SEATTLE—December 12, 2005—If you drive through High Point, you might not notice the feature that is most unusual about this neighborhood.
But if you walk through the redeveloped neighborhood, you may wonder, Why are the planting strips so wide? Why are there sections that look like ditches and why are they landscaped with so many plants? Why do sidewalks look different? And what about those gravel parking areas?
The answer to all of these questions is the same. These features are elements of High Point's natural drainage system. When finished, the natural drainage system will cover all of High Point's 120 acres, making it one of the nation's largest man-made natural drainage systems.
Rainwater from High Point drains into Longfellow Creek, supplying the creek with roughly 10 percent of its water. The old drainage system, after collecting dirt and toxic materials from rooftops, streets, sidewalks, and parking areas, channeled the water into pipes that drained directly into Longfellow Creek. While the water quickly disappeared from the site, the contaminated and wildly fluctuating run-off polluted and eroded the creek, and harmed its ecosystem.
With High Point’s redevelopment, Seattle Housing and the City of Seattle saw an opportunity to demonstrate, on a large, densely populated tract of land, that polluted storm water run-off in urban areas is not inevitable. These agencies wanted to show that with thoughtful design, the trend can be reversed and the environment protected. The system they designed uses the attributes of the soil to ensure that High Point’s runoff, when it reaches Longfellow Creek, is nearly free of contaminants.
The soil is an excellent water purifier. As water enters the ground, plant roots and microorganisms break down and consume most of the harmful chemicals and waste. Once through the soil surface, water moves slowly, sometimes as little as one inch per day. When this water finally reaches the creek below the hillside, it is very clean, and is the same temperature as the creek water.
In a built-out urban environment, it takes a complex plan and a great deal of construction work to let nature act naturally. Natural processes alone may not be sufficient to handle large amounts of rainwater in storms that occur perhaps only once in a hundred years. So, to accommodate these rare events, a system of perforated conveyance piping has been installed deep below the surface. Through these pipes, excess water is channeled into a storm water detention pond at the northeast corner of the site. This pond holds, cleanses, and later slowly releases the water.
The planting strips are landscaped and shaped to serve as the key collection and absorption points for rainwater. Gravel parking areas and porous pavement sidewalks allow rain water from smaller storms to flow through unimpeded. Those sidewalks are actually made from porous pavement – porous because water runs through countless tiny holes directly into the ground below.
In many ways, the new system is expected to function like a forest meadow. Environmental benefits for the creek will be substantial, and residents will benefit as well. The wide planting strips create an unusually pleasant streetscape and help make High Point a beautiful and healthy neighborhood—a neighborhood perfect for a walk.