BY BRIAN LIBBY
In late August, as Hurricane Harvey ravaged Houston with nearly unprecedented levels of rain — some 60 inches through the course of the storm, more than an average year's worth of precipitation there — I got thinking about stormwater infrastructure. When that much rain comes down in such a short time, it's arguably impossible to prevent some type of flooding. But the fact that Houston is comprised of some 600 square miles of unchecked sprawl, with many of its natural defenses against flooding paved over, certainly contributed to the problem.
Portland is not Houston. We are not prone to hurricane-level storms as that Gulf of Mexico-bordering city is, but we also get a surprisingly comparable level of rainfall: an average of 44 inches a year, compared to 50 for Houston, but 164 days of rain compared to just 104 in that Texas city. And as longtime residents know — especially those who lived through the flooding of 1997 — the Rose City is certainly not immune to such potential disasters.
Many of us have followed the progress over the past 15 years of the "Big Pipe" project, completed on the west side of the Willamette River between 2002 and 2006 and on the east side between 2006 and 2011. They are now part of a combined sewer system of pipes, drains, pumps, and other infrastructure that transports sewage and stormwater runoff to the city's Columbia Boulevard Wastewater Treatment Plant. The combined projects reduced combined sewer overflow into the Willamette by 94 percent and into the Columbia Slough by more than 99 percent.
There are also complimentary efforts such as green streets and bioswales, which can help slow and reduce the movement of water into the sewer system. The City of Portland's Tabor to the River program, for example, adds about 500 green streets and 3,500 trees to a 2.3-square-mile area from Mt. Tabor to the Willamette River between SE Hawthorne and SE Powell boulevards. Because of increases in the amount of pavement and other impervious surfaces that exist in the city, coupled with decreases in tree canopy, the volume of stormwater going into the pipes is much greater than the system was designed to manage 100 years ago. These moves not only decrease pressure on the sewer system, but they also save money. Resolving the sewer system problems in the Tabor to the River Program area with only pipe solutions would have cost an estimated $144 million. Adding green infrastructure projects reduced the estimated cost to $81 million.
To learn a little more about the overall system, I spoke with Tim Kurtz, who manages stormwater implementation for the Bureau of Environmental Services.
Portland Architecture: Given the level of disruption and damage caused in Houston by Hurricane Harvey, as well as the increased capacity of storms in this time of climate change, how would you characterize Portland's protective measures to deal with a major flooding event?
Tim Kurtz: For us it’s about levels of service. There will be really huge events that happen that you just can’t fully control. When it rains 40 or 50 inches in 30 hours [like in Houston], that’s almost unimaginable. You have to have a conversation with your ratepayers about the right level of service: where do they want to draw that line? In the combined sewer system, we can have basement sewer backups when we get a heavy downpour. The backups occur when you have so much water entering the sewer that it pressurizes and it flows backwards. Our level of service to protect homes from such backups is the storm we expect to happen once every 25 years. We have a system plan that helps us prioritize where we need to work. As we work through that plan, we move projects into our capital program for construction.
Most of us know a little about the Combined Sewer Overflow issue, the Big Pipe project and various green streets. How would you characterize Portland's capacity now to deal with floods?
We have to decide: what’s that level of control we want to pursue? For Johnson Creek, which flows through southeast neighborhoods our city, we have restored a floodplain by buying homes from willing sellers and that has made a dramatic difference in the frequency of flooding to nearby homes as well as to Foster Road. For Johnson Creek, the city decided the 10-year flood level is what we’d try to manage. It comes down to trying to determine the best benefit we can get while balancing costs to ratepayers in a developed landscape. We do incremental projects at the most critical locations, and try to preserve the capacity that still exists.
How does completion of the Big Pipe change the bureau's focus? What's the priority now?
Until about five years ago we were really focused on completing the combined sewer program. We installed the Big Pipes and many green street facilities to reduce the amount of stormwater runoff and to act as shock absorbers for the combined sewer. That investment is making a big difference and the number of combined sewer overflows to the Willamette River has dropped by about 94 percent, improving river health and opening up the opportunity for more recreation. We’re now working throughout the city, and focusing attention on system planning for the separated stormwater system. Green streets, pipes, and water quality filter vaults are examples of tools in our toolbox, and we work to come up with the best combination for stormwater management in particular locations.
Is dealing with stormwater more a matter of volume or the speed at which it's arriving?
Both. The basement sewer backup issue I mentioned before, is a matter of how fast the water is entering the sewer. So we employ techniques such as green street planters that slow the water down. In the combined sewer overflow tunnels, we want to manage the volume of runoff. If the runoff will eventually feed into a river or stream, we also care about how clean it is. How we clean it depends on how much runoff and how fast it’s flowing, and also what part of the city we’re in.
What are some of the bureau's next steps?
We’re still going to be spending a lot of time working in the combined sewer system, but I really see work in the separated storm system as the next wave. In the combined system, the city owns most of the infrastructure. As we go into the separated storm world, it’s a composite of public and private drainage ways intertwined together. It makes things a lot more complicated. It’s different tools, with different challenges. In the inner east side we can infiltrate water there, while other parts of the city have steep slopes and poorly infiltrating soil where it’s more about how we hold that water back or move it around. We’re looking to create a good balance for managing stormwater throughout the city.
How do Portland's efforts to deal with sewer overflow compare to the systems and approaches in other cities?
It was a blessing that we were able to do our combined sewer overflow work relatively early. Many cities are doing it right now and it’s only gotten more expensive. Any older city that sits on a bay or major river typically has a combined sewer system because it was cheaper to build one sewer for both storm water and sewage than two separate systems. The theory was that sewage would dilute into the river or into the bay. Maybe that worked somewhat when there are 50,000 people living there. But with 500,000 or 5,000,000, it creates a huge problem. Atlanta, Philadelphia, Cincinnati, Kansas City, Chicago, Seattle: they’re all looking at it. It’s a very common and prevalent issue throughout the country.
We obviously have a lot of rainy days, but does it help that Oregon isn't subject to hurricanes and other extreme weather?
It does happen here. The Columbus Day storm in 1962 was the remnant of a cyclone, but it was mostly wind and I don’t think we had a lot of flooding. But we do have big floods like those in the mid-1990s. When there are events like that, and like the one in Houston, it does raise awareness. In the ’90s with flooding here, it really spurred a lot of attention, for example, to Johnson Creek. Much of the work we’ve done there is a result of planning done right after those events. The longer we go between events, it’s easier to forget how big the impacts can be. It’s the city’s role to make sure we continue to make progress.