City of Seattle Natural Drainage Systems
1.
Project Statement
Urban stormwater is a leading cause of water pollution in the U.S. yet cost-effective
solutions to the problem have been elusive. Seattle’s innovative Natural Drainage
Systems approach delivers higher levels of protection for receiving waters at a lower cost
than traditional street and drainage improvements.
2.
Project Summary
Why Seattle is Implementing Natural Drainage Design
For most Seattle and Puget Sound area residents, creeks, lakes and marine waters define
their sense of place. Seattle is home to five species of salmon that have returned to local
waters for the last 10,000 years. Public demand for better stewardship of salmon led the
City of Seattle to promote restoration of creek habitat in the late 1990s and the program
has been a great success.
Yet restoring stream habitat is only part of the solution.
Stormwater runoff from development significantly harms fish habitat and water quality in
cities and counties across the nation. However, most attempts to protect urban streams
from stormwater damage have been ineffective or too expensive.
Impervious surfaces such as rooftops, streets and parking lots do not allow rainwater to
seep into the soil. Traditional stormwater systems are designed to collect rainfall in
gutters along the street and feed it into large underground pipes and storage vaults before
conveying the water to a nearby stream, lake or treatment facility.
In Seattle, that means pollutants generated by urban activities, including pesticides, heavy
metals, motor oil and bacteria from animal waste are carried through creeks into lakes
and Puget Sound, impacting the food chain that supports native marine fish populations.
The sheer volume, rate of flow, and transport of pollution through our creeks prompted
Seattle to look for solutions throughout its urban watersheds, far beyond the stream
channels themselves.
How Natural Drainage Systems Work
Typical stormwater systems are designed to “drain” a city, preventing rainwater from
going back into the ground where it belongs. They exist mainly to move and delay
stormwater, sending it eventually into local water bodies. At best this approach only
transfers the problem, but does not solve it.
City of Seattle Natural Drainage Systems
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Several years ago, Seattle began implementing an innovative approach to drainage that
more closely replicates the natural hydrologic process of a forested watershed in its ‘predeveloped’ condition, and with it, more natural levels of runoff entering creeks.
Natural Drainage System design is based on new science and technology that emphasizes
decentralized, natural infrastructure, rather than traditional pipe and vault systems that
quickly convey huge amounts of polluted stormwater to creeks.
Seattle’s natural drainage projects completely redesigned city residential streets using
open, vegetated swales and stormwater cascades to mimic the functions of nature lost to
urbanization. At the heart of all natural drainage projects is the plants and trees, and deep,
healthy soils that support them. All three combine to form a living infrastructure that,
unlike pipes and vaults, increases in functional value over time.
How Natural Drainage Systems Benefit the Community
The most important achievement of Seattle’s natural drainage projects has been to fix the
problem at its source in an urban setting; not just moving and delaying stormwater, but
reducing and treating it. Techniques for using natural systems to reduce stormwater have
been experimented with elsewhere – mostly in individual buildings, parking lots, and new
suburban developments. Seattle’s program is the first to apply it systematically to a highdensity urban environment.
Seattle’s first natural drainage pilot project (SEA Street) was completed in 2000. This
effort had remarkable success, reducing overall stormwater runoff, providing significant
water quality treatment, and generating wide publicity for low impact development
techniques. In a steeper area of the same watershed, the 110th Cascade pilot was built in
2002. It is a vegetated, creek-like cascade that now intercepts, slows, and filters 26 acres
of stormwater flowing along four residential blocks.
Other pilot projects include the Broadview Green Grid project which involves 15 city
blocks and the High Point Redevelopment Project. The Broadview project will
manage stormwater flow from approximately 32 acres, almost an entire sub-basin of the
Piper’s Creek Watershed. The High Point project is a partnership between Seattle Public
Utilities and the Seattle Housing Authority, which will integrate a natural drainage
system into the High Point project – a 129-acre mixed income housing development
located in the Longfellow Creek Watershed in West Seattle.
The results from Seattle’s first pilot projects, done in collaboration with the Seattle
Department of Transportation, were so compelling that the City made the commitment to
use natural drainage systems as the primary stormwater management approach in all
areas that drain directly into creeks. The amount of interest expressed in the past year by
other cities and counties is a strong indication that this green approach to drainage is
rapidly winning converts locally, nationally, and even internationally. This broad appeal
stems from three types of benefits demonstrated by Seattle’s early pilot projects:
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Environmental Benefits: The first of Seattle’s NDS pilots reduced stormwater runoff
from the target block by 98% over two years. The result was less runoff, reduced
flooding, and cleaner water entering the creek.
Cost Effectiveness: Stopping runoff and pollution at its source in urban neighborhoods
has reduced the need to build more costly pipe systems and regional detention facilities,
and to mitigate for pollutants entering local waterbodies. Rather than deteriorating over
time like piped systems, natural systems become more effective as plants and trees grow,
so total life-cycle costs are reduced. In all, natural drainage systems cost about 25%
less than street redevelopment with traditional curbs, gutters, catch basins, asphalt,
and sidewalks.
Public Appeal: The pilots have proved to be extremely popular with neighborhood
residents, who were involved in all aspects of planning and implementation. The project
incorporated more trees, native plants, and green space into neighborhoods, increasing
residents’ interest and willingness to pay for and maintain such systems. Residents along
SEA Street now help maintain city infrastructure by tending to street "gardens" in front of
their homes. Tree cover helped reduce summer heat while absorbing air pollutants. The
redesigned streets improved pedestrian and auto safety by reducing the speed of traffic.
This project has had a direct positive impact on the homeowners in the area. For the last
decade, a property owner in the Broadview project has had water seep into his basement
each winter when it rains. The project was completed shortly before the record October
20, 2003 record storm which dropped 5 inches or rain on Seattle in a 24-hour period. The
day after the storm, the property owner called to say he was shocked he did not have to
use his sump pump.
The program is replicable in any municipality as either a retrofit of existing streets when
improvement is desired, or as new construction. Developing areas certainly have the
most to gain from the natural drainage approach as “doing it right the first time” leads to
long term cost savings. Natural drainage systems are most appropriate in residential
neighborhoods that drain to water bodies needing environmental protection. These new
projects have been met with enthusiastic approval from the Environmental Protection
Agency and the Department of Ecology as a strategy to achieve regulatory compliance
goals.
Local governments are finding they must incorporate new low impact approaches to
development and stormwater management. The Natural Drainage Systems program is a
cost-efficient model of a sustainable future where stormwater systems mimic natural
processes to solve problems inherent to development.
City of Seattle Natural Drainage Systems
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