North Henderson Basin

What & Why?

Seattle Public Utilities (SPU) has completed two new underground storage facilities as part of the Henderson North Combined Sewer Overflow Reduction project. SPU constructed a 16,000-gallon storage pipe in 57th Avenue South near Martha Washington Park and a 2.65-million-gallon underground storage facility beneath Seward Park to reduce the frequency and volume of stormwater and untreated sewage that overflows into Lake Washington during heavy rain events. This project also included creative design elements and public amenities along the south shoreline of Seward Park such as:

  • An enhanced shoreline with clear views of Lake Washington and Mount Rainier with improved natural aquatic habitat (trees installed on the shoreline were repurposed after being removed by the project);
  • All native plantings were used, in line with the Olmstead’s vision for the park;
  • Other trees that were removed by the project were reused throughout the park for projects such as trail maintenance;
  • An Americans with Disabilities Act (ADA)-compliant pedestrian promenade and pathway that connects to other park amenities;
  • An improved, ADA-compliant parking lot with wider spaces and a turnaround area;
  • New and improved tennis courts;
  • A stained blue line between the tennis courts shows an approximation of the original lake-line location before the Ballard locks were constructed, which dropped the level of Lake Washington to the same level as Lake Union (about 9 feet on average);
  • An odor control system which will treat air from both the new and existing CSO facilities; and
  • A replacement outfall pipe that will carry future overflows farther away from the shore.