Green Roofs - Living Architecture Monitor - Summer 2017 - 32

POLICY

EXPLORING TWO OF SEATTLE'S GREEN
INFRASTRUCTURE PROGRAMS:

SEATTLE GREEN
FACTOR AND
RAINWISE PROGRAM
BY EMMA TAMLIN, GREEN ROOFS FOR HEALTHY CITIES

Seattle, Washington is one of 800 cities
in the United States operating on a
combined sewer system. But over the past
two decades it has been at the forefront
of several green infrastructure initiatives
and was the first city in North America to
adopt a green area ratio program. With
an average of 37.49'' of precipitation per
year, Seattle generates 27,000 gallons of
stormwater runoff from a 1, 200 square
foot roof, and approximately 1 million
gallons from 1 acre of pavement annually.

A

ccording to Seattle's Green Stormwater Infrastructure 2015-2020
Implementation Strategy (GSI), Seattle managed over 100 million
gallons of stormwater annually as of 2015. Their five-year plan calls
for a four-fold increase in green stormwater infrastructure by 2020
and is currently on track to meet their goal of managing 400 million
gallons of stormwater annually by 2020 and has already set the goal for
2025, aiming to manage 700 million gallons of stormwater.
Seattle encourages the implementation of green infrastructure to
manage stormwater and create better quality urban landscapes. The
Seattle Green Factor and the RainWise program are two initiatives that
complement the 2013 City Council and Mayoral request to have all city
departments' work together to meet Seattle's stormwater goals.
LIVING ARCHITECTURE MONITOR / SUMMER 2017 / 32

THE SEATTLE GREEN FACTOR

Seattle's Green Area Ratio program, the Seattle Green
Factor (SGF), is a landscaping requirement for all new
developments that are more than 4,000 square feet, have
more than four dwelling units, or more than 20 new
parking spaces in the neighborhood business districts. The
program was implemented after Ordinance 122311 was
passed in December 2006 and then expanded in 2009.
The SGF's objective is to create landscapes in the
public right of way that both manage stormwater and
offered visual community amenities. According to the
American Society of Landscape Architects, "because SGF
significantly raises the bar for landscaping in affected
zones, landscape design now starts in the initial stages of
site planning, allowing more collaboration between design
professionals; the resulting landscapes are more attractive
and better integrated into site programs and amenity
areas" (Wener, 2007).
The program offers a menu of landscaping strategies
divided into the following categories: Landscaped Areas,
Plantings, Green Roofs, Vegetated Walls, and Water Features. Developers can choose from a variety green stormwater infrastructure tools such as tree canopy, permeable
pavement, green roofs, and rain gardens so long as the final
score meets the minimum requirement for their geographic
area. The score is automatically calculated by inputting data
into the Green Factor Score sheet. Some green infrastructure elements are weighted more heavily than others due to
their enhance stormwater management and amenity space
benefits, such as green roofs and bioretention sites which
are given a score of 0.7, compared to ground covers at 0.2.
The square footage of each element is then multiplied by
the weighting of that element, to provide a total score.
The total score from all of the elements in the worksheet
is then divided by the size of the parcel to calculate the
Green Factor Score. Minimum scores vary based on the
different zones across Seattle with different standards set
for commercial and residential properties (Figure 1) with
residential properties requiring higher green factor scores
than commercial zones.
By 2010, the City of Seattle had implemented 52 green
roof projects totaling 360,000 square feet as a result of
SGF and other green infrastructure strategies and is currently conducting an inventory of green roofs. To ensure
maximum uptake and meet the objectives of implementing
high quality green infrastructure, the Seattle Department
of Construction and Inspections sponsored a number of
courses on green walls, rain gardens permeable paving
systems, and healthy soils.
Seattle has been successful in its green infrastructure
implementation due, in part, to its flexibility in how
developers can meet the requirements. The City believes
it is important to reduce rigidity in regulations and allow
developers to customize solutions to meet the goals of
stormwater management and improve amenity spaces.
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