April 12, 2012 | Vol. 39, No. 15

Hammer Slough mudslide sends silt into Middle Harbor

A mound of mud has been making its way down Hammer Slough since a mudslide near the Public Works office last fall. On Saturday, at an extreme low tide of -3.6, the silt lifted a privately-owned float and at least one boat right out of the water.

Suzanne Ashe
About 600 to 1,000 cubic yards of silt has slipped down Hammer Slough into the Middle Harbor. Authorities are investigating the best course of action to remove it before a -4-foot tide in May.

“Our best guess is that it's 600 to 1,000 cubic yards.” said Harbormaster Glorianne Wollen.

The problem with the mud is that when the tide is extremely low, property that should be afloat, sits on mud instead.

City Councilman Rick Braun also took a look at the site on Saturday. In 2005, Braun did a survey of the Middle Harbor prior to the harbor being rebuilt. The 2005 survey included a hydrographic survey of the harbor basin and a topographic survey of the parking lot and surrounding marine related structures and uplands. That survey showed deep water around all of the floats. Now, some of the floats are almost dry at a minus tide, he said.

“A visual inspection of the mouth of Hammer Slough Creek at a minus tide right now, shows a considerable amount of sediment has washed down the creek this winter,” Braun said. “If the sediment builds any more under that float, the water line may be damaged at low tide. If a portion of the float system sits on the bottom while the rest are still floating, some of the floats may be damaged as well. At low tide, some of the boats will have difficulty leaving and entering the harbor without damaging their propellers and maybe their hulls,” he added.

This is a concern for the City Council, the sediment and what to do about it has been a subject of discussion at the last two council meetings.

According to Wollen, removing the silt can be done one of two ways. It can be removed by suction dredging. This technique could take up to two weeks to remove silt from the problem areas.

The second option would be “clamming” the mud. This is when the mud is scooped up and set on a barge to allow the water to run off. The problem with clamming, Wollen said is that there is no estimate on what a project like this will cost, nor is there money in the budget at this time. One thing Wollen stressed is that it's a project that needs to be addressed right away.

“It's an emergent situation,” she said. “I'm not sure which way we'll go.”

Wollen has not received any reports of damaged property. But there will be another extreme low tide in May.

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