Design period

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1. Design period

2. Design period --- The number of years for which the designs of water works have been done Mostly water works are designed for design period of 20-30 year Population forecasting A rithmetical increase method Geometrical increase method Incremental increase method Decrease rate method Simple graphical method

3. Arithmetical increase method Simple method of population forecasting It generally gives lower results In this method increase in population from decade to decade is kept constant This method is for large cities , which have reached their saturation population

4. Formula Pn = p+nl Pn = population after n decades n = no of years in decades I = avg increment for a decade n year 2013 1989 1947 End year 2023 2004 2015 1 0.5 6.8

5. Year population ? 1931 12000 1941 16500 1951 26800 1961 41500 Increment per decade - 4500 10300 14700

6. Geometrical increase method Percentage increase in population is kept constant This method gives higher results , since % increase never remains constant

7. Year population ? 1931 12000 1941 16500 1951 26800 1961 41500

8. Incremental increase method Arithmetic + geometrical method From census data the actual increase in population is found Then the increment increase in each decade is found Pn = p + n( Ia + Ic ) Ia = Avg arithmetical increase Ic = Incremental Increase method

9. Year population ? 1931 12000 1941 16500 1951 26800 1961 41500

10. year 1931 1941 1951 1961 1971 1981 1991 population 12000 16500 26800 41500 57500 68000 74100

11. INCREMENTAL INCREASE METHOD X = Average arithmetical increase Y= Incremental Increase method

12. Example: Predict the population for the year 2021, 2031, and 2041 from the following population data using the incremental increase method. YEAR POPULATION 1961 860,741 1971 989,726 1981 1,201,897 1991 1,689,890 2001 2,079,270 2011 2,601,071

13. SOLUTION For year 2021 For year 2031 For year 2041

14. Decrease rate method Decelerating growth is assumed to asymptotically approach a saturation population, that is, the maximum population predicted for the geographic area of interest. The saturation population may be based on practical limitations such as the maximum number of dwellings under the zoning restrictions or other constraints.

15. As a rule, the larger a city becomes, the smaller will be the rate of growth from year to year The Saturation population must be estimated. Note: the saturation population is the maximum number of people can inhabit a town based on the physical constraints of buildable land zoning.

16. Formula dp / dt = K d (S-P) K d = P= P 1 + (S-P 1 )(1-e -Kd (t - t 1 ) ) P = population t = time Kd = decreasing rate of increase growth constant S = Saturation population

17. Year population Increase Percentage increase in population Decrease in % increase 1940 8000 1950 12000 1960 17000 1970 22500 Total Average Year Net % increase in population population 1980 32.4-8.8=23.6 22,500 + (23.6/100)*22,500 =27,810 1990 2000

18. GRAPHICAL METHOD

19. SIMPLE GRAPHICAL METHOD In this method, the populations of last few decades are correctly plotted to a suitable scale on graph This extension should be done carefully and it requires proper experience and judgment. The best way of applying this method is to extend the curve by comparing with population curve of some other similar cities having the similar growth condition.

20. COMPARATIVE GRAPHICAL METHOD In this method the census populations of cities already developed under similar conditions are plotted. The curve of past population of the city under consideration is plotted on the same graph. The curve is extended carefully by comparing with the population curve of some similar cities having the similar condition of growth The advantage of this method is that the future population can be predicted from the present population even in the absence of some of the past census report.

21. MASTER PLAN METHOD The big and metropolitan cities are generally not developed in haphazard manner, but are planned and regulated by local bodies according to master plan. According to the master plan the city is divided into various zones such as residence, commerce and industry. The population densities are fixed for various zones in the master plan. From this population density total water demand and wastewater generation for that zone can be worked out. By this method it is very easy to access precisely the design population.

22. Logistical curve method This method is used when the growth rate of population due to births, deaths and migrations takes place under normal situation and it is not subjected to any extraordinary changes like epidemic, war, earth quake or any natural disaster, etc ., and the population follows the growth curve characteristics of living things within limited space and economic opportunity

23. Logistical curve method If the population of a city is plotted with respect to time, the curve so obtained under normal condition looks like S-shaped curve and is known as logistic curve

24. Formula Equation of the logistic curve Ps= Saturation population

25. Saturation population Ps P0 , P 1 , P2 = Population of the city at the time t , t 2 , t 3

26. Problem The population of a city in three consecutive years i.e. 1991, 2001 and 2011 is 80,000; 250,000 and 480,000, respectively. Determine (a) The saturation population, (b) The equation of logistic curve, (c) The expected population in 2021.

27. Solution P = t = P 1 = t 1 = P = t 2 =