By Richard Van Winkle, News of Mill Creek.
At their regular July 22, 2014, meeting the Mill Creek City Council learned that the engineering estimate to fix the 35th Avenue SE flooding problem has almost doubled since February. Their preferred design option includes a large contingency factor and totals out to $6.3 million, which is substantially more than the $3.5 million estimate given to them in February.
After receiving a project status report from Scott Smith, Mill Creek City Engineer, the city council approved his recommendation to proceed with a design “to maintain the existing three-lane section, raise the roadway profile with a pile-supported slab, and incorporate some type of lightweight fill material if beneficial.”
KPFF Consulting Engineers will continue to design the 35th Avenue SE reconstruction project, which will result in a “shovel-ready” project with well-defined cost estimates ready for construction toward the end of the year.
This will make the project more likely to receive state or federal funds to help build the redesigned roadway than if the city were to ask for funds before the design is complete.
The flooding problem
At the point where 35th Avenue SE crosses Penny Creek settlement of the roadway has increased the chance of flooding during times of peak rainfall. The road is built over a thick peat bog that is being compressed by the road’s weight, which causes the road to settle further each year. Engineers estimate that the roadway will continue to settle up to an additional 36 inches in the next 30 years.
Mill Creek can’t ignore the problem because according to a report prepared by KPFF Consulting Engineers, “35th Avenue Southeast is a three-lane major arterial that serves as a critical north-south corridor for regional traffic with an average daily traffic (ADT) of 15,000 vehicles per day.”
The road had to be completely closed due to flooding for a number of days during the winter of 2012-2013. Traffic had to be diverted through residential neighborhoods when heavy rains caused the water level to rise one and a half feet above the roadway at that location. City staff observed peak water levels estimated to be 392 feet above sea level during the heaviest flooding.
However, 35th Avenue SE flooding was much less this past winter and only one lane had to be closed with no traffic diversions in spite of record rainfall in March and April. According to a water level data logger on Penny Creek near the project site, the winter’s high-water level of 391 feet occurred on March 10, 2014.
In June of this year Tom Gathmann, Mill Creek Public Works Director, speculated that a number of factors may have contributed to lower water levels and decreased flooding last winter. He said that city maintenance workers obtained permission from the Washington State Department of Ecology to whittle away at downstream beaver dams in order to increase drainage.
Gathmann said another factor upstream of the roadway that may be reducing flooding is the improved storm water retention system installed by Polygon Northwest as part of their mixed-use East Gateway Urban Village development. The new system uses a “Thirsty Duck” control structure that releases stormwater more efficiently and more slowly.
Designing a fix
In February the Mill Creek City Council approved a $423,223 consulting contract with KPFF Consulting Engineers to develop a detailed design for fixing the 35th Avenue SE flooding issues.
Scott Smith, Mill Creek City Engineer, presented KPFF’s interim pre-design study at the July 22nd city council meeting. He wanted to inform the city council of the team’s progress and get direction on the council’s preferred design approach.
Smith said that predicting the highest wetlands water level was very difficult because the computer model prepared in 2002 by Snohomish County to accomplish this task failed to predict the peak water levels already attained. He said that he was very nervous about using the 393.5-foot water level design guideline recommended by KPFF.
After some discussion the city council agreed that designing the road to be dry with water levels as high as 395 feet would be more prudent than using KPFF’s suggestion.
A number of design options for reconstructing 35th Avenue SE were presented to the city council. According to Smith, “As part of the contract scope of work, the consultant was to evaluate several different options to address the flooding problem on 35th Avenue SE between 141st Street SE and 144th Street SE.”
“The options range from doing nothing to using different fill materials to constructing a new bridge. Each option was assessed on factors such as long-term maintenance, right-of-way and environmental impacts, construction costs, utility impacts and aesthetics.”
Doing nothing was the cheapest design option, but of course it does nothing to alleviate flooding. Because the roadway is predicted to settle 12 to 36 inches over the next 30 years, the roadway flooding will occur more often and last a longer time if nothing is done.
The most expensive option considered was to construct a bridge over the problem area. Although a bridge would completely eliminate flooding, the city council determined the estimated $19 million cost to be excessive.
The roadway reconstruction option selected by the city council elevates the problem section above the 395-foot design elevation. According to KPFF, “The pile-supported slab alternative involves supporting a structural slab with small diameter steel pin piles.”
The 4-inch diameter steel pin piles are driven through the peat bog into stable soil beneath and then a structural slab is constructed with the pin piles as support.
The roadway is then rebuilt on top of the structural slab. This takes pressure off the peat and no additional settling is anticipated.