Seattle’s Natural Drainage System (NDS) consists of stormwater management projects that use low-impact development (LID) strategies to meet multiple goals within street rights of way (ROWs), which account for 25% of Seattle’s total land surface. The projects work by infiltrating stormwater runoff, slowing it temporarily or lessening its volume, filtering, or removing pollutants through the use of soils and native plants, replacing impervious surfaces with pervious, and adding native vegetation.
NDS projects are “green,” using natural elements to mimic the ways of nature that have been lost to urbanization. Its features include open spaces, vegetated swales, stormwater cascades, and small wetlands ponds. Along with the plants and trees, there must be deep, healthy amended soils to support their growth.
While NDS projects may be better for the environment than traditional stormwater management structures, in the long term, they are usually as cost effective or even cheaper than traditional hard infrastructure as well. They also offer the advantage of being more attractive to the public than utilitarian infrastructure. One way in which they achieve this is by providing wildlife habitats.
A Seattle city plan developed the initial concept for the Street Edge Alternatives (SEA) Streets, Seattle’s first NDS project. Darla Inglis of the Low Impact Development Center’s Central Coast office in San Luis Obispo, CA, and Tracy Tackett, P.E., the LID program manager for Seattle Public Utilities (SPU), added to the concept strategies to reduce stormwater peaks and volumes that were being discharged to salmon-bearing creeks.
A Redeveloped Housing Project
Seattle has five NDS projects. The largest and most elaborate is called High Point after its location, which has a rich history. In the 1940s, thousands of workers moved to Seattle to work for Boeing or in the shipyards in support of the war effort. Housing was needed for them and their families, and 700 duplexes were quickly constructed. The large temporary housing project, called High Point, was on the western side of Seattle. From there, residents could enjoy fine views of the downtown skyline and Mount Rainier.
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Photo: SvR Design Company
A porous concrete street and vegetated
swale |
After the war, when defense plant production slowed, occupancy of the duplexes dropped. In 1952, High Point was converted to a public housing project. Eventually it became an obsolete, isolated neighborhood with rundown buildings and many vacancies.
Now renovated, High Point is a key to the renewal of West Seattle. With 129 acres, it is the single largest redevelopment in the city since World War II. It’s also the largest sustainable mixed-use, mixed-income urban neighborhood in the US. Construction of the first phase began in 2003 and the second phase was finished in the spring of 2009.
High Point is the result of a collaborative effort of the Seattle Housing Authority (SHA) and SPU. The US Department of Housing and Urban Development (HUD) and other city of Seattle departments were also involved.
The project’s success demanded a high level of cooperation from all parties involved. Integrating an innovative drainage system into a predominantly townhouse-style housing development had no precedent. Discussions and planning took approximately two years. The agencies agreed to share funding sources to meet the total $550 million cost. Some federal funds—Hope VI funds, intended to redevelop urban housing projects—were used.
“High Point took the complexity of managing multiple goals in the limited right-of-way space to a much higher level than we encountered on the lower-density NDS projects,” explains Tackett. “There were more competing needs—such as underground power, as well as a desire for playable space in the planting strips—in the High Point project, and a narrower right-of-way width. Accommodating all the demands while achieving the stormwater goal of infiltrating the water-quality design event was the most
challenging.”
As an example of the effort required by all parties involved, Tackett recalls “the meeting of approximately 15 folks to decide if a 5-inch curb instead of a 6-inch curb would be permitted.”
Peg Staeheli, ASLA, LEED AP, principal with SvR Design Company, was the landscape architect for High Point. She agrees on takeaway lessons from the project: “It is a study of inches—and true integration of professional disciplines with the owner [SHA] and the Public agency [the city of Seattle]. The design team kept the focus on the place for the people. It has to meet that criterion first; it is not all about stormwater. We wanted great streets for people and little nooks to visit and play. We used the spaces for multiple functions, and it paid off.”
For Staeheli, the most difficult part of the project was a lack of precedent. “In 2001, there were not any examples at this scale and no rules. This was before the Puget Sound Partnership came out with guidelines.” She also notes that the construction phase was tough, with a lot of lessons learned.
Staeheli and other people involved took input from people who had worked on other projects around the country, “especially the folks in Prince Georges County, MD,” she says. But, she adds, “The implementation ideas had not been applied at a scale such as High Point with a mix of ownership.”
Details that were worked out during this project can be used as guidance for future projects. Click here to learn more.
City planners, SHA staff members, the design team, and technical experts used the design charette process to negotiate the use of above- and below-ground streetscape. Balancing community needs with ecological wishes and technical requirements meant that everyone involved had to be willing to consider multiple approaches and often think outside the box.
