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4.10 CHAPTER FOUR

variation in water quality can result from the entrance of pollution from tributary streams
upstream of the proposed intake location, and water quality near one bank may be infe-
rior to the quality encountered midstream or at the opposite shore.
Flood Considerations. It is essential to protect intake structures against flood damage.
The intake structure must also be designed to prevent flotation and to resist the thrust of
ice jams. Flood stages at the intake site should be considered carefully, and a substantial
margin of safety provided. It is essential for the flood design criteria to be clearly marked
on the design drawings. Due to watershed and channel alterations, future flood stages may
exceed those of the past, and so the designer should consider the possibility that the in-
take will be exposed to flood stages in excess of those on record.
Silt and Bed Load Considerations. Many streams carry heavy loads of suspended silt
at times, and heavy material move along the beds of many streams. The intake must be
designed so that it will not be clogged by silt and bed load deposits. Silt, sand, and gravel
can also cause abrasion of pumps and other mechanical equipment, leading to severe prob-
lems at the treatment plant. To help prevent such deposits, jetties may be built to deflect
the principal flow of the river toward and past the face of the intake.

Lake and Reservoir Intakes
Both tower intakes and submerged intakes are employed for drawing water supplies from
lakes and reservoirs. A tower intake may be designed as an independent structure located
some distance from shore in the deepest part of the lake or reservoir. Access to these tow-
ers is typically provided by bridge, causeway, or boat. Towers must be designed to cope
with flotation, wind, wave, and ice forces.
The Metropolitan Water District of Southern California (MWD) owns and operates
the Diamond Valley Lake, which contains, at its maximum water level, about 800;000
acre • ft (990,000 ML) of imported raw water. Diamond Valley Lake serves as an emer-
gency water supply for southern California. The intake tower, shown in Figure 4.7, is the
structure by which water enters and exits the reservoir. The tower is 260 ft (79 m) tall,
100 ft (30 m) long in the direction of flow, and 80 ft (24 m) wide. It includes nine por-
tal tiers, each equipped with two 84-in. (2. l-m) pipes with butterfly shutoff valves. Mul-
tiple tiers provide MWD the flexibility to selectively withdraw the highest-quality water
as reservoir conditions change. The tower is equipped with four movable wire cloth screens
to prevent the entry of fish during a drafting condition. The screens can be positioned
over any of the intake ports selected for operation. Normal hydraulic withdrawal capac-
ity of the intake tower is 1,100 mgd (4,200 ML per day) with a maximum capacity dur-
ing emergency drawdown of the reservoir of 4,500 mgd (17,000 ML per day). The raw
water is chlorinated within the lake's intake facilities to control algae and mussel growth,
and the water is dechlorinated at the tower prior to entry into the reservoir.
The Charlotte-Mecklenburg Utilities Department intake on Lake Norman in North Car-
olina is an in-lake tower facility (Figure 4.8). The intake has an initial capacity of 54 mgd
(204 ML per day) and is expandable to an ultimate capacity of 108 mgd (409 ML per
day), which should satisfy projected demand for 35 years. The intake uses passive stain-
less steel screens mounted on the exterior of the structure's inlet pipes to exclude entry
of debris and fish. The screens are designed with air backwash for cleaning, and redun-
dancy is provided with two 60-in. (l.5-m) source water conduits to shore facilities. This
facility incorporates a special "gazebo" architectural concept, which was specifically se-
lected to blend the intake structure with the park setting.