Spokane city officials say special tanks designed to prevent sewer overflow into the Spokane River did their job during Thursday’s massive rain event.
Combined Sewer Overflow tanks, CSOs for short, are designed to prevent sewage from entering the Spokane River at overflow points during big rain storms or snowmelt.
That’s because in some areas of the city, particularly the south side, the storm drains are connected to the city’s sewer lines. On much of the north side, they are separate.
Under federal mandate of the Clean Water Act, the city has in recent years moved to construct two dozen CSO tanks to hold back water from the storm drains during major rain events, like the one Thursday.
City spokeswoman Marlene Feist says the tanks actually did a pretty good job during that deluge.
“Ten sites did overflow into the river, and frankly, I think that was a pretty impressive result, because I would have expected because of the intensity and amount of water that we were receiving that we would have seen all of those tanks overflow. These systems are designed for good size rainstorms, but not for rainstorms that deliver an inch of rain in 20 minutes,” said Feist.
Feist says some who saw video of the flowing above T.J. Meenach Bridge where cars were getting stranded in the road by water flow, may have misinterpreted that as a sign that tanks were not doing their job.
“The tanks are designed to take excess water that is already in the pipes, but what we saw yesterday was the water wasn’t getting to the pipes. So either the storm drains were clogged or in some cases the storm drains were so full of water they could take no more," she said.
"What we found on the north side where we do have separated storm sewers is most of those join and come out at what is called the Cochran basin outfall right by T.J. Meenach Bridge. And there was so much water that, at one point yesterday, that pipe was flowing by itself at 600 cubic feet per second. And for comparison that is twice what was coming out at Latah Creek to the river at that point,” Feist said.
Feist says future plans call for filtration of the storm water on the north side of the city when those lines are overwhelmed by major rain events to filter that runoff from entering the river, much like the efforts now to prevent the combined sewer and storm water from polluting the waterway.