The construction, engineering, and technical people had to understand High Point’s history and the socioeconomic factors involved in making a diverse neighborhood cohesive. The housing and neighborhood experts had to understand technical details of LID stormwater practices and how everyday living experiences would impact the environment.
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Photo: SvR Design Company
A High Point pond during a 100-year storm in December 2007 |
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Photo: SvR Design Company
The pond one week after the storm |
Tom Phillips, director of SHA, served as project manager for the renovation. “Our goal at High Point was to knit the former post-war housing project back into the fabric of West Seattle, creating a mixed-income, mixed-use, diverse neighborhood typical of Seattle’s thriving lifestyle,” he says.
More than 90% of the old-growth cedar and fir lumber was recycled into new buildings. More than 100 trees, valued at $1.5 million, were saved, and 2,500 new trees were added.
Energy conservation in the new buildings exceeds code requirements by 30 to 50%. All homes meet local Built Green three-star standards. Thirty-five of the homes, called Breathe Easy homes, were specially designed for children with asthma; they have superefficient ventilation systems and other features to reduce environmental asthma triggers.
In this diverse urban neighborhood, 50% of the 1,600 homes are reserved for low-income residents. Houses are clustered, 15 to an acre, which allowed 20 acres to be set aside for parks.
Phillips explains that the project’s aim was “creating a walkable, front-porch experience that would foster a sense of community and neighborliness among the people who live here.”
SHA collaborated with the city of Seattle to achieve approval of the subdivision plan for the NDS that includes a network of swales, ponds, and multifunctional open space throughout the 34 blocks of rights of way. Because the city would be revising its existing stormwater code in 2004, an ordinance was passed in 2003 that allowed High Point to begin work on its stormwater LID features in advance of the new code.
So that the city maintains control of stormwater management, drainage restrictions were included in the subdivision’s recorded documents and future homeowner association covenants. The city decided to enter the permit data into its geographical information system (GIS) for long-term tracking.
To achieve the drainage goals, each block in High Point was assigned an “allowable percent impervious surface coverage.” When each individual project was finished, the contractor filled out an application for a permit certifying that the project met the predetermined allowable amount of impervious surface and did not exceed it. When the permit was granted, the amount of impervious surface allowed for the given block was entered into the city’s GIS. That figure will keep any future redevelopment from exceeding the previously permitted impervious surface thresholds.
Following through to be sure that High Point was built as the sustainable community it was designed to be required five levels of commitment. Staeheli and Richard L. Johnson Sr., civil engineering specialist for the city, explain these levels in their paper on LID practices in Seattle. The first level was adding drainage thresholds in the plat. The second was developing a “drainage covenant” for the plat. The third level was involved developing technical standards for compliance with the drainage covenant.
Looking to the future, the fourth level was “the development of a covenant for maintenance of natural drainage landscape, open space, and ROW.” The fifth level is the Memorandum of Agreement between the city and SHA regarding funding and maintenance of the drainage system.
High Point’s 34 blocks constitute the largest natural drainage system in the US. The system was designed to protect the Longfellow Creek watershed, one of four major watersheds in Seattle. High Point covers 10% of the watershed, which is Seattle’s most productive stream for the spawning of coho salmon in the fall.
Part of Seattle’s Urban Creeks program, Longfellow Creek flows through southwest Seattle, draining a 2,685-acre watershed into the Duwamish River and, eventually, into Puget Sound. It collects runoff from an area of approximately 1,730 acres. Residents of earlier years remember when the creek teemed with fish. Initially, High Point’s NDS will keep the fish count from dropping, despite increasing urbanization of the area, but eventually the salmon should increase in number.
High Point’s system to hold and treat runoff is a series of small wetlands, overflow ponds, and swales, like a meadow. The streets of the neighborhood—some only 25 feet wide—tilt slightly toward one side to draw runoff from roofs, streets, and parking lots. Rain gardens help keep stormwater out of the streets.
Shallow swales totaling 22,000 linear feet, or about four miles, parallel the streets. They contain drought-tolerant native plants, shrubs, trees, and grasses. Each swale is designed to treat the runoff from the street and housing of the adjacent block. This localized focus—treating stormwater as close to its source as possible—is demonstrated throughout the project and is a characteristic feature of an NDS. It cleans, cools, and infiltrates runoff better than the traditional piped and centrally managed system.
Staeheli and Johnson’s report explains that the overall system will “provide water-quality treatment for the 6-month storm and attenuate the two-year, 24-hour storm to predeveloped pasture conditions.”
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Photo: SvR Design Company
A roadside conveyance swale |
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Photo: SvR Design Company
Vegetated swale and porous concrete
sidewalk |
Soil in the swales has been enriched with about 3 feet of compost, making it similar to a forest floor. This was necessary to increase the rate of infiltration and water-holding capacity.
