Page 114 - Water and wastewater engineering

INTAKE STRUCTURES 3-11

The unit space occupied by a bar and the adjacent opening is 4 cm 1 cm 5 cm. The fraction

of the area occupied by the bar is 1 cm/5 cm 0.20 or 20 percent. To account for the bars, the
area of the port opening must be increased by 20 percent or

.
A (120 )(0 9 m 2 ) 1 08 or 11 m 2
.
.
.

Fine Screens. A fine screen is placed downstream of the coarse screen. Its purpose is to collect
smaller material that has passed through the coarse screen but is still large enough to damage
downstream equipment. Generally, it is placed in the low-lift pump station ahead of the pump
intake. Fine screen characteristics are given in Table 3-5 .

Intake Crib
Location. The desired location of the intake crib is in deep water where it will not be buried
by sediment, be washed away, be a navigational hazard, or be hampered by problems associ-
ated with ice. The minimum suggested depth is 3 m from the surface. In rivers, where the depth
exceeds 3 m, the top of the intake should be 1 m above the river bottom. In cases where the water
depth is less than 3 m, the crib is buried 0.3 to 1 m (Kawamura, 2000).

Structure. The Milwaukee, Wisconsin, intake crib shown in Figure 3-4 is a classic design. It is
octagonal in shape. A similar circular design was used for the Cincinnati, Ohio, intake ( Figure 3-7 ).
Other structures include hydraulically balanced inlet cones and inlet drums. The intake is protected
by riprap or a concrete slab.

Intake Ports. In warm climates, the intake crib ports are sized to provide a maximum velocity
of less than 0.3 m/s (Kawamura, 2000). In cold climates, where frazil ice is anticipated, the intake
velocity is limited to less than 0.1 m/s.

Screens. Submerged intakes are screened with coarse screens. The T-screen ( Figure 3-8 ) has
found application in both river and reservoir applications. It is especially employed to prevent
small fish from entering the intake structure. It is cleaned by a burst of air at a pressure of
1,000 kPa.

Conduit
The conduit may be either a tunnel or a pipeline. Generally, it is designed to flow by gravity.

Size. The conduit is sized to carry the maximum design flow rate ( Table 3-3 ). To minimize the
accumulation of sediment the flow velocity should be greater than 1 m/s (Kawamura, 2000). The