Bear Creek Trail
In 2006, the City constructed a new trail along Bear Creek between Redmond Way and Avondale Way NE, behind the Bear Creek shopping center.
The south end of this trail, adjacent to Redmond Way is made of permeable asphalt. The north end of the trail is made of traditional impervious asphalt.
In 2011, a portion of the "traditional impervious asphalt" was replaced with permeable asphalt as part of construction of the new Stormwater Treatment Wetland built adjacent to the trail. If you visit this site, you can see the transition from 2006 permeable asphalt to 2011 permeable asphalt and then to 2006 traditional impervious asphalt.
This site is subject to periodic flooding from Bear Creek, but the pavement continues to infiltrate stormwater. Moss growth is observed on all three sections of pavement, to differing degrees.
What is Permeable Asphalt?
Permeable asphalt offers a powerful tool in the toolbox for stormwater management. In the natural environment, rainfall sinks into soil, filters through it, and eventually finds its way to streams, ponds, lakes, and underground aquifers. The built environment, by way of contrast, seals the surface. Rainwater and snowmelt become runoff which may contribute to flooding. Contaminants are washed from surfaces directly into waterways without undergoing the filtration that nature intended.
For these reasons, managing stormwater is a significant issue in land use planning and development. Stormwater management tools can serve to mitigate the impact of the built environment on natural hydrology. Unfortunately, however, they also can lead to unsound solutions such as cutting down stands of trees in order to build detention ponds.
Permeable asphalt pavements allow for land development plans that are more thoughtful, harmonious with natural processes, and sustainable. They conserve water, reduce runoff, promote infiltration which cleanses stormwater, replenish aquifers, and protect streams.
A typical permeable pavement has an open-graded surface over an underlying stone recharge bed. The water drains through the permeable asphalt and into the stone bed, then, slowly, infiltrates into the soil. If contaminants were on the surface at the time of the storm, they are swept along with the rainfall through the stone bed. From there they infiltrate into the sub-base so that they are subjected to natural processes that cleanse water.