Delridge 99 CSO Gate Retrofit Project

What & Why

As part of the Plan to Protect Seattle’s Waterways, Seattle Public Utilities will retrofit the Delridge 99 basin sewer system to reduce combined sewer overflows (CSO) into the Duwamish River. The Delridge 99 Gate Retrofit Project involves replacing the HydroBrake flow restriction device that currently manages basin flows in wet weather with an automated gate that will better use the existing offline storage tank along the north side of Andover Street.

Following this work, SPU will continue to monitor performance and identify if additional action is needed to limit CSOs to less than 1 per year on average.

What’s happening now?

Construction starts in mid-May, and it is scheduled to take about 3 to 5 months to complete. The construction area is located at the intersection of S.W. Andover Street and 26th Avenue S.W.

During construction, you can expect:

  • Construction noise, dust, and vibrations.
  • Increased construction traffic and staging of large equipment in the area.
  • Slowed vehicle traffic and intermittent lane closures near the construction area.
  • Parking in or near the work area may be prohibited.
  • Pedestrian traffic on the north side of S.W. Andover Street near the work area will be restricted throughout construction.
  • Restoration of pavement and vegetation will occur following construction.

Project goals and benefits

This project will improve SPU’s ability to monitor and control the Delridge 99 basin flows and will improve water quality by reducing the frequency and volume of CSOs into the Duwamish River.

Project Documents