The seawall has served its purpose well but is showing the effects of its age and the corrosive marine environment. As a result of continual deterioration, the seawall has sustained damage to the timber platform from a variety of marine borers called Gribbles and ship worms. While the City continues to maintain the seawall, damage to the timber platform found along its length makes the seawall vulnerable to failure. In addition, the original seawall design did not account for earthquakes. Even undamaged, the seawall would not be able to resist loads associated with liquefaction of the loose soils behind the wall. The damage to the timber platform means that the seawall's vulnerability to an earthquake is greater than previously thought.
It is predicted that the current seawall has a 1 in 10 chance of failure from an earthquake in the next 10 years. Even without an earthquake, the seawall could fail in places as a result of continued deterioration. If the seawall were to fail, sections of the viaduct, the Alaskan Way surface street, and adjacent structures and utilities could collapse or become unsafe. Because of these risks, the seawall needs to be replaced.
Existing Structure and Project History
What is the Elliott Bay Seawall Project?
The Elliott Bay Seawall Project will replace the existing seawall—from S. Washington Street to Broad Street—with a structure that meets current safety and design standards. The project will replace the three types of deteriorated seawall structures along the waterfront, which were constructed between 1916 and 1934 and range in size from approximately 40 to 60 feet wide. The City plans to replace the most deteriorated sections of the central seawall beginning in late 2013, with a second phase of work for the northern seawall following as funding is available. Phase 1 of the project is between S. Washington Street (just south of the Washington Street Boat Landing) and Virginia Street (at the northern edge of Pier 62/63), and Phase 2 is from Virginia Street north to Broad Street (just south of Olympic Sculpture Park). The new seawall’s “service life”—or the amount of time before the seawall may need to be replaced again—will be approximately 100 years.
What is the public safety risk?
The Elliott Bay Seawall Project is a critical public safety project. Failure of the seawall would have significant impacts to the public, the City of Seattle, the Puget Sound region, Washington State, and the nation. Protection from coastal storm damage and shoreline erosion is vital to preserving Seattle’s downtown, the economy, and the region’s quality of life and economic competitiveness. The Elliott Bay seawall:
- Protects Seattle’s downtown waterfront from wind-driven storm waves and the erosive tidal forces of Puget Sound and Elliott Bay.
- Supports and protects major public and private utilities, including power for downtown Seattle and the western seaboard, natural gas, and telecommunications.
- Supports State Route 99, the ferry terminal, and rail lines, all of which transport local commuters and visitors as well as local, regional, and international freight.
Why does the seawall need to be replaced?
The current seawall was not built to withstand earthquakes. The seawall is unable to resist loads associated with liquefaction of the loose soils, caused by seismic activity, on which it is constructed. In an earthquake similar to that of the Nisqually Earthquake in 2001, liquefaction of the fills is expected. The likelihood of such a seismic event occurring in the next 10 years is one in 10. In addition, waves and tidal forces have eroded fill from behind the seawall, creating voids that lead to structural damage and instability of the seawall, the underlying structure of the roadway, and other public and private infrastructure. There is also significant damage to the timber platform from a variety of marine borers called Gribbles and Toredos. Their damage means that the seawall’s vulnerability to an earthquake is increased. Even without an earthquake, continued deterioration causes significant problems.
What is this project’s relationship to the city’s Waterfront Program and the state’s Alaskan Way Viaduct Replacement Program (AWVRP)?
The Elliott Bay Seawall Project is one project within the city’s Waterfront Program, which also includes Waterfront Seattle, utility relocation projects, and a parking mitigation program. Because the seawall is the foundation of the future waterfront, construction of the seawall must be completed before the city moves ahead with construction of the final post-viaduct Alaskan Way or enhancements to the public spaces that will become available with viaduct demolition. Although the seawall was at one time a component of the state’s AWVRP, the bored tunnel is not directly dependent upon the seawall for stability. However, near Washington Street (the southern end of the Seawall Project), the bored tunnel and the seawall are quite close to one another—approximately 30 feet apart—which means that construction coordination in that area is critical. The tunnel boring machine is expected to pass underneath the Viaduct (heading northeast) in late 2013, freeing up space for seawall construction to proceed in that area.
What is Waterfront Seattle?
