Seattle, WA

Seattle Public Utilities

Natural Drainage Solutions

Delridge is a neighborhood in the City of Seattle 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 enters our waterways.

Mukilteo ferry terminal Mukilteo ferry terminal

What CM Design Group Did

CM Design Group was a subconsultant to CH2MHill on this Seattle Public Utilities (SPU) project to improve efficiency and provide the combined benefits of City services by partnering with Seattle Department of Transportation (SDOT) to co-locate Neighborhood Greenways and Roadside Raingardens on approximately 20 city blocks in the Delridge neighborhood of West Seattle within the Longfellow Creek watershed.

CM Design Group was involved in the feasibility analysis in the early stages of the project, and also prepared the parking study. In the design phase, CM Design Group prepared the design plans for two blocks of GSI improvements that included biofiltration facilities with drilled drains, pedestrian improvements, and utility relocations.

Key Components

  • Protects public waterways from polluted stormwater runoff
  • Included drilled drains to enhance infiltration rates
  • Reduces the sewage overflows to Longfellow Creek