RALEIGH, N.C. (AP) North Carolina transportation officials closed the only bridge onto a 60-mi. (96.5 km) stretch of the Outer Banks on Dec. 3 after discovering the deteriorating span posed an immediate safety threat.

The state Department of Transportation said it closed the Herbert C. Bonner Bridge over the Oregon Inlet on North Carolina 12 to protect the traveling public. The DOT declared that the inlet had scoured out sand around the base of supports at the southern end of the bridge, near Hatteras Island.

Subsequent sonar tests and divers found there was more sand gone in that section of the bridge than previously thought, DOT spokesman Mike Charbonneau said. Reopening the bridge will require fortifying the bridge’s support columns and bringing in additional sand, he said.

The Bonner Bridge is the only road access for vehicles between Hatteras Island and the mainland. An emergency ferry will move people and cars across the Pamlico Sound to Hatteras Island, transportation officials said.

“Closing the Bonner Bridge is necessary to keep all travelers safe, but we know it will have a devastating effect on the people who live along and visit the Outer Banks,’’ state Transportation Secretary Tony Tata said in a statement. “We will work to safely reopen this vital lifeline quickly.’’

Once the ferry is operating at full capacity on a full schedule, it will be able to carry 760 cars a day in both directions, the DOT said. The bridge carries as many as 13,000 vehicles during peak travel days during the summer vacation season.

The bridge was designed to last 30 years when built in 1963. DOT began the process of trying to replace the Bonner Bridge in 1989 and awarded a contract of almost $216 million in 2011 for construction that was set to begin earlier this year.

But the new bridge’s building timetable of two to three years was postponed in part by environmentalists, who want a 17-mi. (27 km) bridge that would be one of the longest in the world. Environmentalists contend replicating the current 2.5-mi. (4 km)-long structure ignores its effects on the Pea Island National Wildlife Refuge and the frequent breaches of N.C. 12 by water and sand kicked up by storms.

The proposed 17-mi. bridge would bypass the refuge, but building the second-longest bridge in the United States would cost more than $1.1 billion, federal Judge Louise Flanagan wrote in September in a ruling allowing the shorter span. An appeal of that federal lawsuit and a state challenge still block the start of construction.

Hatteras Island also can be reached by ferry from nearby Ocracoke Island. Tolls on two ferry routes to Ocracoke from the mainland are being waived for island residents, emergency workers and suppliers while the bridge is closed, the state DOT said.

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