Seawall Open House

9 02 2010

I was able to attend the Seawall Open House last week at City Hall where Deputy Mayor Darryl Smith and department staff explained the goals for the seawall replacement, its proposed funding plan, and general information on the waterfront.

Here is a brief of what they presented:

The Purpose of the Alaskan Way Seawall

The central seawall, originally constructed between 1911 and 1936, extends along the western edge of downtown Seattle. It provides structural support to surface streets, the Alaskan Way Viaduct, buildings, and utilities. If the seawall were to fail, sections of the viaduct and adjacent structures and utilities could collapse or become unsafe, damaging Seattle’s waterfront and potentially resulting in loss of life.

The Urgent Need to Replace the Seawall

Seattle’s seawall is showing considerable effects of its age and exposure to the corrosive marine environment. In 2002, SDOT discovered that 40-50% of the timber relieving platform that supports the seawall and street is significantly damaged because of age, the effects of tidal erosion, and subsequent harm caused by marine borers and earthquakes. Results of this study also indicated that 90% of the structure’s cap beams show some degree of deterioration and only 10% were fully intact.

The major risk facing the Alaskan Way seawall is its seismic vulnerability. When the seawall was originally designed and constructed, engineers did not account for seismic events in their design so the existing seawall structure cannot resist seismic forces associated with earthquakes of the magnitude experienced in this region.

Shortly after the 2001 Nisqually earthquake, a 100 foot long by 10 foot wide section of the Alaskan Way surface street settled, exposing additional problems with the seawall, Soil held back by the seawall is likely to liquefy in earthquakes, which is what occurred along this section of the waterfront. The current seawall is unable to prevent this liquefied soil from leaking out into the water, jeopardizing structures built nearby. The likelihood of another seismic event occurring within the next 10 yrs could significantly damage the seawall is one in 10. A study conducted by external structural engineers revealed that another major earthquake would likely cause te seawall to fail. (See this VIDEO)

All subsequent seawall investigations and inspections conducted by the City of Seattle have shown that the seawall continues to deteriorate despite regular maintenance. Marine organisms called gribbles are eating away at the timbers that support the seawall. Inspections conducted in 2005 also found new seawall damage from another marine organisms called the shipworm. They have damaged approx. 55% of the wood panels that protect portions of the seawall. This damage means that even without an earthquake, there is a risk of localized collapses of the sidewalk and street.

SDOT actively monitors the seawall, but the deteriorated condition over time has increased the city’s annual required maintenance and repairs. Since the Nisqaully earthquake, the city had committed over $3 million to temporary repairs of the seawall, which have not dealt with the seismic vulnerability of the seawall.

Design and Permitting

On Nov 18th, 2009, SDOT requested Statements of Qualifications from consultants with experience in innovative seawall design and construction, engineering, environmental analysis and documentation, economic analysis and public involvement. Applications were received earlier in January and a consultant team will be selected and retained in March 2010. Conceptual design will begin in April and will overlap with the Central Waterfront design to ensure the best option for the seawall.

Design of the seawall will be coordinated with the Central Waterfront design and planning process in order to fully integrate seawall design concepts with the waterfront public space and Alaskan Way surface street design.

SDOT is looking to replace the seawall with a long-term solution that meets seismic standards, improves habitat, and provides effective protection for the public, waterfront facilities, and economic activities.

Mayor McGinn’s Seawall Proposal

On Jan 14, 2010, Mayor McGinn announced his intent to accelerate seawall design, permitting and construction, with the goal of completing the project in 4 years instead of the previously scheduled 6 years. Extensive federal, state, and local permitting requirements complicate timely project completion, but SDOT is accelerating work under the city’s control to replace the seawall as soon as possible.

As part of his announcement, McGinn also proposed a May 2010 election on a $242.8 million bond to fund the project. Legislation will be transmitted to the council for consideration in February.

I was impressed the City of Seattle is tackling this problem head on and actually talked a lot about creating habitat along the seawall. They mentioned the habitat enhancement seawall test panels they have been monitoring with UW research scientists and their want to keep that as a key role of the seawall.

We shall see if McGinn’s bond passes….  See more info HERE




One response

21 01 2011
Janette Plunkett

Could the Brisbane flooding have been reduced by a higher or differently designed Brisbane seawall? This article implies seawater caused some of the flooding

Should Seattle increase it’s estimates of worst case high water due to Climate Change patterns?

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