Tuesday – Friday, November 22-25, 2016
Thanksgiving Day rainfall
A pair of strong cold fronts crossed the city starting Tuesday, November 22, and continued through Friday morning, November 25, dropping a total rainfall of about 3.5-4.5 inches. A storm front hung up in the Portland area on Thanksgiving Day, November 24, with parts of the city receiving 1.0 to 1.9 inches in 6 hours. A rainfall rate of 1.0 to 1.5 inches in 12 hours usually causes the Willamette River Big Pipes to fill up and overflow, especially when the heavy rainfall occurs throughout the city. This is what happened on Thanksgiving Day.
The Westside Big Pipe is 14 feet in diameter and 3.5 miles long, while the Eastside Big Pipe is 22 feet in diameter and 6 miles long. Their total capacity is 109 million gallons.
Combined sewer overflow (CSO)
On Thanksgiving Day, the Willamette River combined sewer overflow (CSO) system discharged for almost 17 hours during constant rainfall. The CSO began at about 1:30 p.m. on Thanksgiving Day and lasted until about 7:30 a.m. on Black Friday, November 25. The Willamette River Big Pipes filled up to capacity and overflowed at all 8 outfall relief points on the west and east sides of the river. This resulted in about 159 million gallons of combined sewage and stormwater discharging to the river. No overflows to the Columbia Slough occurred.
By 7:30 a.m. on Black Friday, the Willamette River Big Pipes were safely conveying all the wastewater sent to them from other parts of the system to the Columbia Boulevard Wastewater Treatment Plant in North Portland.
This overflow was the third of 2016, and the first of the 2016-2017 winter season. Before we completed construction of the Willamette River Big Pipes in 2011, we had an average of 50 CSOs each year. Now we have an average of 4-5 each year. For more information about CSOs, what they are and why we still have them, click here.
Wastewater collection and treatment
During this storm, the treatment plant processed 881 million gallons. The treatment plant processed almost 245 million gallons on Thursday and 303 million gallons on Friday. It typically treats 70 million gallons a day.
The wet weather screening facility (pictured above) screens items like leaves, rocks and grit out of incoming wastewater. This facility and the rest of the wet weather process start up when incoming wastewater flow gets to about 120 million gallons.