What is green stormwater infrastructure?

Video courtesy of the Sightline Institute.

In Seattle, heavy rains in the University Combined Sewer Overflow Basin area can cause sewers to overflow into Portage Bay approximately six times per year. King County plans to install natural drainage facilities, called green stormwater infrastructure, to help control these overflows.

Controlling stormwater

As in many cities around the country, the older parts of King County’s sewer system use the same pipes to carry both sewage and stormwater to a treatment plant. During storms, sewer pipes that carry stormwater can fill up with the runoff from roofs, driveways, and streets and be overwhelmed. The system is designed to allow these overflows (a combination of about 90 percent stormwater and 10 percent sewage) into local water bodies. We call this a combined sewer overflow, or CSO. Even though these CSOs help prevent backups into homes and businesses, they pose a risk to public health and the environment.

What is green stormwater infrastructure?

Green stormwater infrastructure, also called natural drainage systems or GSI, mimics nature by slowing or reducing runoff close to its source. It also treats runoff from roads, roofs and parking lots by capturing and cleaning it before it harms our waterways.

Why green stormwater infrastructure?

By installing green stormwater infrastructure in our public rights-of-way, King County can help reduce pollution entering waterbodies; reduce the need for large, industrial water treatment and storage facilities; and add greenery and beauty to our neighborhoods. Cities around the Country are increasingly turning to “green” infrastructure as an effective way to control stormwater.

What does green stormwater infrastructure look like?

King County has examined a wide range of green stormwater infrastructure options. The following options describe what green stormwater facilities could look like in your neighborhood.

Green stormwater infrastructure installed in planting strips

Tree boxes, like the Filterra Bioretention System

Photo courtesy of Contech Engineered Solutions.

The Filterra BioRetention Systems use a small area with short trees or shrubs to capture a high volume of runoff. Water enters the system from curbs to get filtered, cleaned and dispensed into soils.

Permeable pavement

Permeable paving allows stormwater to seep into the ground while maintaining a durable surface for cars and people. A specially-engineered layer of soil under the permeable pavement cleans the stormwater as it soaks in.