Herrera is proud to announce the completion of a milestone on the 5-year project by Washington State Department of Transportation Ferries Division (WSF) to clean up contaminated sediments and promote nearshore habitat restoration by installing a containment cap during construction of the Seattle Multimodal Terminal at Colman Dock.
Located in downtown Seattle, Colman Dock is a multimodal transportation hub visited by millions of commuters, pedestrians, and tourists each year. The busiest hub in the Washington State ferries system, Colman Dock provides general and commercial transportation services, in addition to passenger ferry service and high occupancy vehicles.
To maintain the dock’s critical role in public transportation amidst its aging infrastructure, WSDOT, WSF, the Federal Transit Administration, and the Federal Highway Administration replaced the north timber trestle of Colman Dock with a new concrete trestle made to withstand large earthquakes. A total of 7,400 tons of creosote-treated piles were removed from this critical salmon corridor adjacent to the new Elliott Bay Seawall. Over 30,000 tons on clean sandy gravel has been placed to a depth of at least 2 feet over 6.0 acres of sediments to contain a legacy of pollution from many urban sources, which caused levels of metals and organics to exceed sediment quality standards and pose risks to environmental and human health.
Starting in 2016, Herrera was contracted by WSF to lead a sediment management alternatives evaluation, helping stakeholders evaluate the best alternative for managing surface and subsurface sediments during construction. Sediment borings showed increasing contaminant concentrations with depth to over 20-feet-thick of historical fill materials in some areas. Based on risk and cost factors, thick-layer capping was selected as the preferred sediment management alternative. A voluntary sediment cleanup action plan was prepared that established site-specific sediment cleanup levels and cap design criteria.
In accordance with the cleanup action plan, Herrera prepared contractor specifications and drawings to construct the sediment cap. The in-situ cap design involved a non-removal remediation technique to prevent the spread of contaminated sediments by placing cap material over the sediment surface to provide physical and chemical isolation. In addition, the cap provides functional habitat for the benthic community and migrating salmon, while withstanding erosive forces of ferry boat propellers and winter storms.
Dock and sediment cap construction has occurred in five phases over five years. For each phase, Herrera provided engineering oversight to ensure cap materials and thickness met specifications. We compared pre-cap and post-cap bathymetric data in conjunction with cap material volumes to estimate cap thickness and settlement. Our scientists monitored sediment quality at 16 sites each year to document progress towards meeting cleanup levels and ensure contamination had not spread outside the capping area.
In response to an oil release while excavating within a former seawall bulkhead (fill cell) during the project’s final phase, Herrera collected sediment cores to characterize the oil extent and re-designed the sediment cap to include an organoclay reactive core mat for oil containment. The team also deployed a cap thickness probe during evening low tides at 50 locations under the dock where bathymetric data indicated insufficient cap thickness. These measurements were critical to our final evaluation that at least 95 percent of the voluntary cleanup area had been adequately capped. Herrera will conduct additional sediment chemistry monitoring later in 2022 and beyond to evaluate compliance with the established sediment cleanup levels.
Herrera is delighted to be a part of this project to preserve one of Seattle’s hallmark transportation hubs by reducing contamination and restoring salmon habitat in Elliott Bay. Our team looks forward to continuing our work across Seattle and the Pacific Northwest to promote environmental resilience.