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.
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 ‘pre-developed’ 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 high-density 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:
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.