Page 25 - Water Engineering Hydraulics, Distribution and Treatment

Divide
Yield
2
Area = 1 mi
Water shed, catchment
Rainfall = 40 in./yr
area, or drainage area
Evaporation = 20 in./yr
2
100 mi
Stream flow or runoff = 20 in./yr
= 348 MG/yr/mi
= 952,000 gpd/mi
Reservoir
Water surface = 4.3 mi 2 2 2 1.3 Sources of Water Supply 3
Collection system Average depth = 15 ft
Storage = 13.5 billion gal Figure 1.1 Rainfall, runoff, storage,
= 180 days of draft and draft relations in the
Reservoir and dam development of surface water
(conversion factors:
Pipeline
Purification works Filters 2 2
Pipeline Service 1mi = 2.59 km ;
Distribution reservoir 1in.∕yr = 25.4mm∕yr;
system Municipality 1ft = 0.3048 m; 2
2
1MG∕yr∕mi = 1.46 ML∕yr∕km ;
Distribution system Population = 500,000 1 gpd∕mi = 1.461 L∕d∕km ;
2
2
Consumption = 150 gpcd
= 75 MGD 1 billion gal = 1BG =
= 750,000 gpd/mi 2 3.785 billion L = 3.785 BL;
= 79% of mean annual 1 gpcd = 3.785 Lpcd;
rainfall 1MGD = 3.785 MLD).
35 to 500 gpcd (132–1890 Lpcd), varying radically with (b) From larger, prepared watersheds, or catches,
industrial water demands. Average rates between 100 and stored in reservoirs, for large communal supplies.
200 gpcd (378–757 Lpcd) are common, and a generalized 2. Surface water:
average of 150 gpcd (568 Lpcd) is a useful guide to normal
(a) From streams, natural ponds, and lakes of suffi-
requirements.
cient size, by continuous draft.
The capacity of individual system components is set by
(b) From streams with adequate flood flows, by inter-
what is expected of them. Distribution systems, for example,
mittent, seasonal, or selective draft of clean flood-
must be large enough to combat and control serious con-
waters, and their storage in reservoirs adjacent to
flagrations without failing to supply maximum coincident
the streams, or otherwise readily accessible from
domestic and industrial drafts. Fire demands vary with size
them.
and value of properties to be protected and are normally a
function of the gross size of the community. The distribu- (c) From streams with low dry-weather flows but
tion system leading to the high-value district of an average sufficient annual discharge, by continuous draft
American city of 100,000 people, for example, must have an through storage of necessary flows in excess of
excess of fire standby capacity equal in itself to the average daily use in one or more reservoirs impounded
rate of draft. For smaller or larger American communities, by dams thrown across the stream valleys.
the standby capacity falls or rises, within certain limits, more (d) From brackish and seawater by desalination.
or less in proportion to the square root of the population. Desalination is an artificial process by which
saline water is converted to freshwater. The most
common desalination processes are distillation
1.3 SOURCES OF WATER SUPPLY and reverse osmosis. Desalination is currently
The source of water commonly determines the nature of the expensive compared to most alternative sources
collection, purification, transmission, and distribution works. of water, and only a small fraction of total human
Common sources of freshwater and their development are as use is satisfied by desalination. It is only eco-
follows: nomically practical for high-valued uses (such
as household and industrial uses) in arid areas.
1. Rainwater: The most extensive use is in the Persian (Ara-
(a) From roofs, stored in cisterns, for small individ- bian) Gulf. Mildly saline waters (brackish) are
ual supplies. desalted most economically by reverse osmosis;