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Drainage work may lessen flooding on NE 124th

  • Written by Lisa Allen

Senior engineer with King County Natural Resources and Parks, Chase Barton has spent years studying flooding and flood patterns in the Lower Snoqualmie Valley. Over time, he noticed changes happening in the behavior of winter floodwaters, especially in the area just south of Duvall near the Tolt pipeline. “The water wasn’t draining from the fields after flooding,” he said during a recent tour of the Tolt Pipeline Protection Project which includes drainage improvements.  “Instead of draining out, like it should, water has been sitting in the fields for lengthy periods of time which also means more water stays over Northeast 124th.”

As the population in the Valley has grown, Northeast 124th has become a crucial corridor that often is closed by high water in the winter months, causing long traffic backups.  The project to protect the pipeline and also deal with the drainage issues has been in the works for some time, and finally was given the go-ahead for this year.

Lallen tankReporter Lisa Allen is dwarfed by the huge culvert that was recently installed in the new drainage area next to the Tolt pipeline. (Photo by Craig Garric)Much of the blame for the drainage issues was placed on problems with Deer Creek, which wasn’t moving as it should, partly due to a non-functioning culvert and broken floodgate. So as part of the pipeline project, the creek was dredged out and the culvert replaced with a much larger one.  A new floodgate was also installed that is designed to work automatically as floodwaters go up and down. Part of the regrading process included removing a massive amount of dirt which was taken elsewhere. The culvert location has also been changed which will allow better fish passage and improve habitat, according to King County. Nine acres of native shrubs and trees will be planted on the site this fall and into the winter.

Barton is optimistic that the drainage improvements will help farm fields dry out quicker and increase the days Northeast 124th will be open during flood events. “We will be watching the river closely this winter,” he said.

The broken culvert also caused currents created during flooding to change their paths.  Repairs should help return those currents to where they traveled previously.  Work on the project, other than planting, should be completed early this month.

The $10.2 million project, primarily to protect the Tolt pipeline, began in early summer. The pipeline was at risk of being damaged after years of riverbank erosion. The erosion has caused the riverbank to come as close to 100 feet from the pipeline, said Barton. The project, which rebuilt 1,200 feet of the eroded revetment, was funded by King County Flood Control District. The original revetment (the riverbank rock armoring) was installed on the north bank of the Snoqualmie River in the 1960s to protect the pipeline and adjacent farmlands from erosion. The revetment has been damaged over time and the river channel has been migrating towards the pipeline.

The Seattle Public Utilities (SPU) website states that the Tolt pipeline supplies 30 percent of the drinking water for 1.3 million people in and around Seattle. Seattle itself gets 37 percent of its water from the Tolt Reservoir; Woodinville and Duvall get 100 percent of their water from the Tolt and other Eastside cities also get a large part of their water from the Tolt.

For this project, the county has used rock and wood engineered structures (ballasted wood jacks) that were placed along the riverbank.  “The ballasted jacks are better than tons of rocks,” Barton said. “The structures interlock with each other and offer better integrity and coverage than rocks.”

landAlmost 700 ballasted wood jacks were lowered into the Snoqualmie River to protect the bank from erosion. (Photo by Lisa Allen)The log structures used were engineered specifically for this project, said project manager Craig Garric. But because they are located along the riverbank and not always visible, watercraft safety is also an issue, he noted.
Written material provided by Garric states that King County addresses safety issues that may result from “large wood installation projects.” King County’s procedures, according to the information, for considering public safety when placing large wood in King County rivers requires that the Department of Natural Resources and Parks develop and maintain a list of projects where large wood will be or is likely to be installed on a King County river or stream. This project list is updated every year and made available by request and via the county website (kingcounty.gov/rivers) or email notifications. For the pipeline project, it is noted that recreational use is very low in the vicinity.
However, both Garric and Barton agreed they are concerned about the increase in the use of high speed jet skis on the river and the possible dangers the installations may pose to them.  “But there is a lot of large wood in rivers,” said Garric, “and a lot of wood is used along riverbanks across the county. Public safety is our primary consideration. We are trying to maximize public benefits and minimize risks to the public. We do regular outreach programs to the boating community to get the word out about our projects.”

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