Page 110 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 4
Quantities of Water Demand
Design periods for water and wastewater structures
Table 4.1
Special characteristics
Type of structure
Water supply
25–50
Large dams and conduits
Hard and costly to enlarge
Easy to extend
20–25
When growth and interest rates are low
Wells, distribution systems, and filter plants
When growth and interest rates are high
10–15
Pipes more than 12 in. (304.8 mm) in diameter
20–25
Replacement of smaller pipes is more costly in
the long run
Full development
Requirements may change fast in limited areas
Laterals and secondary mains less than 12 in. (304.8 mm) in
diameter
Sewerage a a Design period (years)
Laterals and submains less than 15 in. (381 mm) in diameter Requirements may change fast in limited areas Full development
Main sewers, outfalls, and interceptors Hard and costly to enlarge 40–50
Treatment works When growth and interest rates are low a 20–25
When growth and interest rates are high a 10–15
Conversion factor: 1 in. = 25.4mm.
a The dividing line is in the vicinity of 3% per annum.
4.2 DESIGN POPULATION 4.2.2 Population Growth
4.2.1 Population Data Populations increase by births, decrease by deaths, and
change with migration. Communities also grow by annex-
For information on the population of given communities or
ation. Urbanization and industrialization bring about social
regions at a given time, engineers turn to the records of
and economic changes as well as growth. Educational and
official censuses or enumerations. The US government has
employment opportunities and medical care are among the
conducted a decennial census since 1790. Some state and
desirable changes. Among unwanted changes are the cre-
local enumerations provide additional information, usually
ation of slums and the pollution of air, water, and soil. Least
for years ending in 5, and results of special surveys sponsored
predictable of the effects on growth are changes in com-
by public authorities or private agencies for political, social,
mercial and industrial activity. Examples are furnished in
or commercial purposes may also be available. US census
Table 4.3 (a) for Detroit, MI, where the automobile industry
dates and intervals between censuses are listed in Table 4.2.
was responsible for a rapid rise in population between 1910
The information obtained in the decennial censuses is
and 1950; (b) for Providence, RI, where competition with
published by the US Bureau of the Census, Department of
southern textile mills was reflected in low rates of population
Commerce. Political or geographic subdivisions for which
population data are collated vary downward in size from the
country as a whole, to its coterminous portion only, individual
Table 4.3 Census populations of Detroit, MI; Providence, RI;
states and counties, metropolitan districts, cities and wards,
and Miami, FL; 1910–2006
townships and towns, and—in large communities—census
tracts. The tracts are areas of substantially the same size and City
large enough to house 3,000–6,000 people.
Census year (a) Detroit (b) Providence (c) Miami
1910 466,000 224,000 5,500
1920 994,000 235,000 30,000
Table 4.2 US census dates and intervals between censuses 1930 1,569,000 253,000 111,000
Year Date Census interval (years) 1940 1,623,000 254,000 172,000
1950 1,850,000 249,000 249,000
1790–1820 First Monday Approximately 10 1960 1,670,000 207,000 292,000
in August 1970 1,493,000 177,000 332,000
1830–1900 June 1 Exactly 10, except between 1980 1,203,000 157,000 347,000
1820 and 1830 1990 1,028,000 161,000 359,000
1910 April 15 9.875 2000 951,000 174,000 363,000
1920 January 1 9.708 2010 714,000 178,000 401,000
1930 April 1 10.250
Source: After US Bureau of the Census.
1940–2010 April 1 Exactly 10
All population values have been rounded to the nearest thousand.