Waterfront Seattle is another independent project within the Waterfront Program sponsored by the City of Seattle that will capitalize on the removal of the Alaskan Way Viaduct and replacement of the seawall to provide a new surface Alaskan Way, improve connections to the waterfront, and develop a series of new public spaces that will serve the entire city and region.
Why is habitat restoration a purpose of this project?
Sixty percent of Seattle’s waterfront is covered by piers and other over-water structures, resulting in stark contrasts between light and dark areas. The light areas show a remarkable diversity of habitat, while the dark areas (under piers) are not amenable to habitat growth. Of particular significance to the project are efforts to improve the salmon migration corridor. Juvenile salmonids journey from the mouth of the Duwamish River along Seattle’s urbanized downtown waterfront. Juvenile salmonids are less susceptible to predators in locations with better habitat conditions, including lighted waters, substrate for plant life, and shallower water. Replacement of the seawall provides a unique opportunity to restore the shoreline ecosystem and improve the migration corridor.
How much habitat enhancement is required? Who decides how much is enough?
Permitting for the seawall project will require specific habitat restoration measures, given the in-water work necessary for seawall construction. While the Department of Natural Resources will likely require that any overwater structure provide 30 percent of ambient light to under-pier areas, the project has an opportunity to showcase best practices for habitat restoration in an urban area.
Design of the Future Waterfront
How have you been working with the Waterfront Seattle team?
The seawall is the foundation for Seattle’s future world-class waterfront and will support and complement designs currently underway by the Waterfront Seattle team. The James Corner design team and seawall designers have collaborated on potential seawall placement to maximize flexibility for the future waterfront. The project’s alternatives have been reviewed with the Central Waterfront Committee and Subcommittees, the Seattle Planning and Design Commissions, and the Central Waterfront Stakeholders Group as well as individual stakeholder groups interested in the seawall design. In addition, Waterfront Seattle and the Seawall Project have developed an integrated public outreach and engagement plan to ensure that the projects are presented to the public in a coordinated fashion.
How flexible are the project’s alternatives at this stage?
Even at 35% design, a great deal of flexibility remains, and we expect the final design process to be an iterative one, based upon collaboration with the Mayor and Council, Waterfront Seattle, the public, and our agency and City partners. This is a once-in-a-lifetime opportunity to shape Seattle, and providing flexibility for ongoing collaboration is very important.
Will we see the seawall?
That depends on the final waterfront design. The seawall structure forms the foundation for the future waterfront, but the face of the seawall [whether that is a vertical face with textures for art or fish habitat, stairs, rocky beaches, or another façade] will allow for a varied public experience and access to the water.
Will people be able to go to the waterfront during construction?
Yes. Two of the City of Seattle’s project goals are to support the economic vitality of the waterfront and to minimize construction impacts. We will work to maintain access to businesses and residences throughout construction, and we anticipate shutting down construction during the high tourist season for waterfront businesses.
How many structural solutions are being considered for the seawall?
Two types of structural or soil stabilization solutions are being considered for this project: drilled shafts and jet grouting. These methods can both be used along the waterfront in a mix-and-match style. At this time, jet grouting is emerging as the preferred method of soil stabilization due to better seismic performance, lower cost, and potentially faster construction.
In terms of sequencing, do you have to remove the old seawall in order to build a new one?
That depends on the location of the new seawall structure. The new seawall can largely be built either in front of or behind the original seawall. In some cases, the existing face would need to be removed, and in other cases the face and seawall structure would remain in place when the new structure is complete.
How will the project deal with historic structures or archeological finds?
The project will meet the requirements of Section 106 of the National Historic Preservation Act and other applicable laws, regulations, and polices. The seawall team has been working with the appropriate historic districts to develop designs and plans addressing historic resources in the project area. Archeological investigations have been completed and will soon be supplemented with additional surveys to help inform future design decisions.
What is the anticipated cost of the Elliott Bay Seawall Project?
In April 2012, the project costs were updated based on 35% design of the staff recommended alternative. The cost for design, environmental analysis, and construction of the central seawall from Washington to Virginia Streets is $300 million. With the progression of design, the project is better defined and more is known about the soil stabilization method (jet grouting), the habitat elements, and other project features, all of which led to cost savings.
How much of these costs are funded?
