Flood-induced sewage geysers - fact or fiction?

Can it happen or not? All your questions answered about sewage geysers. - Photo illustration
Can it happen or not? All your questions answered about sewage geysers.
— image credit: Photo illustration

As if the tough economy weren’t enough to worry about, the threat of flooding in the Green River Valley has residents and business owners on high alert. We’ve got our sandbags, we’ve signed up for CodeRED, and we’ve bought flood insurance. But none of these measures protects us from sewage backups.

Some people have the idea that along with flooding comes something even scarier than the flood water itself - geysers of sewage shooting out of their toilets and kitchen sinks just like "Old Faithful." We can thank science fiction movies for planting this smelly seed in our imaginations.

But how big of an issue is this really? After taking so many calls from people with this horrible image in their minds, I endeavored to get to the bottom of it and report my findings. I recently sat down with Kent’s Public Works Director Tim LaPorte, to ask him the questions so many of you have asked me.

So….for those of you living in fear of sewage geysers, read on to get the real "scoop on the poop."

M: So, is there any such thing as a sewage geyser?

T: Only in the movies.

M: If not a geyser, what about a sewage backup? What could happen in my house?

T: Sewer backups in houses are very rare, even during a flood. If anything at all, it would be more likely for sewer overflows to occur at the manholes in the street which are usually several feet lower than the floor level of a house. In Kent, sewers are a closed and completely separate pipe system from our storm drains. Storm drains empty in to creeks and rivers. So a rainstorm may back up a creek without affecting the sewer system.

For example, Mill Creek backs up regularly at 76th Avenue or at Earthworks Park during heavy rains, and all the while, the sanitary sewer remains operational.

M: What would cause a sewer backup then?

T: First of all, sewage backups due to flooding are very unlikely. However, during a severe rainstorm or a flood situation, water could enter the sewer system through small cracks in the pipes or even the holes in the manhole lids which would add to the pressure on the sewer system but it would still continue to function.

In low-lying areas, sewage many not be able to drain into the sanitary sewers, because the pipes would already be full. It would simply not move. Like I said before, it is more likely you’d see an overflow in the street.

M: So what makes sewage backups so unlikely? People who call are convinced this is going to happen and are in such a panic about it.

T: In the Valley the deepest sewer pipes are well below the water table all year long, so added floodwater pressure is not significant enough to force flow backwards, or literally uphill, in the pipe. All of Kent’s 213 miles of sewer pipes are designed to flow sewage downward by gravity through sloped pipes until it reaches either a lift station, or a King County Metro Sewer Interceptor. From there, it continues to flow downward by gravity until reaching the King County Wastewater Treatment Plant in Renton.

M: So that’s where the phrase “poop rolls downhill” comes from!

T: You could say that. In fact the elevation of the Valley decreases 40 feet between South 277th Street and the Treatment Plant. Whatever is in the pipe is always flowing downhill.

M: You mentioned a lift station? What is its function?

T: Due to Kent’s varied geography, lift stations are required to “lift” sewage from low points to higher elevations where it can start flowing downward again by gravity. Lift stations use electricity to operate the pumps that lift the sewage. Each of Kent’s lift stations has a back-up generator that supplies power in the event of a PSE power outage. That’s why you can still flush the toilet when the power is out.

M: So, people who live on top of the East and West Hills don’t need to worry about sewer backups?

T: Not due to flooding, anyway. If people, regardless of where they live, pour their cooking grease down the drain, all bets are off. Grease solidifies when it cools and builds up in the pipes, which is the leading cause of sewer backups. So, don’t dump grease down the drain. In other words, have a different game plan for that leftover grease from your turkey fryer this holiday season.

M: The Seattle Times recently reported that nearly 800,000 people may be without sewer service. What is this all about if backups are as unlikely as you say?

T: That article would have been more accurate if it said 800,000 people would be asked to minimize their use of plumbing fixtures in their house. The sanitary sewer system would still be operable. If the overall system was receiving a lot of infiltration and inflow of water due to a rainstorm, it would tax the capability of the system so conserving water use would be very helpful. People would still be able to flush their toilets, but it would be best to wait to do the laundry.

M: What can residents and business owners do to prevent sewer backups?

T: They can first refrain from pouring grease down their drains. But during a flooding event, there are some devices that can be installed prior to leaving if an evacuation is ordered. Keep in mind, with these devices in place, you can’t use your toilet. This would strictly be an additional precaution you could take, but again, it only applies on your way out the door.

There are a few options with varying costs and or possible permitting needs. One of the simplest devices to use is an inflatable plug which is available at most hardware or plumbing stores. To use this device a homeowner would need to know where their sewer clean-out is located. People can come to the Permit Center at Kent City Hall for help locating their clean-out. Most homeowners could easily do this job.

M: There you have it – sewage geysers are only a figment of our imagination. I hope this clears up some of the many misconceptions about the possible impact of flooding in our community. So…we can take those sci-fi images and flush them down the toilet, right?

T: Right. A final note….during an update on the repairs being made at Howard Hanson Dam, the U.S. Army Corps of Engineers said there was now a 1-in-25 chance of flooding in the Green River Valley. With the giant sandbags in place the odds of widespread flooding improve to 1-in-33. Previously, the Corps of Engineers said the chance of widespread flooding was 1-and-4. People should certainly rest easier.

Michelle Witham is the community and public affairs manager for the City of Kent.

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