Controlling what is washed away by the rain is a monumental task facing our government, businesses and the nearly 4 million residents of the Puget Sound region. With a million more people expected by 2020, this looms as the costliest and most complex problem faced by those who want to protect the Sound. If it’s on the land, it will end up in the water. Copper, lead, zinc and petroleum byproducts from motor vehicles. Fertilizers and pesticides from our lawns. Bacteria from unscooped pet waste… whatever goes into the storm drain usually comes out in a wetland, a stream, a lake or directly into the Sound.
Urban development has removed many of the trees and natural surfaces that soaked up rain like a sponge. Without those natural barriers, rain not only has the potential to cause flooding and erosion, but it collects pollution and can overwhelm sewer systems. A natural drainage system slows the flow of runoff by making more water soak into the ground.
Within my Seattle neighborhood (my street, North 107th Street, is pictured), streets that run east-west have steep downhill slopes. Before reengineering, rainfall on these slopes was conveyed quickly into ditches along the roadside; there was little time for the roadside soils and grasses to absorb the water. In order to increase the time and space for runoff to be absorbed by the landscape, city engineers designed a series of giant swale cells down one side of the street. Each cell is divided by a series of weirs with flow control notches, creating a „staircase“ for water flowing down the slope. All swales are filled with modified soils to speed absorption and vegetation to slow runoff. The plants‘ root systems help stabilize soils within the swale cells and absorb runoff, removing pollution. In addition, the bacteria within healthy soils help break down carbon-based pollutants like motor oil.
By planting dense groups of fine-stemmed grasses, sedges and rushes and clusters of native shrubs, Seattle planners have designed a system to mimic an undeveloped watershed’s capacity for containing rainfall and cleaning runoff.