Combined Sewer Overflows occur when Portland’s Big Pipe system reaches 100% capacity, causing the excess storm and sewer water to spill into the Willamette River.
PORTLAND, Ore. — Sustained heavy rain is causing sewage and stormwater to spill into the Willamette River, the Portland Bureau of Environmental Services reported Monday. The public is advised to avoid contact with the river during the spill and for 48 hours afterward.
These spill events, referred to by BES as Combined Sewer Overflow incidents, occur when Portland’s Big Pipe system reaches 100% capacity, causing the excess water to be automatically dumped into the river. The system hit capacity at 1:55 p.m. Monday, officials said.
The Portland and Salem region was forecasted to receive about an inch of rainfall Monday, with steady rain continuing for most of the morning.
The overflow water tends to be about 80% stormwater and about 20% sewage, according to the bureau. BES maintains a live Big Pipe Tracker website where residents can monitor the system’s capacity. The tracker showed the system still at 100% as of about 4 p.m.
Overflows occur regularly but infrequently, about three or four times per year on average. The last two incidents were in early November and late December of last year, in both cases due to heavy rain.
The centerpieces of the $1.4 billion Big Pipe project were a trio of massive sewer and stormwater pipes, one on each side of the Willamette River and one along the Columbia River in North Portland, completed between 2000 and 2011.
Overflow incidents dropped dramatically after the third pipe was finished, down from more than 50 per year before 2012, according to the city. They also became shorter, lasting hours at a time rather than days.
The Willamette’s water quality also improved, and the river is now considered safe for recreation except during and immediately after sewer overflow incidents.