Page 92 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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2.14 Collection of Rainwater 55
2.12.5 Intake Conduits and Pumping Stations

Intakes are connected to the shores of lakes and reservoirs (a) by pipelines (often laid with flex-
ible joints) or (b) by tunnels blasted through rock beneath the lake or reservoir floor. Pipelines
are generally laid in a trench on the floor and covered after completion. This protects them
against disturbance by waves and ice. Except in rock, conduits passing through the foundations
of dams are subjected to heavy loads and to stresses caused by consolidation of the foundation.
Intake conduits are designed to operate at self-cleansing velocities of 3 to 4 ft/s (0.9 to
1.2 m/s). Flow may be by gravity or suction. Pump wells are generally located on shore.
Suction lift, including friction, should not exceed 15 to 20 ft (4.5 to 6 m). Accordingly,
pump wells or rooms are often quite deep. The determining factor is the elevation of the
river, lake, or reservoir in times of drought. Placing pumping units in dry wells introduces
problems of hydrostatic uplift and seepage in times of flood. Wet wells and deep-well
pumps may be used instead.

2.13 DIVERSION WORKS

Depending on the geology and topography of the dam site and its immediate surroundings,
streams are diverted from the construction area in two principal ways:
1. The entire flow is carried around the site in a diversion conduit or tunnel. An
upstream cofferdam and, if necessary, a downstream cofferdam lay the site dry.
After fulfilling its duty of bypassing the stream and protecting the valley during
construction, the diversion conduit is usually incorporated in the intake or regula-
tory system of the reservoir (see Figs. 2.4 and 2.13).
2. The stream is diverted to one side of its valley, the other side being laid dry by a
more or less semicircular cofferdam. After construction has progressed far enough
in the protected zone, stream flow is rediverted through a sluiceway in the com-
pleted section of the dam, and a new cofferdam is built to pump out the remaining
portion of the construction site.

Diversion conduits are built as grade aqueducts and tunnels, or as pressure conduits and
tunnels. As a matter of safety, however, it should be impossible for any conduit passing
through an earth embankment dam to be put under pressure; a leak might bring disaster.
Accordingly, gates should be installed only at the inlet portal, never at the outlet portal. If a
pipe must work under pressure, it should be laid within a larger access conduit. To discour-
age seepage along their outer walls, conduits passing through earth dams or earth founda-
tions are often given projecting fins or collars that increase the length of path of seepage (by,
say, 20% or more) and force flow in the direction of minimum as well as maximum perme-
ability. At their terminus near the toe of the dam, moreover, emerging conduits should be
surrounded by rock, through which residual seepage waters can escape safely.
The capacity of diversion conduits is determined by flood-flow requirements.
Variations in the head and volume of floodwater impounded behind the rising dam are
important factors in this connection. Rising heads normally increase the capacity of diver-
sion conduits, and increasing storage reduces the intensity of floods. At the same time,
however, dangers to the construction site and the valley below mount higher.

2.14 COLLECTION OF RAINWATER
Rain is rarely the immediate source of municipal water supplies—a notable example is the
water supply of the communities in the islands of Bermuda, on which streams are lacking and
groundwater is brackish. The use of rainwater is generally confined (a) to farms and towns in