A preliminary geotechnical soils analysis showed that the site’s soil were primarily silty fines to fine sandy silts. These types of soils rate only as “poor” in terms of potential infiltration and storage.
The native plants help filter out toxins before water reaches the 303(d)-listed Longfellow Creek. Runoff is also close in temperature to that of the creek, which is important for the salmon.
Another component of the NDS at High Point is porous concrete for city streets and 2 miles of pervious sidewalks (half of the total sidewalk area) next to many of the swales. Pervious concrete is also used for parking and access on many of the private properties.
32nd Avenue SW between Juneau and SW Raymond streets is Washington’s first porous concrete city street. Composed of a mixture of Portland cement, gravel, and water, it has a “rice cake” appearance. The porous pavement also helps shrubs and trees by allowing air and water to reach their root structures.
Overall, despite the urban density of the area, High Point will have only 60% impervious area. The NDS will handle runoff from most storms, but for the heaviest ones, a traditional concrete infrastructure was installed as backup. Also part of the plan to deal with extremely heavy runoff is a large detention pond.
As part of its Public Arts Program, SPU wanted to include art to emphasize the concepts of sustainable living and natural stormwater management at High Point. The utility commissioned artist Bruce Myers to create several works of art for the neighborhood. Myers added pavement scored with designs of raindrops, raindrop designs on lightpoles, storm grates with maple leaf relief and raindrop designs, and splash blocks embossed with salmon swimming upstream and other creek-inspired motifs. A sculpture of steel-plated dorsal fins of orcas by the artist is located near the main stormwater detention pond.
SPU will use a 20-year, 1.5% Clean Water State Revolving Fund loan of $2,715,000 to pay for High Point’s NDS. It seems money well invested, for SPU accounting projections indicate that NDS systems are at least 25% less expensive than traditional stormwater systems due to lower construction and maintenance costs.
The first phase of High Point’s NDS was finished in 2005 and the second in 2008. SPU began monitoring the first phase of the system in January 2007. Data collected will be released in 2010.
The Urban Land Institute selected High Point as one of five outstanding developments worldwide, a 2007 Global Award for Excellence winner. The project was one of only eight in the US to be given an American Institute of Architects’ Show You’re Green award. The 2007 Pacific Coast Builders’ Conference awarded High Point its Gold Nugget for the Best Master Planned Community of the Year, and the American Society of Civil Engineers, Region 8, chose High Point for its 2006 Outstanding Major Project Award for Site Development.
Staeheli admits to being surprised by the awards and attention that High Point has drawn. “The Seattle Housing Authority and the design team did not set out with the goal to win awards—the vision was a great place where people would want to live and grow a community,” she says. Her office alone has given more than 100 tours of High Point to groups ranging from Boy Scout troops, to engineering professionals from dense urban areas in Japan.
For Staeheli, the best part of High Point is observing people as they walk around the neighborhood, obviously pleased with their surroundings. “The project continues to demonstrate a great collaboration from planning through permitting, construction, and ongoing maintenance,” she says.
Other NDS Projects
Seattle’s first NDS project, SEA Streets, is on 2nd Avenue NW between NW 117 and 120th streets. Designed to help protect the Pipers Creek watershed in northwest Seattle, it was completed in the spring of 2001.
Impervious surface was reduced to 11% less than a traditional street. Vegetated swales were added to detain and filter runoff. More than 100 evergreen trees and 1,100 shrubs were planted.
A distinctive feature of SEA Streets is the curvilinear roadway, which measures just 14 feet wide (18 feet wide at the intersections). The narrower roadway naturally slows traffic and is wide enough for two standard-size cars to pass each other slowly. To accommodate commercial and service trucks and emergency vehicles, the edge of the roadway has no curb. The 2 feet of grass shoulder on each side is reinforced to withstand the load of larger vehicles.
Angle and parallel parking stalls are grouped between swales and driveways. Most properties have alley access with parking. The sidewalk, which is located along only one side of the street, also follows the curvilinear design.
The swales and surrounding areas are graded and planted with native wetland and upland plants. Granite boulders and various sizes of washed river rock were added for both function and beauty. Native soils were mixed with organic compost to reduce the use of water and fertilizer. Native clay, 6 inches deep, was used to line the swales, helping to ensure that plants could survive during dry summer months. The clay is less likely than a fabric liner to dry out.
The primary drainage goal of SEA Streets was to use surface detention to reduce peak runoff rate and volume from a two-year, 24-hour storm event (1.68 inches) to that of predevelopment conditions. To meet that goal, stormwater was detained for the entire contributing drainage area (street right of way and properties along the east side, totaling 2.3 acres), not just for new or redone impervious area.
Early designs called for using native soils to allow infiltration throughout the length of the block. That was later found to be unsuitable because some homes had a groundwater intrusion problem. So that basements of these residences would not be flooded, some of the swales were lined with the aforementioned 6 inches of native clay.
