Long ago, before Seattle was a thriving metropolis, stormwater runoff wasn't much of an issue. It was all trapped by trees and absorbed into the ground, so there was very little runoff to concern anyone. As the city grew, and more streets and rooftops came into being, stormwater was sent towards creeks, causing flooding. In the late 1990s, the city developed Street Edge Alternatives Streets (SEA), which is an alternative street design that uses grading, soil science, plant selection and layout combined with traditional drainage infrastructure to function more like an undeveloped ecosystem. It also provides neighborhoods with sidewalks and traffic calming. By reducing impervious surfaces to 11 percent less than a traditional street, providing surface detention in swales, and adding new trees and shrubs, SEA Streets is designed to provide drainage that is more like pre-development conditions. As a pilot design, the street will be monitored over time to determine its success. In 1998 Seattle Public Utilities began exploring street drainage alternatives. Their objectives were to decrease runoff peak flow and volume; minimize impervious areas; reduce vehicle speeds; and change the paradigm that curb gutter/sidewalks are necessary. "Less impervious surface makes it so there is less runoff to begin with," said Senior Landscape Architect with Seattle Transportation, Shane DeWald. "The water coming off paved surfaces allows water to be held and to be allowed to move slowly down stream from one end to the other." The 'SEA Streets' project, designated for the Pipers Creek Watershed in northwest Seattle, was initiated as part of the Urban Creeks Legacy program, they also wanted the pilot project to reduce the impact of stormwater on local creeks. Thirty possible sites were selected in the Pipers' Creek Watershed. Six blocks submitted petitions to be a part of SEA Streets. Engineering criteria and resident interest were compared for those six sites, resulting in the selection of the 11700 block of 2nd Avenue NW. Landscape Elements DeWald was involved more during the design of this project than your average streetscape design. One reason was because the sidewalk design is not your typical design. It is a curvilinear design that helps calm traffic. Also large-scale trees were retained and vegetation was relocated to meet homeowners needs and project goals. "A typical streetscape has uniform plantings and generic treatment along the corridor for continuity," DeWald said. "On this project, because we had a strong objective to decrease the amount of impervious surface, we had a larger area to landscape with far greater opportunities. We had to make a strong connection between street improvements and the adjacent property." The landscape design complements the drainage system function and focuses on native and salmon-friendly plantings. The system is unique in its use of grading, soil engineering, plant selection and layout as components that function much like they do a natural ecosystem. According to DeWald, more than 1,400 shrubs with 16 different species, both evergreen and deciduous were planted. She said about 75 percent of the plantings were natives in order to have a nice blend between things that would be habitat oriented versus things that might blend more easily with residential landscapes. Over 1,800 groundcover plants with five different species were planted. Ten species of perennials and about 750 ferns along with 1,500 wetlands plants were included. There were 114 trees placed as well. "The plant schedule looks like an accounting sheet with all these columns and rows," DeWald said. "I laid it out so that each property is identified for a certain quantity of plant rather than actually trying to lay it out on a plan sheet to show each plant. The only thing identified on the plans is where the trees go, other than that the other plant material was field located to allow for the best opportunity to allow them the best watering and sun exposure." The plant survival has been great. According to DeWald only about five percent of the plants have been lost. She attributes that to the work of the Seattle Transportation crew and the great soil preparation by the contractor involved who was "very conscientious and did great prep work for us." Street Improvements The curvilinear roadway was one of the more prominent features of the project. The project team was focused on reducing the paved surfaces where possible. The roadway is wide enough for two standard size cars to pass each other slowly. To address larger vehicles or emergency vehicles that need to pass, the edge of the roadway has no curb and two feet of grass shoulder on each side that is designed to accommodate vehicle loading. The two foot concrete border defines a stream-like alignment, serving both a safety and functional purpose. It provides tight control of final paving elevations, which is necessary for the drainage system, and visually defines the roadway edge. Parking concerns were addressed by having a majority of the spaces at 45 degree angles and some at 90 degrees, while some were designated as parallel spaces. Residents were involved in helping determine how much parking was needed. Most properties have alley access with parking. The sidewalk also follows the streets curvilinear design. It is along only one side of the street to reduce the impervious area. "Residents really see the sidewalk as one of the