Traditionally, most U.S. cities have rarely considered changing street sizes and shapes, primarily due to concerns over parking space. However, wide streets create large amounts of impervious surfaces, and thereby reduce the ability of soils to absorb storm water. Speeding across blacktop, sidewalks, and through underground pipes, water that has picked up pesticides, animal waste, and motor oil empties into nearby streams, creeks, and basins. The results are dead fish, disrupted food chains, and polluted water.
Residents of the City of Seattle decided that it was time to address the impact of human activity on water quality. The City's Natural Drainage Systems program uses native plants and forest species to convert excessively wide streets to "walk[s] in the park." Increasing the ability of the ground to drain water slowly, and manageably, Natural Drainage Systems provides inexpensive combinations of plants and narrower roads to stop the overwhelming pollution of nearby water.
In the past decade, increased resident support for cleaner waterways led to an evolution of responses by Seattle's Department of Public Utilities. In 1998, through the Urban Legacy Program, millions of dollars were used to clean creek perimeters, channels, and salmon spawning areas. Creek clean up, however, did not stop storm water from running off of roofs, streets and building lots. Seattle recognized the need for proactive measures as opposed to simple damage maintenance. The City devised Natural Drainage Systems, which now serves as the current "key design approach" for 20 percent of the City.
Natural Drainage Systems recreates forest ground and surfaces that mimic their function. Roadside swales--ground depressions planted with sponge-like, wetland plants--increase absorption and form natural water barriers. Simultaneously, the construction of narrow and more curving streets has increased porous surfaces and reduced conventional street expenses. Whereas street maintenance costs increase over time as the result of aging materials, pipes, and drains, Natural Drainage Systems actually becomes more effective over time, as plants and trees mature.
Areas beyond these neighborhoods benefit from natural drainage technology. Results from Natural Drainage Systems in Seattle include a 98 percent runoff reduction on 2.3-acres of residential development. A drainage project in the 110th Cascade Neighborhood showed that even a change from conventional drains to a gradual stair-like, slope of plants and ground levels, could dramatically slow the flow on 26 acres of storm water, along four blocks. With narrower, curving streets that promote slower car speeds, pedestrian safety has increased. Opportunities for residential involvement in planning and maintenance have fostered stewardship and common environmental goals.
At the national and even international levels, Natural Drainage Systems holds the potential to diminish water-resistant surfaces and save millions of dollars. In Seattle, the program costs 10 to 20 percent less than conventional asphalt, sidewalk, and ditch utilization. For cities that stretch for miles, the economic savings of this program would provide financial room for adaptations to the needs of a region, and for necessary collaborations among departments and organizations. Setting new street-side standards, Natural Drainage Systems employs an inexpensive idea in urban planning, relying not on overly voluminous drainage, but on designing long-lasting ecosystems which, by nature, function best to drain and sustain.