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The 'Big Pipe' keeps sewage out of the Willamette River -- except when it rains heavily

DAVID F. ASHTON - This is the sewer system outfall pipe at Ross Island, to spill combined rainwater and sewage to the river when the system overflows in heavy rain. Any sewage in the Willamette River south of this point is coming from cities to the south.Heavy rains caused Portland's combined sewer system to overflow to the Willamette River around 9:15 a.m. Sunday morning, April 8, from several outfalls.

Because of increased bacteria in the water, the public is advised to avoid contact with the river north of the Ross Island Bridge for 48 hours after the rain has ended. The overflows were still continuing later Sunday-- and after a dry Monday there will be more rain most of the following week.

Combined sewer overflows (CSOs) are rare, according to the Portland Bureau of Environmental Services, and occur during periods of heavy rain or snowfall. A CSO is about 80 percent stormwater and 20 percent sewage.

BES says that since completing the Big Pipe project in 2011 -- a 20-year $1.4 billion program to reduce overflows -- the number of CSOs have dropped by 94 percent to the Willamette River, and 99 percent to the Columbia Slough.

The Big Pipe project constructed a series of improvements, from disconnecting downspouts on residences to allow rainwater to be absorbed naturally into 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, says BES, CSOs occurred to the Willamette River from multiple outfalls an average of 50 times a year, with some instances lasting days. Now, overflows occur an average of four times per winter season, and once every three summers.

This was the first overflow of 2018, and in fact the first since October 22, according to the Bureau.

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