Page 41 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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4 Chapter 1 Introduction to Water Systems

Divide

Yield
Area 1 mile 2
Water shed, catchment Rainfall 40 in./year
area, or drainage area Evaporation 20 in./year
100 mile 2 Stream flow or runoff 20 in./year
348 MG/year/mile 2
952,000 gpd/mile 2

Reservoir
Water surface 4.3 mile 2
Average depth 15 ft
Collection system
Storage 13.5 billion gal
180 days of draft
Reservoir and dam
Purification works Pipeline Filters
Pipeline Service
Distribution reservoir
system Municipality
Distribution system Population 500,000
Consumption 150 gpcd
75 MGD
750,000 gpd/mile 2
79% of mean annual
rainfall
Figure 1.1 Rainfall, Runoff, Storage, and Draft Relations in the Development of Surface Water
2
2
2
(Conversion factors: 1 mi 2.59 km ; 1 in./yr 25.4 mm/yr; 1 ft 0.3048 m; 1 MG/yr/mi
2
2
2
1.461 ML/yr/km ; 1 gpd/mi 1.461 L/d/km ; 1 billion gal 1 BG 3.785 billion L 3.785 BL;
1 gpcd 3.785 Lpcd; 1 MGD 3.785 MLD.)
1.2 REQUIRED CAPACITY
Water supply systems are designed to meet population needs for a reasonable number of years
in the future. The rate of consumption is normally expressed as the mean annual use in gallons
per capita daily (gpcd) or liters per capita daily (Lpcd), and seasonal, monthly, daily, and
hourly departures in rate are given in percentages of the mean. In North America the spread in
consumption is large: from 35 to 500 gpcd (132 to 1,890 Lpcd), varying radically with indus-
trial water demands. Average rates between 100 and 200 gpcd (378 to 757 Lpcd) are common,
and a generalized average of 150 gpcd (568 Lpcd) is a useful guide to normal requirements.
The capacity of individual system components is set by what is expected of them.
Distribution systems, for example, must be large enough to combat and control serious
conflagrations without failing to supply maximum coincident domestic and industrial
drafts. Fire demands vary with size and value of properties to be protected and are nor-
mally a function of the gross size of the community. The distribution system leading to the
high-value district of an average American city of 100,000 people, for example, must have
an excess of fire standby capacity equal in itself to the average rate of draft. For smaller or
larger American communities, the standby capacity falls or rises, within certain limits,
more or less in proportion to the square root of the population.