Seawall meets its nemesis, the gribble

Home » Seawall meets its nemesis, the gribble

Originally published July 20, 2012, on

There’s a piece of wood on desks down at City Hall. The chunk, with its dozens of air pockets and traverse cavities, resembles a rotting wedge of Swiss cheese. But, in reality, the crumbling wooden chunk is a piece of the Seattle seawall, the nearly mile
and a half wall that separates downtown Seattle from the salty waters of Elliott Bay.

That barrier, built between 1916 and 1936, extends from South Washington Street to Broad Street. It was never designed to withstand earthquakes or heavy tides. Nor to resist invasion by generations of gribbles, a tiny marine worm.

The city has been patching breaks its seawall for decades, but there is no getting around reality – the Seawall is crumbling. The situation is little better than if we were trying to hold back the waters of the bay with day-old newspapers and duct tape.

If there were an earthquake – a one-in-ten risk in the next decade – or if there were a tsunami, the seawall would simply fail. Just think what a failure could mean: the waters of the bay lapping up against the Pike Place Market, the streets of Pioneer Square flooded, transportation throughout the city at a standstill, electrical outages, sewer backups, lives lost and property damaged.

After careful assessment, and much investigation of solutions, the Seattle City Council voted last week to place the issue on the November ballot. Voters will consider a 30-year bond issue, authorizing $290 million to replace the badly deteriorated seawall with jet grouting and also rebuild city-owned Piers 58, 62,and 63. The piers, too, are crumbling, no longer able to safely support events such as summer concerts.

Oddly enough, the cause of these monumental problems is a tiny creature, the so-called gribble worm, a marine isopod ranging from one to four millimeters in length (it would take more than 20 to cover a penny). The wood-boring species is a distant
relative of shrimps and crabs. But, unlike more succulent crustaceans, gribbles have long been a scourge to sea-going communities, boring their way through ships, docks, and jetties.

National Geographic, writing about gribble worms, reports that Christopher Columbus was forced to delay a return trip to Spain because gribbles had rendered his ships unseaworthy.

Today most piers and boats are constructed with materials that are resistant to gribbles, a species that – for all its destructive capability – actually performs a useful ecological service. Unlike air-born fungi, they are able to consume woody debris that drifts down rivers and into the sea. Some scientists speculate that they could some day be useful, employed to break down wood and straw into liquid biofuel.

Pictures of gribbles — known scientifically as Limnoria quadripunctata — are instructive. They are pale, colorless critters. They burrow into wood, creating a larger surface area that becomes available to microorganisms. When gribbles return to old burrows, they enlarge their burrows and dine on the colonies of microorganisms that have taken up the surface area. That’s what breaks down the wood and causes it to become spongy and friable. Pick up a piece of the seawall and it might crumble in your grasp.

There are 56 species of gribbles. They have seven sets of legs and four sets of mouth appendages. As a city staffer once joked: “Just like my first husband.”

But what’s happened to Seattle’s Seawall obviously is no joke. It’s a disaster that we’ve been staving off, but it is only a matter of time until the inevitable collapse occurs.

The remedy – building a new seawall using jet grouting, a solution that’s impervious to damage from the wood-boring gribbles – has been placed squarely in the hands of the voters. Despite competing needs, city planners say construction of the seawall needs to start in September of 2013 in order to be ready for the viaduct to come down in 2016. There’s a timing crunch and the need for fast action to divert a possible disaster.

Let’s not let the gribbles have the final say, or the last laugh.