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280 Chapter 8 Pumping, Storage, and Dual Water Systems
3. If the bottom elevation of a storage reservoir must be below normal ground surface,
it shall be placed above the groundwater table. At least 50% of the water depth
should be above grade. Sewers, drains, standing water, and similar sources of pos-
sible contamination must be kept at least 50 ft (15 m) from the reservoir. Gravity
sewers constructed of water main quality pipe, pressure tested in place without
leakage, may be used at distances greater than 20 ft (6 m) but less than 50 ft (15 m).
4. The top of a partially buried storage structure shall not be less than 2 ft (0.6 m)
above normal ground surface. Clearwells constructed under filters may be ex-
empted from this requirement when the design provides adequate protection from
contamination.

8.5 ELEVATION OF STORAGE
Storage reservoirs and tanks operate as integral parts of the system of pumps, pipes, and con-
nected loads. In operation all the parts respond to pressure changes as the system follows the
diurnal and seasonal demands. Ideally the storage elevation should be such that the reservoir
“floats” on the system, neither emptying nor standing continuously full. In systems with in-
adequate pipes or pumps, or having a storage reservoir that is too high, the hydraulic gradient
may at times of peak demand fall below the bottom of the reservoir. When this occurs, the
full load falls on the pumps and system pressures deteriorate suddenly.

8.6 TYPES OF DISTRIBUTING RESERVOIRS
Where topography and geology permit, service reservoirs are formed by impoundage, bal-
anced excavation and embankment, or masonry construction (Fig. 8.7). To protect the
water against chance contamination and against deterioration by algal growths stimulated
by sunlight, distributing reservoirs should be covered. Roofs need not be watertight if the
reservoir is fenced. Open reservoirs should always be fenced. Where surface runoff might
drain into them, they should have a marginal intercepting conduit.
Earthen reservoirs, their bottom sealed by a blanket of clay or rubble masonry and
their sides by core walls, were widely employed at one time. Today, lining with concrete
slabs is more common. Gunite, a sand-cement-water mixture, discharged from a nozzle
or gun through and onto a mat of reinforcing steel, has also been employed to line or
reline them. Plastic sheets protected by a layer of earth have also been used to build
inexpensive but watertight storage basins. Roofs are made of wood or concrete. Beam
and girder, flat-slab, arch, and groined-arch construction have been used. Where con-
crete roofs can be covered with earth, both roof and water will be protected against ex-
tremes of temperature.
Inlets, outlets, and overflows are generally placed in a gate house or two. Circulation
to ensure more or less continuous displacement of the water and to provide proper deten-
tion of water after chlorination may be controlled by baffles or subdivisions between inlet
and outlet. Overflow capacity should equal the maximum rate of inflow. Altitude-control
valves on reservoir inlets (Fig. 8.8) will automatically shut off inflow when the maximum
water level is reached. An arrangement that does not interfere with draft from the reser-
voir includes a bypass with a swing check valve seating against the inflow.
Where natural elevation is not high enough, water is stored in concrete or steel stand-
pipes and elevated tanks. In cold climates, steel is most suitable. Unless the steel in rein-
forced-concrete tanks is prestressed, vertical cracks, leakage, and freezing will cause rapid
deterioration of the structure. Ground-level storage in reinforced concrete or steel tanks in
advance of automatic pumping stations is an alternative.