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SR520
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.
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.
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