Westside Ditch

The Westside Ditch Diversion is located on the Yakima River at river km 267.4 [river mile 166.2], near Thorpe, Washington. Water is diverted from the Yakima River into the Westside Ditch Canal. The carrying capacity of the canal is about 2.8 m3/s [100 cubic feet per second (cfs)]. Canal flow varies from 0.6 to 2.8 m3/s (20 to 100 cfs) and is regulated at the canal headgates located about 0.5 km upstream of the Westside Ditch Screens. The screening facility (Figure 1) diverts fish that have entered the canal and directs them back to the Yakima River. Trash racks located in front of the headgates "filter" out large debris that could damage the screens or interfere with flow control through the screen facility.


Figure 1. Flow Control Structure and Fish Bypass System in the Westside Ditch Canal Fish Screening Facility


The screening facility houses four rotary drum screens with axes parallel to the length of the structure. Each screen is about 3.7 m (12 ft) long and 1.8 m (6 ft) in diameter. Screen mesh openings are 3.2 mm (1/8 in.). Water depth at the screens is maintained at about 1.6 m (5 ft). The fish bypass is located in the flow control structure at the downstream end of the screening facility. Water and fish diverted past the front of the screens pass through the fish bypass slot and out the fish return pipe. Flow through the fish return is 0.6 m3/s (20 cfs). A waste water channel is installed along the forebay wall opposite the drum screens to prevent flooding and canal bank erosion. No water flows out the waste water channel at normal canal forebay level.

The rotary screens are installed at an angle of 26 to canal flow. This orientation is designed to provide a sweeping-velocity-to-approach-velocity ratio equal to or exceeding 2:1. The maximum allowable approach velocity is 0.15 m/s (0.5 fps). Screen orientation and flow velocity differential help direct fish to the fish return pipe and back to the river.