Green Utilities Pave the Way for Seattle Transit and Stormwater Management Project

June 21, 2011
[WaterWorld Online Exclusive] Urban project managers are increasingly turning to "green" utilities as a natural approach to restoring dysfunctional ecosystems in densely populated municipalities...

Among the biggest challenges facing urban areas are stormwater management and the attendant flooding and pollution that are endemic in heavily paved environments. Urban project managers are increasingly turning to "green" utilities as a natural approach to restoring dysfunctional ecosystems in densely populated municipalities. Unique combinations of trees, soil, ponds and sustainable man-made construction products are increasingly being used to address these problems ecologically.

The "green" utilities consist of open soil, large trees and rain gardens. Together, they recreate a natural system for water management in an ultra urban setting.

Forward thinking communities view trees as solar-powered, natural technologies. The focus on mature trees is shifting from a beautification tactic to a solution for long-term, sustainable communities. Trees, and the soil they grow in, act as powerful tools for improving the rate, quality, and volume of stormwater runoff.

Creating favorable growing conditions in urban areas enables trees to mature and provide natural stormwater management. Key ingredients to growing large urban trees include large quantities of high-quality uncompacted soil, adequate drainage and irrigation, and aeration.

According to a U.S. Forest Service study, a large tree with a 30" trunk diameter provides 60-70 times the ecological services of a tree with a 3" trunk diameter. The study explains that the leaves of large trees provide greater rainwater interception. The bigger the tree, the more interception it provides.

Another major benefit provided by mature trees is evapotranspiration, the movement of water to the air from sources such as the soil, canopy interceptions and water bodies.

These tree attributes served as the basis for the Aurora Shoreline Project, a large transit/stormwater management project in a suburb of Seattle. With an average annual precipitation of 37 inches, the Washington State community wanted a better way to manage water on-site along with traffic.

Artist's rendering of sidewalk plan.Finding a Soluble Solution in SeattleShoreline City municipal planners needed an eco-friendly solution for the Aurora Shoreline Project, a three-mile section of Route-99 located between 155th and 185th Streets.

Being a major highway, Route-99 had a traditional stormwater system built in the 1960s, and a better infrastructure was required to address traffic safety, stormwater runoff, and water pollution (such as phosphorus) flowing into Lake Washington.

Major problems faced the project planners, who were working within a limited right-of-way and large paved areas with weak runoff control. The plan called for additional traffic lanes, wider sidewalks, up-to-date stormwater controls -- all with sustainability requirements.

The challenge was designing a green infrastructure system that could handle the high amount of polluted stormwater that ran directly into Lake Washington. The project aimed to slow the rate of runoff, reduce the total volume, and remove contaminants and heavy metals before they entered the lake.

The Shoreline city council called for an urban forest near the water basin for the purpose of naturally intercepting and filtering rainwater runoff. In addition, there would be rain gardens to filter and manage water, as well as beautify an unattractive vehicle-focused corridor. Sidewalks and paths alongside the road would encourage pedestrian and bike traffic.

The transit project, which launched in January 2010, will be implemented in three phases through 2012.

Calling in the Contractors
For project design, the city hired Omaha-based engineering consulting firm HDR and Seattle-based Otak, a premier architectural firm providing water and environmental services. Otak Senior Associate Curtis LaPierre designed the natural, sustainable, eco-friendly solution for the Aurora Shoreline Project.

Cutaway of Deep RootSilva Cell underground soil and stormwater management system.LaPierre and his team configured an infrastructure that integrated modern techniques and materials, including permeable pavers (called Eco-Priora), curb cutting, and an underground soil and stormwater management system (called Deep RootSilva Cell).

Most of the impervious sidewalks around the highway are being replaced with pervious pavement, which will allow for stormwater filtration. Room was also needed for vehicle traffic, as well as large soil volumes to support the "green" utilities. While many man-made products were selected for the transit project, the streetscape design includes natural features such as large trees and rain gardens to intercept and route excess water.

Making Room for Utility Trees
The stormwater from the road enters the rain gardens through curb cuts. It is also absorbed into the soil where it is soaked up by the trees and plants and recharges the groundwater.

Silva Cells were installed under the permeable pavers adjacent to the rain gardens.Silva Cells, the underground soil and stormwater system, were installed under the permeable pavers adjacent to the rain gardens. Rain that falls directly on permeable pavers or spills from adjacent sidewalks and parking lots enter the underground system. The cells, which essentially create an underground planter area, contain the soil volume that nurtures the trees and also slows and treats the water before it is piped out of the system.
The project utilizes about 800 Silva Cells in a one-layer system, adding 8,000 cubic feet of soil volume for the trees and 1,600 cubic feet of open pour space to store and treat stormwater.

A typical urban tree that doesn't utilize an underground growth technology has an average life span of 13 years, meaning that the tree will have to be replaced at least 3 times over a 50-year period. A short tree lifespan deprives the "built" environment of many benefits that trees provide.

Traditional methods of planting trees -- putting them in 4 x 4 cutouts with no additional soil volume -- also can become very expensive to a community. By utilizing newer methods of tree planting such as a soil delivery system, municipalities can actually save an estimated $25,000 in environmental benefits to the community and avoid the hassle of replanting over the same 50-year period.

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