Water Supply and Urban Drainage Unit 2.pdf

Its a module for engineers for water supply design

1. POPULATION FORCASTING
The present population of a town or a city can be best
determined by census.
Generally census is official surveys at some year intervals
say about 10 years.
Smaller period surveys can also be conducted by state
governments or local bodies.
These data is then used for predicting the future population
of the city
The design will be done on the basis of projected
population at the end of the design period

2. Methods of Population Forecasting
Arithmetical increase method
Geometrical increase method
Incremental increase method
Decreasing rate of growth method
Graphical method
Logistic Curve method
Growth composition analysis method
Ratio and correlation method

3. 1)Arithmetical increase method:
Generally applicable to a large and old city, whose
scope of further expansion has reached to
saturation limit
For small, average or comparatively new cities, likely
get low result than actual value
Is based upon the hypothesis that the rate of growth
is constant
Where; = the rate of change of population with time.
k = constant
P
n = P
0 +
k(t
n – t
o) P
n = population at some time in the future
P
0 = present population
t = the period of projection

4. For a population survey in a decades based
P
n = P
o + n
Where; P
n = population at some time in the future
P
0 = present population
n = no. of decades b/n present and future
average population increase in the past

For a given population after: one decade, P
1 = P
o + 1.
two decade, P
2 = P
1 + 1. = P
o + 2.
three decade, P
3 = P
2 + 1. = P
o + 3.
X X

5. Exercise
The present population of a city is 100,000. During the
previous consecutive decades the populations were 86,500,
90,500, and 95,000. Compute the expected population after
two, three and four decades. Using Arithmetical increase
method

6. 2)Geometrical increase method
(uniform percentage growth):
The percentage growth rate increase is assumed to be constant.
 This method gives higher values and should be applied to a new
industrial town at the beginning of development only for few
decades
Can be expressed as

For n decade, P
n = P
0 (1 + )
n

Where; P
0 = present population
P
n = the population at the end of n future decades
r = assumed growth rate that can be computed from the past
known population data in the following two ways; 100
r

7. growth rate
i.- Increase in population/initial population * 100, computed
for each decade, and their average may be taken as the
assumed constant per decade increase(r) in either:
a- the arithmetic average i.e r = OR

b- the geometric average i.e. r = , and

ii. Where; P
1 = initial known population
P
2 = final known population
t = no. of decades (period) b/n P
1 and
P
2
t
rrrr
n ...
321 t
nrrrr *...***
321 1
1
2
t
p
p
r

8. Example:
Present 50,000
Before one decade 47,100
Before two decades 43,500
Before three decades 41,000
Probable population after one, two and three decades using a
Geometrical increase method?

9. Quiz

1.The population of a city census in 1970 was 120,000 and the
second census which is carried out in 2005 is 160,000
estimate the population in 2025 and 2040
2.Estimate the population For the design period water supply
system
Treatment Plant with design period 27 years
Distribution system with design period 36 years
Pumps and service reservoirs with design periods 16 years
Using Geometric increase method, Arithmetical increase
method and Decreasing rate of growth method . The
following data are known (All the times are in EC)



43
Year 1961 1971 1981 1991 2001
Population 93,000 111,000 132,000 161,000 182,000

10. 3)Incremental increase method
For an average size town under normal condition the growth
rate is found to be in increasing order .
Depending upon whether the average of the incremental
increase in the past data is positive or negative.
The population for a future decade is worked out by adding
the mean arithmetic increase ( ) to the last known
population and added the average of the incremental
increase ( )
This method is a modification of arithmetical increase
method.
Give somewhere b/n the results given by ‘arithmetic
increase method’ and ‘geometric increase method’, and is
thus considered to be giving quite satisfactory results.
X Y

11. Mathematically
P
n = P
o + n +
Where; P
n = population after n decades
P
0 = present population
= average increase of populations of known decades
= average incremental increase of the known decades
-----------------------------------------------------------------------------------------------
Example;-
Present 50,000
Before one decade 47,100
Before two decades 43,500
Before three decades 41,000
Probable population after one, two and three decades using a
Incremental increase method?
X 
2
1nn Y

12. Exercise
The population of 5 decades from 1960 to 2000 are given in
table below. Estimate the population the population in 2025
2040 and 2056 by incremental increase method

Year 1960 1970 1980 1990 2000
Population 25,000 28,000 34,000 42,000 47,000

13. 4) Decreasing rate of growth method:
population goes on reducing, as the cities reach towards saturation
Average decrease in the percentage increase is worked out, and is then
subtracted from the latest percentage increase for each successive
decade
Gives rational results
Is only applicable in cases where the rate of growth shows a downward
trend.

Mathematically
P
n = P
n-1 (1+(W – nS)/100
Where P
n = Population after n decades
W = % increase of the population of the present
S = Average decrease in the % increase

14. Example;-
The population of 5 decades from 1960 to 2000 are given in
table below. Determine the population in the year 2010,
2020and 2030;

15. Reading Assignment
 Graphical method
Logistic Curve method
Growth composition analysis method
Ratio and correlation method

16. Factors Affecting Population Growth
Some of the possible factors influence the estimated population are:
 Unforeseen circumstances such as discovery of oil, mine, etc.
in the vicinity of the town
Sudden increase in religious importance and historical temples
Nature causing accidents of earth quake, floods, epidemics,
frequent famines, etc.
Development of industrial and other activities
Improvement of transport facilities and economic changes
Connection of the town with some very big and important
cities
Political changes in the adjoining country and nearness to the
national borders

17. Design Period of Water Supply Components
Design Period Is the time required to run any system unit to its full
capacity after its construction or installation
Depends on:
Useful life of component structures,
Amount & availability of additional investment likely to be
incurred for additional provisions,
Rate of interest on the borrowings and the additional money
invested,
Ease and difficulty that is likely to be faced in expansions to be
undertaken in future dates,
Anticipated rate of population growth, including possible shifts of
communities, industries and commercial establishments and
The likelihood that they will be rendered obsolete by
technological advances.

18. If this period is too long, the capital expenditure incurred
will block the money for long time without being utilized and
that present population of the town is over taxed.

Again, if it is too short, to meet the increased demands
expansions/extensions may require and frequent
constructional activities will cause disturbance and prevent
smooth functioning of water supply system.

19. The Design Period Water supply components are designed
to meet the requirements of the following periods
1.Storage Dams………………….........………………………..………..50 years
2.Infiltration works...........……………………………………………..30 years
3.Pumping units:
-pump house ………………………………........................………30 years
- electric motors and pumps………………………….................15 years
4.Water treatment units……………………………...........................15 years
5.Pipes and appurtenances
(in mains and distribution systems) .........................… ....30 years
6. Clear water reservoir, and service reservoirs.........................30 years