R520 Flyover Ramp Bike Trail
In 2008, WSDOT completed construction of the flyover ramp where SR520 crosses Redmond Way. A bike trail along this ramp was constructed using permeable asphalt.
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.