In fall 2011, Redmond extended 185th Ave NE north to connect with Union Hill Road. With funding from a Washington State Department of Ecology grant, the road was outfitted with rain gardens and porous pavement concrete sidewalks. The intention of both of these low impact development techniques was to reduce the amount of additional runoff created when a forested area was converted into a road. The rain gardens were also designed to treat the runoff leaving the road.
The swales are along the east side of the new road, and the porous sidewalks are installed on both sides of the road.
In addition, the grant-funded project includes monitoring the rain gardens to see how well they remove pollution from runoff. The monitoring includes flow weighted composite sampling of both the runoff entering and the runoff leaving the rain gardens. The swales were lined with a plastic membrane to make sure what is collected/tested from underneath the rain garden was the same water that entered the rain garden.
Final Monitoring Report
Monitoring of the bioretention swales concluded in fall 2013. The final report for the project is below. In general, the swales demonstrated a full year of flushing, exporting of some priority pollutants. In the second year the test swale met standards for basic treatment (80% TSS removal) but did not meet standards for enhanced treatment (Cu and Zn removal). The swale exported phosphorous, but that is common for bioretention with under drains.
185th Avenue NE Final Monitoring Report
Mid-Monitoring Project Report
Pollutant Export from Bioretention Soil Mix (2012) - Herrera Environmental Consultants