Page 59 - Water Engineering Hydraulics, Distribution and Treatment

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2.12 Intakes
then equals one-third the height H of the reservoir surface
itself is not endangered in times of flood.
above the entrance sill to a rectangular channel, and the rate
of discharge Q becomes
2.12 INTAKES
3/2
√
Q = 2/3CbH 2gH/3 = 3.087CbH
Depending on the size and nature of the installation, water
where b is the width of the channel and C is an entrance
is drawn from rivers, lakes, and reservoirs through relatively
coefficient varying from 1.0 for a smooth entrance to 0.8 for
simple submerged intake pipes, or through fairly elaborate
an abrupt one. A trapezoidal channel with side slopes of 1:2
tower-like structures that rise above the water surface and
discharges
may house intake gates; openings controlled by stop logs;
racks and screens, including mechanical screens, pumps, and
(2.13)
v
v
v
compressors; chlorinators and other chemical feeders; ven-
where Q = 8.03Ch 1/2 (H − h )[b + z(H − h )] (2.12) level. They must be so designed and operated that the dam
turi meters and other measuring devices; even living quarters
2
2
2 1∕2
3(2zH + b) − (16z H + 16zbH + 9b ) and shops for operating personnel (Fig. 2.3). Important in the
h = (2.14)
v
10z design and operation of intakes is that the water they draw
be as clean, palatable, and safe as the source of supply can
Best hydraulic but not necessarily best economic effi-
provide.
ciency is obtained when a semicircle can be inscribed in the
cross-section.
2.12.1 River Intakes
Flow is uniform below the entrance when friction and
channel slope are in balance. Otherwise, flow becomes Understandably, river intakes are constructed well upstream
nonuniform and channel cross-section must be adjusted from points of discharge of wastewater and industrial wastes.
accordingly. A weir within the channel produces a backwater An optimal location will take advantage of deep water, a sta-
curve. ble bottom, and favorable water quality (e.g., if pollution hugs
Side-channel spillways occupy relatively little space in one shore of the stream), all with proper reference to protec-
the cross-section of a valley. The crest more or less parallels tion against floods, debris, ice, and river traffic (Fig. 2.14).
one abutting hillside and can be made as long as wanted. The Small streams may have to be dammed up by diversion or
channel, into which the spillway pours its waters, skirts the intake dams to keep intake pipes submerged and preclude
end of the dam and delivers its flows safely past the toe. If it hydraulically wasteful air entrainment. The resulting intake
is blasted out of tight rock, the channel can be left unlined. pool will also work well as a settling basin for coarse silt and
Crest length and channel size are determined in much the allow a protective sheet of ice to form in winter.
same way as for washwater gutters in rapid sand filters.
As shown in Fig. 2.13, shaft or drop-inlet spillways con-
sist of an overflow lip supported on a shaft rising from an 2.12.2 Lake and Reservoir Intakes
outlet conduit, often the original stream-diversion tunnel. Lake intakes are sited with due reference to sources of pollu-
The lip can be of any desired configuration. A circular lip tion, prevailing winds, surface and subsurface currents, and
and trumpet-like transition to the shaft form a morning-glory
spillway that must lie far enough from the shore to be fully
effective. By contrast, a three-sided semicircular lip can be
placed in direct contact with the shore; accessibility is its Washwater
advantage. The capacity of shaft spillways is governed by Pump discharge discharge
their constituent parts and by flow conditions including air Backwash inlet
entrainment and hydraulic submersion. Hydraulic efficiency
and capacity are greatest when the conduit flows full. Model Bar rack
studies are useful in arriving at suitable dimensions. 1–2 in. Outer
spacing
Flashboards or stop logs and gates of many kinds are casing
Flow well Inner
added to spillways to take advantage of storage above crest Wet
Sectional plan casing
of intake cowl
Pump
Inlet column
Headwall
Well screen
Diversion
Riser
tunnel
Outlet conduit Stream
Plug
Figure 2.14 River or lake intake with vertical pump and
Figure 2.13 Shaft spillway and diversion tunnel. backwashed well-type screen.