Denny Way CSO sediment remediation project
completed February 2008

To protect public health and the environment, King County removed contaminated sediment from one acre of the Elliott Bay floor offshore of Seattle's Myrtle Edwards Park and backfilled the area with clean materials to enhance the marine habitat. This cleanup is part of King County's continuing efforts to remove historic contamination and restore habitat in Elliott Bay.

Contaminated sediment was removed by this dredge barge. Nov. 29, 2007

The project site was contaminated by 40 years of combined sewer overflows (CSOs). When flows exceeded the sewer system's capacity during heavy rains, diluted sewage mixed with storm water discharged from the Denny Way Combined Sewer Overflow (CSO) outfall. The site is no longer affected by the Denny Way CSO due to a 2005 King County CSO control project which diminished the frequency of discharges and relocated the CSO outfall further offshore.

Project overview

A mechanical dredge on a barge excavated the area and loaded a second barge with contaminated sediment. The second barge transferred the sediment to trucks and trains for transport to a landfill. Roughly 14,000 cubic yards of sediment was removed. After dredging, the excavated area was backfilled with clean sand, armor rock and habitat-enhancing gravel to match the seabed's existing grade and improve the site as shallow-water nearshore habitat.

King County is committed to protecting the environment throughout the cleanup. To reduce sediment dispersal and turbidity, King County crews used careful dredging methods that disturb the least amount of sediment possible. Transport barges carrying excavated sediment were lined with filters to limit contaminants draining into Elliott Bay. In addition, the project's timing avoided salmon migration periods. Inspectors on site monitored water quality around the work area to ensure these precautions are fully employed and sufficient.