nicest parts of the project," DeWald said. "By having the sidewalk curvilinear in nature it blurs the line between sidewalk and right of way. It makes yards visually more sizeable. "The curvilinear street is effective with traffic calming and many people in the early morning and evening just walk the street, not using the sidewalk because it is so attractive to pedestrians." Because the street doesn't run in a straight line and because it is narrower, full-size canopy trees were able to be planted on both sides of the street even though there are power lines on one side. "In Seattle, we commonly have overhead wire limitations on one side of a residential street," DeWald said. "We always end up with this lopsided look with small trees or inappropriately planted trees that have been cut short on one side and nice big canopy trees on the other. In this case we were able to keep both sides of the street the same even though we didn't have the capacity to underground the wires. It's very nicely balanced." Drainage Improvements The drainage improvements combine contoured swales with traditional drainage infrastructure to regulate the flow and discharge of storm water. A unique combination of hydraulic engineering, soil science and botany created a more natural system. "We specifically included a strip of grass along the edge of the street giving a nice uniform level spreader function," DeWald said. "Water coming off the street hits the grass and is slowed down preventing the development of real erosion along the edge of the slopes of the roads. "By having grass as a first catch, it prevents it from happening. In terms of water quality, one of the biggest elements on the project was the fact that we had so much additional space to accommodate trees, which are particularly effective to attenuate stormwater and catch runoff before it becomes runoff." The root system of the trees of the trees keeps the soil amenable to infiltration over time so the soil was designed for good plant growth characteristics and to begin with. Said DeWald: "I have every confidence that the plants will keep soil good for the long term." One of the primary drainage goals was to use surface retention or detention to reduce the two-year, 24-hour storm event peak runoff rate and volume to pre-developed conditions. The method used for achieving this goal was to maximize the stormwater time of concentration and the sites retention volume, without compromising the homeowners access and parking needs on the street. Using a long flow path length and high surface roughness along the flow path, it increased the basin's time of concentration. All areas within the right-of-way without hard surfaces were used toward detention. There are three flow control structures on this project. A catch basin regulates the flow of storm water for all six swales. Culverts hydraulically link them so that during larger storm events, water will be detained in the swales on the east and west side of the street. This system provides the storage capacity for up to a two year- 24 hour storm event. The swales work by utilizing an infiltration trench. Flow is regulated through the gravel material and into an eight inch slotted pipe. Like all three flow control structures, there is an overflow bypass that will allow larger than two year storm events to bypass the system when full. The "Sea" Experience This project cost $850,000 because it included a more extensive design and communications budget due to the out of the box nature and the need to work closely with the residents on the design. It is estimated that future projects would be expected to compete with traditional street improvements. One thing that DeWald found from this project was that working closely with residents was key. "I would say I initially looked at it as being pretty complicated," she said. "The more involved I got in it the more I liked it. Early in my career I did custom residential work so this was a step back in time for me. "If I was responsible for dozens of projects like this, it could be overwhelming." The biggest challenge for DeWald was this project being the first of its kind. She said that people didn't know what to expect and she had to keep people from losing faith in the project. "Having completed the project, it will serve as such an important tool to reduce the hand holding on future projects because people can at least get an idea of where we're headed," DeWald said. "I thought the success of the project was being able to adapt to the conditions. I just assured everyone that they would be happy when we were done. "I'm not big on high tech representations for what things will look like for something as experimental as this." So will there be other SEA Streets? The staff at Seattle Public Utilities is currently developing a second pilot project site to address elements that were not feasible to test the SEA Streets site. The issue of porous pavements for streets, paths and parking are being discussed for the next project. "There have been other projects that were forerunners to this that we looked at such as the idea of the engineered soil in the planting strip as a means to process the runoff from sidewalks," DeWald said. "Those projects involved more traditional strips and uniformity. Projects with certain features now have certain design concepts of the SEA Street." John Arnesen, Project Manager for SEA Streets contributed to this article.