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Heavy rains trigger Combined Sewer Overflows of runoff and sewage from outflow pipes.

PORTLAND TRIBUNE FILE PHOTO - An outflow piple that carries Combined Sewer Overlows into the Willamette River.Saturday's record-breaking rain triggered a city warning Sunday morning to avoid the Willamette River because of sewage overflows.

At 9:15 a.m. on April 8, Portland's Bureau of Environmental Services warned the public to avoid the river downstream from the Ross Island Bridge for 48 hours after the overflows end.

KOIN 6 Meteorologist Joseph Dames said downtown Portland eclipsed the one-day rainfall record for April 7 with at least 1.23 inches of rain. That caused the sewer pipes that carry both runoff and sewage to overflow from several outfalls into the river.

The overflows were continuing Sunday afternoon, BES said. The volume was not yet known.

Combined sewer overflows are rare and occur during periods of heavy rain or snowfall. A CSO is about 80 percent stormwater and 20 percent sewage.

According to BES, since completing the Big Pipe project in 2011, the number of CSOs have dropped by 94 percent to the Willamette River and 99 percent to the Columbia Slough.

The 20-year, $1.4 billion project constructed a series of improvements, from disconnecting downspouts on residences to allow rainwater to be absorbed naturally in the ground to the construction of big pipes on both sides of the river and along the slough to store and convey large quantities of flows to the Columbia Boulevard Wastewater Treatment Plant.

Before the project, CSOs occurred to the Willamette River from multiple outfalls an average of 50 times a year, with some instances lasting days. Today, overflows occur an average of four times per winter season and once every three summers, BES says.

Sunday's overflow was the first of 2018 and the first since Oct. 22.

You can find out more information about CSO events at www.portlandoregon.gov/bes/31030.

KOIN News 6 is a news partner of the Portland Tribune and contributed to this story.

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