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Officials hope they've solved the Rivergreen erosion problem

Ongoing riverbank erosion issues have plagued the city for years now


It’s been six years since Wilsonville first attempted to shore up a section of Willamette River bank in the Rivergreen neighborhood.

And it’s only now that city officials are able to talk about a long-term solution with any degree of confidence. An emergency attempt at erosion control only worked temporarily in 2009, leading to another go at it in 2011. This, too, did not prove satisfactory, leading to the present day and a third attempt.

“It ended up being a project where a fix was constructed, and it was not successful,” said Wilsonville Community Development Director Nancy Kraushaar. “So then we had to go back and it’s taken many, many years to get the solution we have in the ground out there today.”

The outfall in question is located at the south end of Willamette Way East, a quiet residential street that connects with Wilsonville Road. The outfall provides stormwater runoff from nearby streets into the Willamette.

After it was originally built, however, it did not take long for nearby homeowners to realize that the combination of stormwater and groundwater seepage was conspiring to steadily undermine the riverbank.

Improvements included construction of a bio-swale and rerouting stormwater discharges to another section of riverbank. It also involved bank stabilization treatments to repair erosion-damaged areas and prevent additional erosion.

“Doing nothing was really not an option,” Wilsonville Mayor Tim Knapp remarked at a June 2 city council work session. “Because major bank failure, due to ignoring a problem, could cost us a whole lot of money over time. And, secondarily, we hopefully have learned something about outfalls and about what kind of standards we need to be holding construction to when we’re doing outfalls, and I’m thinking about projects we currently have going on along the river.”

The most recent fix cost the city $1.3 million and was driven, in part, by the threat of litigation on the part of adjacent property owners. The whole episode also has had the effect of changing city standards on stormwater drainage.

“We’ve already changed our standards to provide detention,” Kraushaar said. “This is an area that has no detention, so it fills up the pipes and goes ‘whoosh,’ on down to the river; there’s no trying to meter it and hold it back. Stormwater management has come a long way in the last 15 years, and we’re still working on how to do it better.”

Councilor Scott Starr noted that prior to the initial 2008 fix there actually was a small detention pond that ultimately failed for lack of capacity.

“It just didn’t work,” Starr said. “So now do we have detention there? Or is it more of a strip of land?”

Kraushaar answered that the current set-up does not have a standard detention pond, rather a smaller pool similar to another erosion control effort carried out on the Willamette in the Morey’s Landing neighborhood a short distance to the north.

“We have some little pools, kind of like we have at Morey’s landing, except it’s much shorter, to account for the velocity of the water,” she said. “There’s still the swale in the open space there, and as of tomorrow we’re still working with the neighborhood to get that seated nicely for them.”

Other erosion control devices, such as woody debris, are designed to blend into environment, she added.

Unfortunately, given the size of the Willamette River — the 19th largest in the United States by volume of water — new erosion problems are cropping up nearby and may require a similar course of action.

“We’ve got erosion problems further to the west in the same area with a different outfall,” Kraushaar said. “This is causing erosion in the bank, so we’re looking at temporary erosion protection and scoping out what that’s going to take.”



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  • 22 Sep 2014

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