Seawall Design Alternatives Completed

7 06 2011

Replacing the seawall that protects downtown Seattle is a complex project, but it has now moved one step closer to construction as the City has completed defining the range of alternatives that will go into the Draft Environmental Impact Statement. The two alternatives bracket the options, from a relatively lower cost project that provides basic structural stability and core habitat enhancements, to a higher cost alternative that pulls the seawall inland to provide increased habitat and design flexibility. The City will likely select a hybrid of these two choices that will provide the best solution to address critical public safety needs while integrating the long-term vision for the Central Waterfront, habitat and environmental enhancement, improved public access, and economic vitality of the waterfront, while being mindful of costs for the project as a whole.

The existing seawall includes three types of structures. The southern two zones, in front of Pioneer Square and the Ferry Terminal, are gravity walls with piles below them. The third zone, in the center of the project, has a timber relieving platform and a steel master pile covered with a wood facing. The fourth zone, in front of the Aquarium, has a pre-cast concrete wall face tied to the timber relieving platform. Each of these existing structures helps to define the two potential wall alignments.

There are two alternative designs for the 3,840 feet of seawall scheduled to be replaced. In Alternative A, the seawall stays near its current location. In Alternative B, the wall is pulled back from 15 feet in the southern two zones to as much as 75 feet adjacent to the Aquarium, allowing options for a different relationship between the water and land. Along with these alternative wall alignments, there are two types of structural solutions for replacing the seawall. The least expensive option involves jet grouting to improve the soil behind the wall. The second option, using drilled shafts, is a more proven technology but also more expensive.

The tradeoffs in flexibility, cost, and relative risk will all be analyzed in the coming year, and a decision is expected in 2012. Construction of the central seawall will begin in the latter half of 2013 and last approximately three years. During construction, there will be a plan for preserving access to waterfront businesses. Construction will occur during the winter months, both to keep the waterfront open during the summer and because of restrictions on working in water during fish migrations. During construction, traffic on Alaskan Way would be moved under the existing viaduct.

Much analysis and many decisions remain before the Seawall project is ready to go out for funding. The current level of design has increased the projected cost by at least $50 million, and possibly as much as $100 million or more. Reasons for the cost increases include the greater expense for drilled shafts, adding 234 feet of length to the project at the northern end, and shifting some costs to this project that had formerly been assigned to the Central Waterfront project.

The increases in projected costs suggest that it was probably a wise decision not to go to a public vote in 2010 or 2011, since the ballot measure would not have been sufficient to cover the costs, and the City would have had to either go to a second vote or find funds elsewhere. Unfortunately, the increased cost will make a successful 2012 ballot measure even more challenging.

The City continues to pursue two possible funding resources. The King County Flood Control District Board has agreed in principle to fund $30 million of the project, although that will have to be confirmed at the time that the funds are needed. And the Corps of Engineers is continuing to work in conjunction with the City and there is still a possibility of federal funding, depending on the national political picture.

Repairing and replacing the seawall remains a high priority for the City. The City is approaching it carefully and thoughtfully to make sure that the right project gets selected and completed.

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