Bioswales: How They Help Us

The urban landscape of today’s cities and towns have increasingly become more densely populated and as a result the open areas have largely disappeared to be covered in greater expanses of concrete and asphalt. These miles of road, sidewalk, and parking lot create what is called impervious surfaces.

Tanner Springs Park, Portland, OR
Impervious surfaces are defined as typically man-made surfaces that prevent water from seeping into the ground to recharge local water aquifers. During heavy rains and after prolonged periods of snow, these surfaces allow the excess water to wash pollutants such as oil, grease, pesticides, and fertilizers directly into storm drains which then deposit these pollutants into rivers and streams.

Rooftops of houses and other buildings act as impervious surfaces as well. Consider that 1 inch of rain that falls over 1 square foot of impervious surface creates 0.6 gallons of water. Now consider that a roof of 2,100 square feet would then produce 2,100 x 0.6 = 1,260 gallons of water per inch of rain. Then multiply this number by thousands of buildings in an average city to give an idea of the volume of water making its way into rivers and streams that otherwise would enter local aquifers.

Now enter the bioswale. The typical bioswale is constructed with an engineered combination of different soils, man-made structures to encourage seepage, and vegetation such as trees, shrubs, grasses, and perennials that together act as a sponge for excess water runoff. Additional benefits of bioswales include:

  • Acting as a filter for pollutants that would otherwise make their way into streams and rivers.
  • Reducing the levels of sedimentation of local rivers and streams by keeping soil in its original location.
  • Reducing the strain on man-made storm water systems by directing excess water down into local aquifers.
  • Creating an aesthetically pleasing environment for the communities adjacent to such sites.
  • De-emphasizing the street as a place for cars to one for both cars and pedestrians.

SW Lincoln St and Pacific Hwy W, Portland, OR
Clearly the benefits of creating bioswales outweigh the costs and are preferable to installing bare curbs on our city streets both for the health of our rivers and streams but also for the health of our neighborhoods.