Seattle, Washington Department of Transportation (Seattle DOT) Elliott Bay Seawall Project
The City of Seattle (City), through its Department of Transportation (SDOT), began a project to replace an aging seawall in Elliott Bay that protects and supports critical transportation infrastructure from coastal storms and shoreline erosion. The original seawall was built between 1916 and 1934 atop timber piles and is at risk of failure in the event of an earthquake due to years of deterioration of the timber caused by waves and tidal forces. The updated seawall will have a minimum 75-year lifespan, provide protection for critical infrastructure (taking sea-level rise into consideration), meet current seismic standards, and improve natural habitat and salmon migration pathways.
This resource was featured in the May 20, 2016, ASAP Newsletter.
"Washington State has incorporated resilience measures into the final design of three transportation projects, including the Alaskan Way Viaduct project that is designed to provide greater resilience and function under projected future changes in climate."
In designing the new seawall, SDOT used sea level rise projections provided by the University of Washington’s Climate Impacts Group (CIG). CIG developed localized projections of six to fifty inches of sea-level rise in the years 2050 and 2100 under different climate scenarios. CIG recommended that the 2100 projections be used because of the assets long lifespan, critical nature and low-risk tolerance for failure. SDOT used the low-probability, high-impact estimate for sea-level rise in the region when planning the seawall. The fifty-inch low-probability, high-impact estimate was developed by modifying the Intergovernmental Panel on Climate Change (IPCC) global sea-level rise projections under a high emissions scenario with three additional factors: higher levels of ice loss from Greenland and Antarctica, seasonal changes in atmospheric circulation in the Pacific, and vertical land deformation. Using the highest predictions from the CIG study, the mean higher high water level in 2100 would reach within three feet of the current seawall elevation, so SDOT determined that it was not necessary to build the new seawall higher than the current structure. Instead, in designing the seawall and adjacent roadway, SDOT will focus on reducing the risk of inundation caused by tidal backwater during extreme events such as a storm surge during high tide.
The seawall supports transportation infrastructure including Alaskan Way and its pedestrian and bicycle paths, the Alaskan Way Viaduct, the Colman Dock Ferry Terminal, and the Burlington Northern Santa Fe rail lines. Much of this infrastructure is at risk of collapse if an earthquake occurs because the original structure was not designed to account for earthquakes and all sections are weakening from daily flooding with the tides. Additionally, the original seawall was not constructed to reach the depths of the stable, non-liquefiable soils that would have provided a solid foundation. The new seawall will be constructed utilizing a jet grouting technique, which involves injecting grout or cement into the ground where it will mix with and stabilize the soils behind the seawall face and protect against liquefaction in the event of an earthquake. After jet grouting is complete, the seawall superstructure will be installed to provide support for the new wall face and the cantilevered sidewalk above. Finally, the utilities and roadway will be restored and the new sidewalk installed.
The cantilevered sidewalk and the seawall face will be designed to improve aquatic habitat and important salmon migration pathways. The sidewalk will contain glass blocks to allow sunlight to get to the water below, facilitating habitat growth and lighting the pathway for the salmon that migrate through Elliott Bay along the seawall. The seawall face will be textured to mimic the natural environment at each tide level, further enhancing habitat and facilitating growth better than a smooth surface would.
The City issued a Final Environmental Impact Statement (FEIS) pursuant to the State Environmental Policy Act (SEPA) in March 2013. Since then, updates were recommended to the Preferred Alternative, warranting a Draft Supplemental Environmental Impact Statement (DSEIS), which was released in December 2013, to consider the updates. The updates included changes to the project design and construction sequencing and approach, but the changes were not substantial enough to consider it as a new project alternative. The three major project components described in the original FEIS remain the same: a new seawall, aquatic habitat improvements, and enhanced upland areas.
Construction of the seawall is occurring in two phases, with the older and more vulnerable Central Seawall (Phase 1) being built first, followed by the North Seawall (Phase 2). Construction on the central portion between University Street and Yesler Way is expected to finish in mid-2016, and construction on the northern portion between Pike and Pine streets is expected to continue through mid-2017. The seawall replacement is part of the Seattle Waterfront Program, which also involves taking down the Alaskan Way Viaduct and a variety of Core Projects, including a new pedestrian promenade, a two-way cycle track, a new Alaskan Way to accommodate all modes of travel, two rebuilt public piers, and new parks and pathways.
The project is estimated to cost $350 million. $290 million of the project was funded by a voter-approved bond measure in 2012.
This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on March 18, 2016.
Publication Date: November 2013
- City of Seattle, Washington
- Seattle Department of Transportation
- University of Washington
- Best practice
- Case study