February 15, 2012

DOT finds scouring problem with one of Bonner Bridge’s pilings


Despite the overall effectiveness of a recent erosion-prevention project at the Herbert C. Bonner Bridge, scouring has become a concern in one section south of the channel span.

Sterling Baker, division maintenance engineer for the state Department of Transportation, said that while the safety of the bridge has not been compromised, measures have to be taken to prevent further erosion.

Installation around bridge pilings of 4-foot concrete structures called A-Jacks was completed in November.  By slowing down the water, the structures, which look like oversized children’s jacks, are designed to encourage sand to collect around the piles, providing support.

But in a recent inspection, Baker said, divers found scouring at Bent 159, located under the bridge 1,000 feet south of the high rise.

“Most of the areas are building up sand,” he said. “But there is one area that may have lost sand.”

The bridge, which is monitored regularly, was inspected on Friday and again on Monday, Baker said. It appears that the scour at the one bent has gotten a foot or two deeper, he said.

Although it is not clear why the problem is occurring, Baker said, there is suspicion that water pressure and currents have been affected by the breach opened in Pea Island by Hurricane Irene in August.

“We think the dynamics of the inlet have changed since we got the other inlet further south,” he said. “In this particular location, there’s not as much sediment settling out… and no sand has accrued like we anticipated.”

After discussion last week with its hydraulics unit, DOT has been working with the U.S. Army Corps of Engineers to get dredged sand placed in the scoured area.  Baker said the project would use some of the funds provided for the A-Jacks project.

Roger Bullock, chief of navigation in the Corps’ Wilmington District, said that the Corps hopes to be able to “point place” material pumped from the channel by a small multi-purpose vessel that can be used to dredge.  

Bullock said that the agency is in the process of coordinating with environmental agencies. If the Corps gets the go-ahead, he said the project will likely start soon and take about a week.

Steve Shriver, the Corps’ survey team leader in Wanchese, said that he is aware of the scouring situation at the bridge, but he hesitated to attribute its cause to the Pea Island breach.

“It’s hard to tell,” he said, adding that the inlet is never the same from one day to the next.

“Oregon Inlet is so dynamic,” he said. “There’s always a little change in the direction of the current . . . It’s always something new, basically."

Baker said that divers will continue to inspect the bridge, especially after every storm.  While emphasizing that the current situation presents no danger, he said that if DOT believed there was a risk to the public, the bridge would be shut down until the problem was addressed.

The aging Bonner Bridge will be replaced by a new bridge, parallel to the current span.  Contractors have already been chosen and work is scheduled to begin on the replacement late this year. The new bridge is expected to open for traffic in 2015.

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