The DeepRoot Silva Cell system was installed to provide proper soil volume for trees while also cleaning at-source stormwater before draining into Eagle Harbor
A half hour’s ferry ride from Seattle, Bainbridge Island initiated a downtown renovation project in 2010 with the purpose of overhauling the utility network, widening the sidewalks for pedestrian walkability, and incorporating sustainability features along the main traffic artery of Winslow Way. The city and design team at SvR agreed to embrace green infrastructure as a project priority, selecting Silva Cells to assist in the growth of seven new street trees uniquely planted among the diagonal parking zones; likewise, the Silva Cells treat on-location stormwater before entering the sewer system and draining into Eagle Harbor. Over a decade later and the street trees are thriving in this award-winning project, providing locals with welcoming greenery and a flourishing shady canopy.
Number of Silva Cells: 560 (1x)
Amount of Soil Volume Per Tree: 800 ft3
Number of Trees: 7
Type of Project: Streetscape, Stormwater
Project Designer: SvR Design Company
Project Contractor: Tucci & Sons
Installation Date of Silva Cells: 2011
The city of Bainbridge Island, located across Puget Sound from Seattle, maintains a small-town community vibe despite its urban adjacency. Its main downtown artery, Winslow Way, features local shopping and restaurants — and in 2010, after years of discussion among residents and city officials, an improvement plan was approved that would overhaul the street and address a number of public concerns.
The primary issues were the area’s water quality and outdated utility network. The city, partnering with SvR Design, took the utility improvement necessity (water distribution, sanitary sewer, and storm sewer replacement) as an opportunity to entirely overhaul Winslow Way, ensuring that the water draining into Eagle Harbor was cleansed of street runoff. Likewise, city planners wanted to make the street more walkable, providing wider sidewalks for pedestrians.
Green infrastructure was also an important element in the design plans, as the city wanted to incorporate vegetative features along Winslow Way. The DeepRoot Silva Cell system was chosen for its sustainability benefits and to help address many of the local concerns, serving not only as a stormwater treatment tool but as a way to support seven street trees: new plantings positioned in the road, alongside the diagonal parking stalls and offering locals an inviting green atmosphere and shady respite from the summer heat.
The Silva Cell system was integrated into the roadway to maximize tree canopy (which visually narrows the street and calms traffic) and to manage stormwater runoff. A total of 560 (1x) Silva Cells were installed along Winslow Way, providing each of the seven new trees with 800 cubic feet of soil volume.
Silva Cells allowed the design team to plant the trees in the angled parking zones and therefore slow vehicle traffic. The street trees, located nine feet beyond the face of the sidewalk, allow space for large canopies desired by the community and away from the adjacent zero lot line developments.
Treating area rainwater was also a primary objective of the design team. Urban stormwater runoff was sending pollutants — including bacteria, oils, heavy metals, and chemicals — into the bay. In response, the city chose to adopt new stormwater planters and raingardens, in addition to the Silva Cell system where the pervious concrete above allows stormwater runoff to enter and pass through the bioretention soil media, which both cleans the water and provides irrigation for healthy, robust root growth. An underdrain below the Silva Cells connects to the public storm drain and conveys runoff that passes through the system to counteract poorly draining native soils and prevent stormwater from migrating into the basements of adjacent buildings.
The Silva Cell system’s versatility allowed several design challenges to be addressed at once. More than a decade after the project’s completion, the improvements have been universally hailed as a success — indeed, the initiative won the 2013 Award of Excellence for Best City Project. Winslow Way is a valued community destination with an ever-increasing tree canopy and is — according to Dan Burden of the Walkable Livable Communities Institute — “one of the best remade streets I’ve seen anywhere in America.”
-2013 Award of Excellence for Best City Project