*TM 5-813-1/AFM-88-10, Vol. 1
CHAPTER 7
INTAKES
openings and conduits will not be clogged by bed-load
7-1. General
deposits. An additional problem, caused by suspended
The intake is an important feature of surface water
silt and sand, is serious abrasion of pumps and other
collection works. For fairly deep streams, whose flow
mechanical equipment. Excessive silt and sand may
always exceeds water demands, the raw water collection
also cause severe problems at treatment plants. Liberal
facilities generally consist of an intake structure located
margins of safety must be provided against flood
in or near the stream, an intake conduit and a raw water
hazards and also against low-water conditions.
A
pumping station. Often the intake and pumping station
depression dredged in the stream bed to provide
are combined in a single structure. On smaller, shallow
submergence is not a solution to the lowwater problem
streams, a channel dam may be required to provide
because it will be filled by bedload movement. A self-
adequate intake submergence and ice protection. Inlet
scouring channel dam may be the only means of
cribs of heavy timber construction, surrounding multiple-
assuring adequate water depth. As an alternative to
inlet conduits, are frequently employed in large natural
unusually difficult intake construction, gravel-packed
lakes. For impounding reservoirs, multipleinlet towers,
wells and horizontal collector infiltration systems located
which permit varying the depth of withdrawal, are
in the alluvium near the river are often worthy of
commonly used. Hydraulically or mechanically-cleaned
investigation. Water obtained from such systems will
coarse screens are usually provided to protect pumping
usually be a mixture of ground water and induced flow
equipment from debris. Debris removed from screens
from the stream.
must be hauled to a landfill or other satisfactory disposal
site. It may be necessary to obtain a permit for
construction of an intake from both State and Federal
7-3. Ice problems
agencies. If the stream is used for navigation, the intake
In northern lakes, frazil ice (a slushy accumulation of ice
design should include consideration of navigation use
crystals in moving water) and anchor ice (ice formed
and of impact from boats or barges out of control. A
beneath the water surface and attached to submerged
permit from the U.S. Army District Engineer is required if
objects) are significant hazards, while on large rivers,
navigation is obstructed.
floating ice has caused damage. Intake design must
include ample allowances for avoiding or coping with
these hazards. The intake location and inlet size are
7-2. Capacity and reliability
important aspects of design. Excessive inlet water
The intake system must have sufficient capacity to meet
velocities have been responsible for major clogging
the maximum anticipated demand for water under all
problems caused by both sand and ice. Inlet velocities in
conditions during the period of its useful life. Also, it
the range of 0.25 to 0.5 feet per second are desirable for
should be capable of supplying water of the best quality
avoiding ice clogging of intakes. Where ice is a problem,
economically available from the source. Reliability is of
river intakes must have the structural stability to resist
major importance in intake design because functional
the thrust of ice jams and the openings must be deep
failure of the intake means failure of the water system.
enough to avoid slush ice which has been reported as
Intakes are subject to numerous hazards such as
deep as six to eight feet. Frazil and anchor ice can also
navigation or flood damage, clogging with fish, sand,
cause difficulties, but on rivers, floating ice is usually the
gravel, silt, ice, debris, extreme low water not
greater hazard. Steam heating has been employed to
contemplated during design, and structural failure of
cope with ice problems at some northern lake intakes.
major components.
Many streams carry heavy
Nonferrous materials are preferred for cold-climate inlet
suspended silt loads. In addition to suspended silt, there
construction because their lower heat conductivity
is also a movement of heavier material along the bed of
discourages ice formation.
the stream. The intake must be designed so that
7-1
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