Existing Conditions. An illustration describing how rainwater currently flows into road grates, occasionally overflowing the combined sewer system in the Barton Basin which sends millions of gallons of untreated wastewater into the Puget Sound.
West Seattle neighborhoods get the rundown on rain garden project to reduce Sound pollution
Residents living in the Sunrise Heights and Westwood neighborhoods of West Seattle, whose streets will eventually be transformed into a series of rain gardens to reduce combined sewer overflows reaching Puget Sounds, met with King County officials on Wednesday night, Feb. 9 for an update on the project and an opportunity to ask questions.
Since September of 2011 work crews have been on the streets surveying the land – both surface and subsurface, digging testing wells, evaluating soil types and depths and groundwater locations in an effort to ultimately build a system of rain gardens, or “bioretention swales” to get technical. The goal, according to King County, is “to capture and reduce the amount of peak stormwater flows that would enter the combined sewer system by up to 15 million gallons a day.”
The field work has caused a few neighborhood headaches due to the sound and clutter of heavy machinery drilling into the earth on residential streets, and the purpose of the Feb. 8 meeting was the explain the progress and purpose of the commotion.
When the rain falls hard and overflows occur, the Barton Pump Station south of Lincoln Park is the destination before pouring into the Sound. According to a King County study done in 2008 there are an average of four untreated overflow events each year that dump four million gallons into the Sound. The county is angling towards the Department of Ecology goal of no more than one untreated discharge per year.
When those overflows occur, King County said, on average, 90 percent is stormwater and 10 percent is sewage.
Why Sunrise Heights and Westwood?
According to the county, “these neighborhoods are ideal for placing biorention swales … because 45 percent of stormwater runoff in the Barton basin comes from these areas.” In addition to the areas significant runoff contribution, it was also chosen because the streets are fairly flat and the planting strips fairly wide (making the installation of rain gardens much easier).
The Sunrise Heights and Westwood neighborhoods are basically confined by S.W. Othello St. to the north, 29th Ave S.W. to the east, S.W. Barton St. to the south and 34th Ave S.W. to the west.
The Feb. 8 meeting was a chance for Associated Earth Sciences, Inc. geologists (hired by the county) to explain what they have found in their field testing, which included drilling 29 test borings and wells to various depths from 20 feet to 209 feet.
The findings revealed a bit of natural history for the area as well as informing the rain garden plan of attack that is designed to capture surface water only; not water captured by storm drains and ran directly into the combined sewer system.
According to the Washington State Department of Ecology, the Puget Sound region was under a mile of ice 20,000 years ago as the Vashon Ice Sheet moved in from Canada. In Seattle, the ice cover was 3,412 feet … a little higher than five Space Needles stacked on top of each other. The pressure of those glaciers forming the earth and meltwater carving the land as the glaciers retreated formed the topography we see today.
An AES geologist explained how that history created the layers in the Barton Basin. Just below the topsoil is a layer called the Vashon Lodgement Till, often called hardpan, which is characterized by low permeability.
“This is what most of you will see at the subsurface: a material almost as hard as concrete … about 20 feet thick,” the geologist said. Since water cannot get through the layer it ends up in the combined sewer system.
Below that is the Unsaturated Vashon Advance Outwash, “the sediment that was coming off the front of the glacier as it was advancing, so it is river deposits essentially.” Water can flow through this layer easily, so the plan is to collect water in the rain gardens and get it to this layer, bypassing the hardpan.
Below that is the Saturated Vashon Advance Outwash, where the areas regional water table resides. The layers continue from there, but these are the three geologists are focused on.
On the surface, field work included looking at the shape of roads (do they crown or are they flat?), steepness of streets, and the landscape of homes in the area. King County takes that data to create color coded puzzle pieces which they use to simulate heavy rainfall to determine the water’s path. From there, they can decide on locations for the rain gardens.
Once rain garden locations are determined King County will contact homeowners whose planting strips will be converted into biorention swales – defined as using “soils, vegetation and trees to capture stormwater and infiltrate it deep into the ground, where it naturally recharges our groundwater supply,” according to a handout at the meeting. There are 60 blocks in the test area, and King County predicts 40 blocks will have swales installed.
Depending on the thickness of that non-porous hardpan layer, either deep infiltration pits (where the layer is relatively thin) or deep infiltration wells (where it is thick) will guide the water caught by swales into the subsurface.
King County officials said they will take care of watering and maintaining the rain gardens, so the responsibility will not fall on residents. Some people will have to move their garbage, compost and recycling containers to a different location for pickup after the transition, but county officials assured it would not be a significant walk.
Early February signifies the shift from field testing to the design phase. King County will work on preliminary designs through April (with another community meeting in March), finalize their design by the end of 2012 and begin construction in 2013. The project is expected to be complete in 2015.
Kristine Cramer is King County’s community contact on the project. She can be reached at (206) 263-3184 or firstname.lastname@example.org.
For more information online, visit the project page found here.