Sizing the swales to detain the predevelopment two-year, 24-hour storm event, while not allowing infiltration, required the use of very small outlet orifices. For proper maintenance, the engineers wanted outlet orifices to be larger than one-half inch. To reduce the likelihood that these small orifices would clog, a sump was included in the flow-control structure. Working with the site’s flat topography (1% longitudinal slope) and the minimum orifice size restraint, the engineers chose to use only three control facilities on the block.
The swales were hydraulically connected into three groups, with each group controlled by a flow-control structure. Detention volume achieved by swales is 2,500 cubic feet, which is 37% more volume than would have been required by the city’s drainage ordinance.
Another goal was conveying the site’s 25-year, 24-hour storm event, which is the design requirement for all city of Seattle drainage systems. Using a long flow path and high surface roughness along that path increased the detention basin’s time of concentration.
Monitoring for two years showed that SEA Streets’ green features reduced total volume of stormwater runoff by 99%. The project cost $850,000. This amount included funds for an extensive public education program, as few of the residents were familiar with this type of project.
Ten blocks south of SEA Streets, in the Pipers Creek watershed, lies the 110th Street Cascade project. Completed in 2003, the project manages stormwater from a multiblock section of unimproved residential streets between two main traffic arterials, Greenwood Avenue N and 3rd Avenue NW. 110th Street receives runoff from approximately 21 acres of the upper watershed. Unlike the SEA Streets project, which manages stormwater only from the roadway and adjacent properties, the 110th Street project also has to manage runoff from the surrounding area.
This NDS project was needed because the previous drainage system of asphalt-lined ditches and culverts moved too much water downstream too quickly. The volume of runoff was eroding the banks of Pipers Creek.
Because NW 110th Street slopes significantly downhill to the west, the project created extra challenges for its designers. The solution was a series of stair-stepped natural pools that slow runoff, reduce flooding, and allow infiltration, which traps pollutants before they reach the creek.
Most of the homes in this area face streets that run north to south. Only two or three homes face NW 110th Street, which means that fewer parking spaces and driveways interfere with the large swale. But fewer homes has also meant fewer residents to help maintain the native plants on the sidewalk (south) side of the street, opposite the swales on the north side.
The native plants used in the 110th Street Cascade project are those that require the least care. Planted along the edges of each swale are evergreen trees, sword ferns, salal, and willow. Grasses, sedges, and rushes have filled in the bottoms of the swales.
It’s harder for plants to grow on steep slopes, so the plants in this project were set into “green walls.” These clay soil bricks, wrapped in UV-resistant fabric, help the young plants get established. Plants are watered during extremely dry summer months. Organic mulch is applied once a year to protect the plants’ roots.
Large, round sedimentation catch basins are located at each end of the swales. These basins store excess runoff, releasing water into the system at a slower rate. Sediments have time to settle to the bottoms of the basins, which are cleaned once a year.
The project’s designers also installed a weir wall to separate each swale cell into two levels. Runoff that accumulates on the upstream side of each weir wall eventually flows downstream through a notch in the top of the wall. This feature creatures a steady, slow rate of runoff on the very steep slopes.
Immediately south of the 110th Street Cascade project is the Broadview Green Grid project. Details of this third NDS project, which began in 2003 and was completed in 2004, are included in the article “Urban Retrofitting” in the March/April 2009 issue of Stormwater, Seattle’s fifth NDS project is Pinehurst Green Grid, which covers 12 city blocks in the northeast section of the city. The project was created to improve the quality of runoff to Thornton Creek and thus improve the habitat for salmon.
Although 15 parks border Thornton Creek, it drains 7,322 acres and runs through some of the most developed areas in the Puget Sound region. Northgate Mall, America’s oldest shopping mall, is in the area.
Drainage reaches Thornton Creek via the Kramer ditch, which receives runoff from about 135 acres. The Pinehurst subcatchment is 83 acres, 49 acres of which directly contribute to the project area.
Pinehurst Green Grid was finished in the fall of 2006. Landscaping was completed in April 2007. The project manages runoff for the full area drainage basin up to the two-year storm event (1.68 inches in 24 hours).
These five NDS projects have been well received by the environmentally conscious residents of Seattle. Another NDS project in the Venema Creek watershed is planned.
“For the coming years, the NDS program will focus on the use of green stormwater infrastructure for combined sewer overflow control,” says Tackett. For people new to NDS concepts, she suggests reviewing the information at www.werf.org/livablecommunities.
“It’s critical for any agency considering surface green stormwater infrastructure facilities to construct a couple of fairly large-scale pilot projects,” she says. “There’s nothing like a real project’s plan sets being circulated to all the various parties with interest in the right of way to really get all the issues on the table.”