The West Side Combined Sewer Overflow Tunnel (also West Side Big Pipe) is a tunnel in Portland, Oregon, United States. It is to receive and store overflow from the combined sewer system before it can reach the Willamette River. The main tunnel is 14 feet (4.3 m) in diameter and 3.5 miles (5.6 km) long for a capacity of 2,850,000 cubic feet (81,000 m3) and connects to dozens of smaller sewer overflow interceptors along the west side of the Willamette River.
The tunnel is a storage and transport system to minimize sewer overflows from reaching the river. Instead, the CSO tunnel receives overflows, stores and transports them to the Swan Island Pump Station. Portland's 1930s sewer design combines street and surface runoff with sewage into a common sewage system which is overwhelmed during heavy precipitation. The original system handled overflows by sending excess flow into the river.
The tunnel is 120 to 160 feet (37 to 49 m) below ground level. It passes under the Willamette River between the NW Nicolai Street shaft (45°32′27″N 122°41′51″W / 45.54072°N 122.69751°W / 45.54072; -122.69751 (Nicoli shaft)) to the confluent vertical shaft on Swan Island (45°33′11″N 122°41′44″W / 45.55302°N 122.69565°W / 45.55302; -122.69565 (Swan Island confluent shaft)), which also receives the East Side Big Pipe. From Nicolai, it travels roughly south close to Front Avenue. There are vertical shafts at Upshur (45°32′11″N 122°41′11″W / 45.53643°N 122.68642°W / 45.53643; -122.68642 (Upshur shaft)Coordinates: 45°32′11″N 122°41′11″W / 45.53643°N 122.68642°W / 45.53643; -122.68642 (Upshur shaft)), Ankeny (45°31′20″N 122°40′12″W / 45.52234°N 122.66992°W / 45.52234; -122.66992 (Ankeny shaft)), and Clay streets (45°30′42″N 122°40′31″W / 45.51163°N 122.67530°W / 45.51163; -122.67530 (Clay Street shaft)). The Clay Street shaft receives the Southwest Parallel Interceptor, a 3-to-6-foot (0.91 to 1.8 m) pipeline which runs along the west Willamette shore for 3 miles (4.8 km) to Virginia Avenue and Taylors Ferry Road 45°28′13″N 122°40′21″W / 45.47015°N 122.67240°W / 45.47015; -122.67240 (SW Parallel Interceptor south end).
The project is a part of the Willamette River combined sewer overflow expansion program. Construction occurred from November 2002 to September 2006, and became fully operational December 2006.
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—John Ashbery (b. 1927)