Civil Engineering Reference
In-Depth Information
to preclude the entraining of air. The ports should also be several feet above the bottom
of the stream so that sand and gravel being transported on the bottom will not be
drawn into the intake. In order to meet these requirements, it is usually necessary to
locate the intake in the deepest part of the stream and away from the shore, particularly
if the river is subject to large fluctuations in stage.
Intakes in small streams frequently require the construction of small diversion dams
for the dual purpose of providing a sufficient depth of water at all flows to divert water
into the intake port and a settling period in order to reduce the turbidity of the water.
A small period of quiescent flow will also permit suspended leaves and wood either
to rise to the surface or to sink if they have become waterlogged, and it will favor the
formation of sheet ice in cold weather and thus reduce the difficulties of ice.
Both bar racks and mesh screens are frequently used on the openings into the intake
structure. Bar racks with spacings of 2 to 4 inches (51 to 101 mm) protect the intake
from large floating objects.
Screens are used to protect against floating materials such as leaves. They should
have not less than 2 and sometimes have as many as 8 meshes to the inch (79 to 315
per meter), depending upon the character of the floating matter in the water. Screens
should be of corrosion-resistant metal and easily removable. Screens should have a
velocity of not more than 3
1
⁄
2
inches / sec (8.89 mm / s). Low velocities and small
openings are necessary to prevent the entrance of fish.
In cold climates, ice troubles are reduced in frequency and intensity if intake ports
lie as much as 25 feet (7.62 m) below the water surface and entrance velocities are
less than 3 to 4 inches / sec (75 to 100 mm / s). At such low velocities, frazil ice, leaves,
and debris are not entrained in the flowing water, and fish are able to escape from the
intake current. Also, the use of fiberglass-reinforced plastic (RFP) for intake bells has
been reported to be effective in reducing frazil ice adherence.
7
Bottom sediments may be kept out of intakes by raising entrance ports 4 to 6 feet
(1.22 to 1.832 m) above the lake or reservoir floor. Ports controlled at numerous depths
permit water-quality selection and optimization. A vertical interval of 15 feet (4.57 m)
is common.
8
Submerged gratings are given openings of 2 to 3 inches (51 to 76.2 mm).
Specifications for screens commonly call for 2 to 8 meshes to the inch (79 to 315 per
meter) and face (approach) velocities of 3 or 4 inches / sec (76.2 to 102 mm / s). Typical
intakes are shown in Figures 8-6 and 8-7.
Impacts of Zebra Mussels
The zebra mussel,
Dreissina polymorpha,
is regarded as the most potentially damaging
natural intrusion into the United States' water distribution systems in years.
9,10
Believed
to have been introduced to the Great Lakes in 1985 or 1986, they have now spread
throughout the central states as shown in Figure 8-8. This fast growing, freshwater
mollusk can rapidly clog submerged intakes to water supplies, as shown in Figure 8-
9. Water intakes provide a hospitable environment for the mussels because of the
continuous flow of water that contain algae, phytoplankton, and other organisms that
represent the food chain for mussels. The water also provides a continuous source of
veligers, the free-swimming early-life form of the mussels that settle out of the water
column and begin to attach to substrates. Among the requirements needed to support
