Green LID Alley

Project status-CN

The Green LID Alley project will re-construct two alley’s with permeable pavement.  Replacement of the Front Street and Georgiana Street Alley between N Albert Street and N. Eunice Street, and the 4th Street and 5th Street Alley between Washington Street and Chambers Street.  The replaced alley sections will consist of porous asphalt and pervious concrete placed over a significant permeable base course.  This project uses Low Impact Development (LID) techniques which mimics the natural environment allowing stormwater to soak into the ground as if there were no paved road surface.

What is Permeable Pavement?Permeable Pavement

Permeable pavement (also known as pervious and porous pavement) looks very much like ordinary pavement except it has small openings that allow water to pass through and eventually soak into the ground. Permeable pavement significantly reduces the amount of stormwater runoff compared to conventional pavement. It can be made of concrete, asphalt or pavers, and can be used for roads, sidewalks, trails, parking lots, and other surfaces.

Permeable pavement has been used throughout the United States since the mid-1970s.  Because it is a best management practice and a low impact development technique, cities, counties, and developers in the Puget Sound area increasingly use permeable pavement to manage stormwater. Permeable pavement systems not only reduce stormwater runoff, but also enhance water quality.

How does it work?

Diagram of porous asphalt

Permeable pavement is similar to conventional pavement, but some of the fine material is not added to the mix, resulting in spaces where water can pass through. After water drains through permeable pavement, it is held in a stone recharge bed, and then slowly soaks (infiltrates) into the ground. 

What are the benefits of permeable pavement?

• Decreases the amount of runoff leaving a site

• Improves water quality by filtering pollutants from runoff

• Recharges the groundwater supply

• Reduces the speed of stormwater runoff

• Requires less need for other stormwater infrastructure such as piping, ditches, catch basins, and ponds

• Reduces ice formation during cold weather

• Reduces spray from tires, making for safer driving conditions

How much does it cost?

Permeable pavement may cost a bit more than conventional pavement to install. However, the total infrastructure cost will be less, because fewer stormwater pipes and catch basins will need to be constructed, making permeable pavement a cost-effective way to manage stormwater.  This project is funded in part by a Department of Ecology  grant of $461,250 for construction, project management and inspection.  A 25 percent match is required which will be funded by the Stormwater Utility.