plant to the distribution system.

In the first stage water is transported by gravity or by pumping or by the combined action of both, depending upon the relative elevations of the treatment plant and the source of supply.

In the second stage water transmission may be either by pumping into an overhead tank and then supplying by gravity or by pumping directly into the water-main for distribution.

Free Flow System

In this system, the surface of water in the conveying section flows freely due to gravity. In such a conduit the hydraulic gradient line coincide with the water surface and is parallel to the bed of the conduit. It is often necessary to construct very long conveying sections, to suit the slope of the existing ground. The sections used for free-flow are: Canals, flumes, grade aqueducts and grade tunnels.

Pressure System

In pressure conduits, which are closed conduits, the water flows under pressure above the atmospheric pressure. The bed or invert of the conduit in pressure flows is thus independant of the grade of the hydraulic gradient line and can, therefore, follow the natural available ground surface thus requiring lesser length of conduit. The pressure aqueducts may be in the form of closed pipes or closed aqueducts and tunnels called pressure aqueducts or pressure tunnels designed for the pressure likely to come on them. Due to their circular shapes, every pressure conduit is generally termed as a pressure pipe. When a pressure pipe drops beneath a valley, stream, or some other depression, it is called a depressed pipe or an inverted siphon.

Depending upon the construction material, the pressure pipes are of following types: Cast iron, steel, R.C.C, hume steel, vitrified clay, asbestos cement, wrought iron, copper, brass and lead, plastic, and glass reinforced plastic pipes.

Hydraulic Design

The design of water supply conduits depends on the resistance to flow, available pressure or head, and allowable velocities of flow. Generally, Hazen-William's formula for pressure conduits and Manning's formula for free flow conduits are used.

Hazen-William's formula

U=0.85 C rH0.63S0.54

Manning's formula

U=1/n rH2/3S1/2

where, U= velocity, m/s; rH= hydraulic radius,m; S= slope, C= Hazen-William's coefficient,

and n = Manning's coefficient.

Darcy-Weisbach formula

hL=(fLU2)/(2gd)

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