Seattle Public Utilities

Delridge Natural Drainage


Natural drainage system cells and plantings in the South Delridge neighborhood.

What & Why

The Delridge natural system project reconstructed several blocks of roadway in the South Delridge neighborhood to include natural drainage systems and pedestrian improvements. Delridge is a neighborhood with a combined sewer system where the sewage and stormwater normally flow in one pipeline. When it rains, the volume of stormwater can exceed the capacity of the system, sometimes leading to sewage overflows to Longfellow Creek. These overflows not only spill sewage into our waterways, they also spill polluted stormwater runoff that flows off rooftops, streets, and other hard surfaces. The natural drainage system process slows and filters polluted runoff every time it rains. Natural drainage systems can slow and reduce polluted runoff by capturing it and cleaning it before it harms our waterways.

The project added natural drainage system cells along 17th Avenue Southwest between Southwest Henderson Street and Southwest Kenyon Street. The natural drainage system cells are maintained by SPU Operations and Maintenance to ensure stormwater functionality.

Features and benefits

By placing natural drainage systems along the best walking and bicycle routes, the project:

  • Protects our waterways from polluted stormwater runoff, which helps keep families and marine life healthy
  • Provides neighborhood improvements such as pedestrian and bicycle safety
  • Reduces demand on existing pipes and sewage treatment facilities
  • Reduces energy and operating costs because stormwater will not need to be pumped to the treatment plant


April Mills, project manager
(206) 733-9816