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4.28 CHAPTER FOUR

FIGURE 4.17 T screen used to modify intake to improve fish protection.

mm). A 150-psi (10.2-atm) air-burst cleaning system can be actuated by head loss across
the screens, timed cycle, or manual operation.
The Geren Island Treatment Facility serves the city of Salem, Oregon. The intake,
which is on the North Santiam River, is sized for a maximum flow of 147 mgd (556 ML
per day). This intake facility, shown in Figure 4.18, conveys flows to the treatment fa-
cilities and includes a concrete intake structure, trash racks, twelve 36-in. (0.91-m) sluice
gates, and two 84-in. by 60-in. (2.1-m by 1.5-m) control gates. A fish screening system
prevents fish from entering the treatment system. The fish screening system consists of
stainless steel fish screens with 0.069-in. (1.75-mm) slot openings, ultrasonic level sen-
sors, and an air-burst cleaning system. Design was based on NOAA fisheries screening
criteria, and meets the criteria with approach velocities of less than 0.4 ft/s (12.2 cm/s)
and sweep velocities at least equal to the approach velocities.

Flotation Considerations
Designing intake facilities requires careful evaluation of uplift forces to ensure that the
structure is stable over the full range of anticipated water surface elevations. Two general
methods for resisting uplift forces are deadweight and foundation ties. The most common
approach to stabilizing uplift forces is to design the structure with enough deadweight to
resist all possible uplift force. Deadweight can be provided by tying the structure to a con-
crete mat or by filling cells of the structure with crushed rock or other heavy material.
Foundation ties or rock anchors drilled into foundation rock are also occasionally used to
tie down an intake structure. With either method, a safety factor against uplift of at least
1.2 should be provided.

Geotechnical Considerations
Geotechnical investigations must be completed as the first step in designing an intake. In-
vestigations typically include borings unless adequate data are already available from a