Surface Water Management

Public Works

LID Demonstration Parking Lot

Project Description
The 1.5 acre site is immediately adjacent to a tributary to Kulshan Creek. The former movie theatre on this site was demolished in 2008. Untreated stormwater discharged directly into the Kulshan Creek system. This parking lot retrofit project incorporated several LID techniques to meet stormwater management criteria including four bioretention cells, porous asphalt and porous concrete surfaces. Traditional asphalt areas drain into the bioretention areas.

Project Goals
Reduce the volume of stormwater runoff and pollutants generated by impervious surfaces and to demonstrate to the public viable alternatives to conventional development /construction methods. Located amidst City and County Services the site provides constant exposure to the public and serves as an LID education and outreach tool .

The LID Demonstration Parking Lot was constructed in 2014 and funded by a Department of Ecology Stormwater Retrofit and LID Grant GL1200598 and several County Departments including the Drainage Utility.

Although not grant-eligible, an LED lighting system was installed and a pedestrian bridge was recycled from a previous County project and serves as access to the new lot.

	This LID project includes a variety of systems that mimic or preserve natural drainage processes to manage stormwater and provide water quality treatment. Porous materials, installation techniques and use of bioretention soils all promote infiltration and eliminate discharge into creeks, ditches and storm drains.
	There are four cells totaling 0.1 acres of infiltrating raingardens. Each were excavated over 3 feet deep to allow for 12” drain rock, 18” bioretention soil mix and composted soil.
	Rain now falls on 0.75 acres of 3 different pervious surfaces, it will filter through the pavement sections and into gravel reservoirs below. Additional water quality treatment is provided by the underlying native soils.
	LID techniques allow for infiltration of all stormwater onsite. Stormwater from our 0.4 acres of impervious asphalt will sheet flow into adjacent bioretention cells for treatment, evapotranspiration, flow attenuation, and storage prior to infiltration.
	Before project: All runoff from the impervious asphalt went directly into the creek tributary untreated.
	After project: Stormwater infiltrates through permeable pavements and rain gardens to be treated before entering groundwater.