text and photo by Bo Normander

Since the 1990s the city of Seattle – a major coastal seaport in the North West of the U.S. – has invested heavily in cleaning up and reducing severe pollution of the water environment. Some of the ideas to help prevent water pollution can be of great inspiration to cities in Europe.

Most of the city of Seattle has a unified sewer system. Under heavy rain conditions the overflow of rain mixed with sewage runs into rivers and other water bodies as the sewer system cannot take the load. Since the 1950s this has caused great environmental degradation to the waters and the area’s wild salmon has become an endangered species.

As a consequence the city of Seattle adopted a reinforced Storm Water Management about 10 years ago. The aim is to improve the environmental condition of Puget Sound and the rivers in the city, and as the most important indicator the wild salmon should return to proliferate.

The solution that has largely been worked with over the past 10 years is to remove as much rainwater from the sewer system as possible. This is done by imitating the forest’s ability to handle rain water through absorption, leaching, and evaporation. A natural forest absorbs about 99% of the rain water, and only 1% runs off to creeks and rivers. In an average city with many paved and built-up areas, 30 to 40% of the rain water runs off.

An example of a bio swale - a green roadside depression that absorbs rainwater and looks pretty - in the High Point development of Seattle.
(click for larger picture)

Some of the components used in Seattle include rain gardens, underground rainwater drainage systems/fascines, bio swales, green roofs and permeable pavements. It is also an important goal to plant trees to restore 'tree canopy', both to increase evaporation and reduce summer temperatures.

High Point is a low-building housing area in Seattle that has been redeveloped to create mixed-income homes with different types of ownership as well as integrating an ambitious plan for rain water management. All green areas have been designed so they can absorb rainwater. This includes rain gardens, which are planted depressions.

Streets slope to one side and direct water into planted ‘bio swales’ in the roadside that can absorb the water. Under more severe rainstorms a piping system under the bio swales holds water and slowly leads it into a large pond. The swales collect, absorb and filter rainwater from streets and houses into the ground before going into the city drain. At the same time the planted swales contribute to a nice and green neighbourhood.

In High Point porous pavements on streets and sidewalks allow water to pass through into the ground. The porous streets are slightly more expensive than "normal" streets but help reduce the amount of water running into the drainage system. Also the conservation of large and old trees and planting of new ones provide immediate stormwater protection as trees absorb large quantities of water both above and under ground.

Some streets in the High Point district are made with a special permeable pavement that allows water to absorb into the ground and hence reducing run off.

In the newly developed Thornton Creek housing area of Seattle, a landscaped wetland - or raingarden - helps to clean and absorb rainwater under heavy rain falls. Previously the Thornton Creek was running underground in man-made pipes but now parts of the creek has been exposed again and reengineered into an attractive, green raingarden. Experts have selected specific plant species to promote a natural water purification, as well as to create a natural balance and thus minimize maintenance of the raingarden. So what is now looking like a nice park is in reality a high-tech water treatment facility that takes up large amounts of rainwater and rinses it before it is lead on through Thornton Creek and into Lake Washington.


Raingarden in Thornton Creek, Seattle.
(click for larger picture)