The project

King County plans to install facilities – called green stormwater infrastructure (GSI) – in north- central Seattle to help control combined sewer overflows. The number, type and location of green stormwater infrastructure facilities will be determined by an alternatives analysis and design process. All green stormwater infrastructure installed would be maintained by King County.

Project Study Area

The project will select sites for installing green stormwater infrastructure within the Project Study Area (below). The Study Area is a part of the University Combined Sewer Overflow Basin, made up of a network of pipes carrying sewage and stormwater that – in the event of a heavy storm – can overflow into Portage Bay and the Lake Washington Ship Canal. Controlling stormwater within the Study Area will reduce the amount of overall stormwater in the basin, and ultimately help reduce the number of overflows. Green stormwater infrastructure would also help reduce the size of a future large stormwater storage facility.

Click to enlarge the map.


In this first phase of the project, the County will consider what types of green stormwater infrastructure we should build and where to install these facilities within the Study Area.

  • Quarter 4 – 2017: Data analysis begins
  • Quarter 4 – 2018: Seek public values on green stormwater infrastructure benefits and challenges
  • Quarter 1 – 2019: Data analysis complete and locations chosen
  • Quarter 3 – 2019: Begin designing green stormwater infrastructure projects
  • Quarter 1 – 2021: Complete designs
  • 2021 – 2023: Implementation (bidding for work, selecting a contractor, construction)