Stormwater in a pilot low impact development (LID) project in Pierce County near Hylebos Creek is successfully being infiltrated and filtered of contaminants despite some problems with construction of the LID elements and poor soils at the site.
Curtis Hinman from the Washington State University Puyallup Research and Extension Center explained how an 8 acre development with 35 homes was constructed in 2002 as a pilot project to evaluate the potential of low impact development techniques to mimic native hydrologic function and protect the nearby Hylebos stream. The project was built using bioretention swales or raingardens along all the roadways, previous concrete along the road shoulders, compost amended soils around the homes, and a compost amended sloped dispersion area for any remaining stormwater runoff.
Curtis explained that there were problems with the project because the builder did not have much guidance on how to build the raingardens, the largest raingarden ended up being lined because of sewer concerns, and the site had poor soils for infiltration. Despite these challenges when post development stormwater was evaluated the results showed that the site was exceeding the goals for stormwater reduction and infiltration. The LID features in the development combined with the compost amended slope removed 96 percent of the stormwater on site. In addition a water quality analysis showed that metal contaminants in the water were at non detectable levels and were much less than metal levels in stormwater runoff in typical residential developments.