The Cost and Benefit at 617 East Arques Ave.
Silicon Valley’s third-largest city after San Jose and Fremont, Sunnyvale is home to LinkedIn and Google, among other notable tech companies. In 2014 the city added 85 new townhomes with underground bioretention. SummerHill Homes at 617 East Arques Ave. were part of the latest wave of housing additions for renters and buyers in the South Bay of San Francisco. When the units were in early stages in 2013, the developer had to decide how the homes’ stormwater would be managed to adhere to the Bay Area’s C3 requirements. Onsite? Traditional, aboveground management? Bioswale? Or perhaps belowground with the not so traditional, innovative technology, Silva Cell? After careful evaluation, it was decided that the best way to achieve the water management would be through the installation of 380 (3 layer or 3x) units of Silva Cell. East Arques Avenue would go on to be one of the first stormwater projects in the Bay Area. The East Arques project is unique in that it unites goals that are historically at odds with each other: meeting stormwater requirements and preserving land for development.
Every drop of rainwater from the entire site, including the rooftops, is piped through the Silva Cell system, amounting to a treatment area of 5,000 sq. ft. This meets the state requirements and simultaneously provides a shared usable space aboveground for its new owners.
The C3 guides have a calculation requirement of taking the total project areas and ensuring 4% of the impervious square footage flows to the treatment area. In an effort to store, treat, and collect all runoff, water was piped under the street, sent to a catch basin area, and then pumped back into the courtyard space where Silva Cell systems had been installed under the grass. This is used to simultaneously treat the four trees onsite. This also provides additional walking, gathering, and communal shared space for the SummerHill Homes’ residents.
Developers look to utilize every square foot of land in a new housing development—whether that land is for another condo, another parking space, or another amenity that will bring monetary value to the real estate. Carving out valuable land for stormwater treatment is typically not at the top of their list.
In this case, the developer—SummerHill Homes—was able to satisfy the need for housing on the Arques site along with the regulatory need to adhere to California’s C3 stormwater requirements. Leading the way in engineering and design, Carlson, Barbee & Gibson (CBG) fulfilled a requirement for open space and stormwater treatment areas in the same location—underneath a communal grassy courtyard with one tree at each corner.
Ryan Hansen, the lead engineer on the project team noted that in these types of new development plans where space is at a premium,
“It is all about utilizing shared space for the community and not taking up land with traditional bio-retention areas which you cannot typically walk on or use.”
Traditional bioretention design is aboveground in a bioswale or rain garden and is typically used when ample land is available.
Silva Cell, with its unique and easy-to-implement design was able to accomplish the same amount of treatment and retention as traditional methods but allowed SummerHill Homes to repurpose and gain back space they otherwise would have had to forfeit. Now it can be enjoyed as additional walking, gathering, and communal shared space for the residents.
Here is the latest image of the project site. As the dozens of residents at 617 East Arques go about their daily lives, tree roots are growing strong, stormwater is being treated, and soil volume is enhanced underground courtesy of the 380 Silva Cells.
Number of Silva Cell: 380 3X Silva Cell 1
Amount of Soil volume: 3500 ft3
Treatment Area: 5500 ft2
Number of trees: Four
Type of Project: Integrated trees & Stormwater
Project Civil Engineer: CBG
Installation Date of Silva Cells: January 2015