Seattle seawall threat is serious enough to act now

One of the first briefings I received after being elected mayor was on the deteriorating state of the downtown Seattle seawall. It was alarming.

Mostly built in 1934 of wooden platforms, steel-sheet piling, concrete and dirt from the Cedar River, the seawall has worked as intended for more than 70 years. It has extended the city's waterfront, provided a foundation for north-south traffic on Alaskan Way (including the Viaduct) and made it easier to load and unload cargo at the Port of Seattle.

But decades of wear — including attack on the timber by marine borer "gribbles" and damage from the 2001 Nisqually Quake — have weakened the structure to the point of serious danger. Already, 50 percent of the existing wall is damaged. In many places, the timber-cap beams that provide the seawall's foundation have been reduced to a fraction of their original size or disintegrated altogether.

This seawall was not engineered to withstand an earthquake. It is built on dirt and sand fill that became unstable during the last quake. During another, the area would be at risk of further liquefaction, exacerbating the existing damage and likely bringing down the seawall and Viaduct while destroying much of the waterfront and lower downtown.

The risk, the Seattle Department of Transportation estimates, is that there is a one in 10 chance of failure within the next 10 years, even if the seawall was in original condition. Given the serious structural damage that has been sustained, we know the risks are even higher.

Even without an earthquake, seepage and the constant deterioration by gribbles pose a threat. As I was told in one briefing, the wall is held together at this point mainly by gravity.

This is not acceptable. Based on what I know now, it would be irresponsible not to immediately address this basic issue of public safety.

On the need there is wide agreement. But a few good questions have been raised this week, including on this page, about the details of my proposal to replace the seawall.

I am meeting with the City Council Monday morning for a briefing and more complete discussion of my plan. In advance of that session, I'd like to answer some of the concerns.

The undercurrent in many of the questions seems to be a suspicion that I'm "up to something," or that this seawall discussion really is a veiled discussion about the waterfront-tunnel proposal. Let me state clearly: No.

As I said last week, the seawall isn't related to the Alaskan Way Viaduct replacement except as it relates to safety. Rather than tying these issues together, my intention is to separate them. Regardless of whether there is a tunnel in our future, the fact is that we need to replace the seawall. And even with the deep-bore tunnel, the plan has always been to replace the seawall first.

The issue here is whether we address these serious safety issues and accelerate design and construction of the new seawall, or accept further delay.

I believe the seawall is vitally important. We can't allow it to get caught up in the politics or long construction schedule of the tunnel.

I invite your input on this issue at my Web site, http://www.seattle.gov/mayor/.

Mike McGinn is mayor of Seattle.