The project is fully funded for the environmental process and design through 2012.The City Council and Mayor have proposed a bond measure for November 2012 in the amount of $290 million, which would provide the funding needed to complete Phase 1 of the Seawall Project (from Washington to Virginia Streets) and additional resources to address other critical infrastructure needs along the waterfront. Approximately $60 million has already been appropriated for the Seawall Project through City funding ($30 million) and a grant from the King County Flood Control District ($30 million).
Working with the U.S. Army Corps of Engineers
What is the federal interest in this project?
Given the local, regional, and national significance of this project, the U.S. Army Corps of Engineers has partnered with the City of Seattle to determine the extent of the federal interest. A feasibility study for this project was initiated in 2004 with the signing of a feasibility cost sharing agreement between the Corps of Engineers and the City of Seattle. When the feasibility study is complete, we will know if there is a federal interest in the project and will be able to identify the maximum allowable federal contribution to the project.
What are the potential benefits of this continued partnership?
If the Corps finds a federal interest in the Elliott Bay Seawall Project and the project is authorized by Congress, the Corps may fund up to 65 percent of replacement costs of the seawall based on the federally preferred plan. While the City of Seattle retains the ability to select a locally preferred alternative, the Corps’ cost-sharing would be defined by the federal plan, which is generally the least-cost alternative.
Can the City of Seattle expedite construction?
Due to the City’s goal of addressing the public safety risk as quickly as possible, the City is exploring options with the Corps to advance the allocation of its non-federal cost share of project implementation to expedite construction of Phase 1, which is the most critical safety need.
Are you maintaining the seawall schedule? What’s next?
The City’s accelerated schedule anticipates beginning construction in late 2013. The work of the technical team is on track to address this urgent safety need. The critical schedule drivers at this time are the environmental and permitting processes, which run through the Regulatory Branch of the Army Corps of Engineers. We are currently awaiting a significance determination from the Corps, which will identify the type of environmental analysis the Corps will require and will set the project schedule moving forward. More will be known in May.
Why is this schedule different from the Waterfront Seattle design schedule?
The schedule is appropriate to address the critical public safety need of the seawall; now is the time to complete this important infrastructure project in order to maintain sequencing for the City of Seattle between this project, the removal of the Alaskan Way Viaduct, and development of the public spaces and streets that will be designed as part of Waterfront Seattle. Replacing the seawall provides the foundation for new opportunities along the Seattle waterfront.
How has the public been involved in the design process to date?
Since 2010, the seawall team has been working with a broad cross-section of the public to develop three alternatives. In 2011 alone, the team participated in over 100 events, briefings, and informational sessions. These interactions have included ongoing conversations with the Central Waterfront Stakeholders Group, over 50 organizational briefings to community groups, a business operations survey of 400 businesses in the project area, public open houses and comment periods, as well as fairs and festivals in the community (reaching over 1,000 people!). Simultaneously, our partnership with Waterfront Seattle—and the seawall’s integral role as the foundation for the waterfront—has connected the seawall team with thousands more interested Seattleites. The project team will continue to work with the public to refine alternatives through final design and to plan for construction.
What construction contracting approach will the City use?
The City will pursue a General Contractor/Construction Management (GC/CM) approach. Having the expertise of the contractor on board early will help the City to develop a more comprehensive construction schedule and allow the contractor to serve as a partner in stakeholder engagement to keep the waterfront vibrant during construction.
In 2008, the City and the University of Washington began the field work phase of a collaborative scientific research project investigating opportunities to enhance the marine habitat surrounding the seawall. To accomplish this, test panels were designed to increase habitat complexity and installed at three locations on the seawall. Results from this study will help determine shapes and textures that may work to enhance the marine habitat function of the seawall face.
Eighteen (18) panels were installed along the seawall with six (6) different shapes and textures at each of the three locations – Clay Street, Vine Street, and between Pier 62 and the Seattle Aquarium. In addition to the panels, “troughs” were installed that extend out from the face of the seawall. The intent of the trough is to mimic shallow water sediment habitats that have largely been lost along the Seattle shoreline. The potential benefits could include higher production of marine life and trapping additional sediment and organic matter.
The University of Washington has been sampling the test panels and troughs for a two year period. What they found so far is a succession of colonizing algae on the test panels, mobile invertebrates and barnacles. Monitoring will continue through 2011.