Civil Engineering Reference
In-Depth Information
Once conservation measures are implemented and people become accustomed to
the lifestyle change, conservation becomes a very reliable water supply alternative in
terms of its consistent and long-term availability. However, many communities rely on
conservation measures to weather droughts. Full-time conservation restricts the ability
to further conserve in emergencies, effectively reducing the ''factor of safety'' of a
water system.
Seawater and Brackish Water
Recent developments in membrane treatment have lowered the cost of desalinating
brackish water and seawater to competitive levels in some communities. It follows that
over the next 5-10 years, further advances are likely to make desalination the supply
of choice in many coastal communities. Seawater supplies are virtually unlimited, and
environmental factors are less critical than for fresh water supplies, so treated seawater
may be one of the most reliable supplies. Brackish water from agricultural drainage
and near-coastal surface water and groundwater sources are also becoming viable sup-
plies in some communities. Brine disposal concerns limit the feasibility of inland
desalination, however.
DESIGN PERIODS FOR WATER SOURCES
The quantity of water from a source(s) should be adequate to supply the total water
demand of a community, as well as a reasonable surplus for anticipated growth. An
analysis of the elements making up the total water demand of a community should be
conducted, including but not limited to the following items: location, climate, popu-
lation growth, type and character of community, fire protection, air conditioning, me-
tering practice, cost of water, water quality, and pressure on mains.
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Surface water systems either use a source of water that is continuously adequate
in quantity to satisfy present and reasonable future demands, or they convert an inter-
mittently inadequate source into a continuously adequate supply by storing water dur-
ing periods of surplus for use during periods of insufficiency.
In the case of multipurpose reservoir projects, the various demands for water should
be carefully integrated. For an impounded source, allowances should be made for
required water releases, evaporation seepage, and losses due to siltation. For major,
costly projects, such as impoundments, which require difficult planning, property ac-
quisition, and financing, the design period should be at least 50 years.
If the source of supply is located some distance from the point of use, a long supply
pipeline is required. A study should be made of the economic size of the pipeline,
taking into consideration cost of construction, expected future growth, and cost of
operation based on power costs and other factors. Parts of the supply works that can
be expanded without difficulty or excessive costs may be designed for a shorter period,
such as 10 to 20 years. Examples include intakes, pumping stations, or certain pipe-
lines.
Major projects, such as centralized well field and long transmission lines, may not
be suited to construction in phases without incurring excessive extra costs, or they
may involve difficult planning, property acquisition, or financing. For such projects, a
design period on the order of 50 years may be appropriate.
