Page 91 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch02_029-060.qxd 8/2/10 9:14 PM Page 54

54 Chapter 2 Water Sources: Surface Water

Offshore wind

Location of
intake structure
Open the bottom ports

Clean bottom water

Onshore wind

Open the Clean surface water
top ports Location of
intake structure
Figure 2.15 Effect of Onshore and
Offshore Winds on Water Quality
at Water Intake

equipped with gates, conduits, and machinery not only for water supply but also for regu-
lation of low-water flows (including compensating water); generation of hydroelectric
power; release of irrigation waters; and control of floods. Navigation locks and fish ladders
or elevators complete the list of possible control works.

2.12.3 Submerged and Exposed Intakes
Submerged intakes are constructed as cribs or screened bellmouths. Cribs are built of
heavy timber weighted down with rocks to protect the intake conduit against damage by
waves and ice and to support a grating that will keep large objects out of the central
intake pipe.
Exposed intake gatehouses, often still misnamed cribs, are tower-like structures built
(a) into dams, (b) on banks of streams and lakes, (c) sufficiently near the shore to be con-
nected to it by a bridge or causeway, and (d) at such distance from shore that they can be
reached only by boat (see Figs. 2.3 and 2.4). In dry intakes, ports in the outer wall admit
water to gated pipes that bridge a circumferential dry well and open into a central wet well
comprising the entrance to the intake conduit. In wet intakes, water fills both wells. Open
ports lead to the outer well, whence needed flows are drawn through gated openings into
the inner well.

2.12.4 Intake Velocities and Depths
In cold climates, ice troubles are reduced in frequency and intensity if intake ports lie as
much as 25 ft (7.5 m) below the water surface and entrance velocities are kept down to
0.30 ft/s (0.9 m/s). At such low velocity, ice spicules, leaves, and debris are not entrained
in the flowing water and fish are well able to escape from the intake current.
Bottom sediments are kept out of intakes by raising entrance ports 4 to 6 ft (1.2 to 1.8 m)
above the lake or reservoir floor. Ports controlled at numerous horizons permit water quality
selection and optimization. A vertical interval of 15 ft (4.5 m) is common. Submerged gratings
are given openings of 2 to 3 in. (5 to 7.5 cm). Specifications for screens commonly call for
two to eight meshes to the inch and face (approach) velocity of 0.30 ft/s (0.9 m/s). Wet
wells should contain blow-off gates for cleaning and repairs.