Page 282 - Water Engineering Hydraulics, Distribution and Treatment

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Pumping, Storage, and Dual Water Systems
Chapter 8
cities, are adopting dual systems. They began in arid areas
but they are present also in Oregon and Washington, the
(c) where frazil ice may be a problem, holding the veloc-
ity of flow into the intake structure to minimum, generally
country’s wettest states. In Florida, with about 48 in. per year
(1,219 mm/yr) of rainfall, some 450 communities have dual
not to exceed 0.5 ft/s (0.1524 m/s); (d) inspection of man-
holes every 1,000 ft (304.8 m) for pipe sizes large enough
systems. Their popularity is based largely on the drinking
to permit visual inspection; (e) occasional cleaning of the
water supply conservation they provide.
For new communities, a dual system with one system
inlet line; (f) adequate protection against rupture by drag-
ging anchors, ice, and so on; (g) ports located above the
for drinking water only, and the other for all nonpotable
purposes, including fire protection, would address not only
bottom of the stream, lake, or impoundment, but at sufficient
depth to be kept submerged at low water levels; (h) where
drinking water supply problems, but water quality problems
as well. In addition, such systems would be far less costly
shore wells are not provided, a diversion device capable of
than conventional systems. Communities would be able to facilities for release of less desirable water held in storage;
keeping large quantities of fish or debris from entering an
afford membrane treatment because the amount of drinking intake structure; and (i) when buried surface water collec-
water to be treated to high quality would be considerably tors are used, sufficient intake opening area to minimize inlet
smaller than overall rates of water to be treated. headloss. Particular attention should be given to the selec-
One such system was adopted 10 years ago for a new tion of backfill material in relation to the collector pipe slot
suburb of Sydney, Australia, called Rouse Hill, involving size and gradation of the native material over the collector
250,000 people with a first stage for 100,000 residents. It has system.
been operating successfully since. This approach is partic- A raw water pumping well shall meet the following
ularly beneficial for systems that are obliged to take water requirements: (a) it shall have motors and electrical controls
from sources of poor quality. located above grade, and protected from flooding as required
The problem is much more difficult to address in existing by the reviewing authority; (b) it shall be accessible; (c) it
communities that are growing or retrofitting new systems. shall be designed against flotation; (d) it shall be equipped
All cities are site specific. Where new distribution systems with removable or traveling screens before the pump suction
are being added, installing the stainless steel pipes gradually well; (e) it shall provide for introduction of chlorine or other
would be beneficial but costs would need to be assessed to chemicals in the raw water transmission main if necessary for
determine whether they are feasible. The high costs of the water control; (f) it shall have intake valves and provisions for
conventional pipes and future problems with water quality backflushing or cleaning by a mechanical device and testing
may justify the higher initial costs. for leaks, where practical; and (g) it shall have provisions for
From a public health perspective we cannot afford to withstanding surges where necessary.
continue our current practices. Dual systems and the water An off-stream raw water storage reservoir is a facil-
supply conservation they provide, along with distribution ity into which raw water is pumped during periods of good
systems carrying only well-treated drinking water, dictate water quality and high stream flow for future release to water
the need to study new approaches. treatment facilities. These off-stream raw water storage reser-
voirs shall be constructed to assure that (a) water quality is
protected by controlling surface runoff into the reservoir;
8.8 RAW WATER INTAKE STRUCTURES (b) dikes are structurally sound and protected against wave
AND RAW WATER PUMPING WELLS
action and erosion; (c) intake structures and devices meet
In accordance with the Ten-States Standards (Recommended requirements stated above; (d) point of influent flow is sep-
Standards for Water Works, 2007), design of an intake arated from the point of withdrawal; and (e) separate pipes
structure shall provide for (a) withdrawal of water from more are provided for influent to and effluent from the raw water
than one level if water quality varies with depth; (b) separate storage reservoir.
EXAMPLE 8.12 DESIGN OF RAW WATER INTAKE STRUCTURE
Design a concrete raw water intake structure with two equal-size square gates under the following assumed engineering conditions:
a. Elevation of the intake structure top (maximum) = 87 m (msl = mean sea level).
b. Elevation of maximum reservoir water level = 82 m (msl).
c. Elevation of normal reservoir water level = 81 m (msl).
d. Elevation of minimum reservoir water level = 80 m (msl).
e. Elevation of the intake structure bottom (planned) = 60 m (msl).