Gribbles gobbling seawall along Seattle waterfront

The gribble is small, it is common, and it is ugly.

Roughly the size of a flea, it possesses 14 legs and four pairs of moving mouth parts. It is often a brownish shade of gray and will roll up into a ball when threatened. Give the gribble a chance and it — along with tens of thousands of brothers and sisters — will munch on docks and wooden ships and piling until they are so weakened they crumble into the sea.

Here in Seattle, the gribble is becoming a celebrity — and a political pawn in the battle over replacing the aging Alaskan Way Viaduct.

The tiny crustacean, which has been the scourge of seaports for centuries, is being blamed for the deterioration of the seawall that runs along the downtown Elliott Bay waterfront. The area is prime tourist territory. It also helps support the viaduct, Seattle's fragile elevated highway that was damaged in last year's earthquake.

Showing off one of five holes city workers cut into a sidewalk at Alaskan Way and University Street, Mayor Greg Nickels pronounced the damage so significant that immediate action is required to prevent disaster.

"An already-strong case for replacing the viaduct and the seawall is now overwhelming," he said. "This is a huge threat to the public safety, a huge threat to our transportation structures and our economy."

Nickels is pushing for the seawall to be rebuilt, which would cost $750 million to $1.5 billion, according to city estimates. As a proponent of light rail and replacing the viaduct with a tunnel running under much of the waterfront, Nickels would like more money for Seattle projects to be included in a regional transportation-improvement package that may go before voters in November.

The damage to the seawall was discovered soon after last year's Nisqually earthquake, which liquefied some of the soil under the seawall and the viaduct. In three of the holes the city dug, inspectors found evidence of gribbles, which swarm unprotected wood in the sea and eat their way through it at a rate of about 2 centimeters a year. Infestations by the creatures are not new. In the past, they were known to devour ships right out from under sailors.

Richard Miller, director of roadway structures for Seattle Transportation, said the damage means the seawall could collapse if struck by another large earthquake. The seawall was built in 1934 out of concrete, dirt, steel and fir timbers to solidify a huge landfill operation that added several hundred feet to the Seattle shoreline.

A steel curtain was supposed to seal thousands of timbers used to support the wall from the sea. Over time, holes were punched in the steel, allowing water — and the gribbles — to mix with the wood.

When it was built, the wall was expected to last between 50 and 70 years. Now at 68, the wall is near the end of its usefulness, Miller said. But he said it could still be expected to hold up for 20 to 50 more years, barring any major quakes.

"It looks reasonably good right now," he said. "... But now it's more critical. It would be extremely weak in the event of an earthquake."

To anyone walking or driving along the wall, which runs from the edge of the water to the viaduct, any damage is concealed under concrete, dirt and asphalt. But even for those who have studied the issue, noticing the damage is not easy.

Before the news conference to announce the gribble as a public enemy, the mayor stared at a number of rotted timbers placed on the sidewalk for members of the news media to photograph. As tourists streamed by, Nickels looked over the wood and posed a question to a city worker: "This is bad, right?"

John Zebrowski: 206-464-8292 or jzebrowski@seattletimes.com.