It’s an edict that has passed down through generations: do not build houses –– or highways –– on sand. But along the Outer Banks of North Carolina, sand is all that engineers have to build upon.
So sand is what they are using to rebuild a section of Hatteras Island that disappeared in Hurricane Isabel in September. The storm created a watery gap of approximately 1,700 ft. (518 m) and buckled or totally washed away approximately 4,000 ft. (1,213 m) of N.C. Route 12. In almost record time, Great Lakes Dredge and Dock Company assembled floating and earth-moving equipment to repair the breach.
The U.S. Corps of Engineers engaged the Oak Brook, IL, company to do the work on the basis of a “letter contract” that set things in motion before a formal $6.2-million agreement was worked up.
Three quarters of the cost was borne by the Federal Emergency Management Agency and the rest by the state. Moving of sand began Oct. 17 and is was completed on Nov. 1. With sections of N.C. 12 now open to residents, work on building dunes is continuing.
The project had to get under way fast because the breach isolated some islanders. The break in the highway south of Cape Hatteras left Hatteras Village residents without a way to the mainland.
Some longtime islanders are used to this sort of thing. Tropical storms and nor’easters hit the East Coast eight or 10 times a year on average, some of them with deadly force.
North Carolinians say a 1933 storm had many characteristics of Isabel, hitting at high tide and washing deep inland. They still talk about the Ash Wednesday storm of March 1962, which also breached the barrier island. Hurricane Dennis in 1999 was the most recent major washover, though that was not a total breach.
The latest washout is in the general area of six “hot spots” where erosion always is a concern, North Carolina Department of Transportation (NCDOT) officials said, though it is a little bit farther up the road than where NCDOT expected to have a problem.
N.C. 12 runs perilously close to the ocean along that stretch, a ribbon of asphalt laid on a spit of sand maybe .5 mi. (.8 km) wide that separates the ocean at that point from Pamlico Sound. The storm eroded protective dunes before eating its way through the barrier island itself.
As quickly as the storm blew itself farther north, Corps engineers were flying overhead evaluating how to repair the damage. Great Lakes Dredge then was contacted because its equipment already was in the area. The company was doing some major channel work farther south in the state.
Besides proximity, the company was chosen because it has proven to do “excellent” work, said Penny Schmitt, a Corps public affairs officer in Wilmington, NC.
Still, the Corps hurry-up request was staggering. The company’s hydraulic division engineer, Sam Morrison, said the Corps asked a lot of the company to pull together needed equipment on such short notice.
“The Corps put an enormous task in front of us –– a monumental task,” said Morrison, noting that the project required 35,000 ft. (11,000 m) of 30-in. (76.2 cm) pipeline, a dredge, supporting tugboats and other water craft and earthmoving equipment to shape the site.
“Under normal conditions, it would have taken us one to two months,” he said. “But one of the reasons the Corps came to Great Lakes is that we are a company that has the resources to perform in an expeditious way.”
The company’s fleet of dredges and other marine equipment has performed such work all up and down both coasts of the United States.
Repair of the Hatteras Island breach is entirely a sand operation. Schmitt was asked if heavier materials like rock would be dropped in the new inlet as a foundation for the sand.
“No, no, no, no,” she replied emphatically.
State law precludes use of hardened material or structures that might normally be employed. Officials resist such permanent fixes because the natural beauty of the Outer Banks is sacrificed when they are above water and the fragile ecosystem is put at risk beneath the water’s surface.
Besides, they don’t work all that well. Don Conner, Division 1 engineer of NCDOT, said his crews found in the new breach obvious pilings for a bridge or barricade structure.
“There apparently had been an inlet at some point in the past,” Conner said. “It filled up on its own.” He added that “nothing indicates this one is filling up on its own.”
Conner’s division will repave N.C. 12 beginning this week. Two lanes of asphalt will again be laid atop the compacted sand.
Aside from stiff regulations, replenishing the barrier island’s sand is complicated by other environmental and historical concerns. One of them is unexploded ordnance from World War II that fell to the ocean bed off Hatteras. The Corps is understandably reluctant to start sucking up sand in an area where a dredge might also suck up a bomb.
So engineers looked to the Sound for sand. Underwater surveys led the Corps to an 18-ft. (5.4 m) deep ferry channel 6 mi. south of the breach. Some 550,000 cu. yds. (421,000 cu m) of sand was required to fill the breach. A core sampling of the underwater borrow pit in the channel indicates it contains about three times that much sand, according to Schmitt.
She added that getting the sand from the channel has the positive effect of also dredging the channel, which was filled slightly by the storm. Thus, Outer Banks travelers will benefit twice from one operation.
In early October, Great Lakes Dredge moved into the Sound its cutter section Dredge Illinois, a 320-ft. (98 m) long and 67-ft. (20.4 m) wide specialty craft. Four tugs and workboats also were moved into place.
The Illinois can dig to a nominal depth of 80 ft. (24.3 m). Among its custom features is an American pedestal crane fitted to the deck. Because the sand was pumped farther than the 20,000 ft. (6,066 m) the dredge is designed to accommodate, a floating booster pump was employed.
The operation actually used three pump units. The first, a 1,500-hp (1,100 kW) ladder pump, dangles under water between the dredge and the borrow pit.
The second pump, a 7,200-hp (5,400 kW) unit, is on the dredge itself. A third, the booster pump floating between the dredge and the shoreline, also is rated at 7,200 hp.
The mechanics of moving tons of sand along miles of pipeline is keyed to specific gravity, Morrison said. “It’s all determined by how much pump pressure you have and how far you have to pump it.”
If the pressure applied to a solution can create constant turbulence and forward motion, sand will keep moving. If the movement slows to a critical level, the line can become plugged.
Dredge engineers constantly gauged movement of the material, monitoring the density and velocity of transfer and the pipeline’s incoming and discharge pressures. If a problem developed, said Morrison, a lighter solution could be created — that is, more water could be added — which eventually might work its way to the trouble spot and mitigate the plug.
Core sampling in the borrow pit indicated the coarseness of the sand was desirable for pumping. Almost any grade of sand can be pipelined, the engineers said, but production is less efficient with some grades than with others.
The sand slipped through the pipe at a rate of 15 to 20 ft. (4.6 to 6 m) a second. That means the trip from borrow pit to island breach took nearly half an hour.
In an hour, 1,200 to 1,500 cu. yds. (917.5 to 1,150 cu m) of sand was relocated, Morrison said, which is the equivalent of 150 truckloads of sand. Because two crews kept the sand moving 24 hours a day, typically between 25,000 and 30,000 cu. yds. (19,000 and 22,900 cu m) of sand was shifted northward 6 mi. by day’s end.
At the discharge point, the solution spilled out at elevation into inlet waters with the sand settling to the bottom. “Training dikes” on the seaward side of the inlet were built first to protect the fill area against the current that cuts through it at 6 to 7 nautical knots.
As the project continued and the level of dumped material rose, sand piled up at water’s edge at high tide and was pushed into the breach at flat tide. Caterpillar bulldozers pushed it.
Three Cat D-7 dozers and one D-8 worked the sand. A Cat 966 outfitted with a timber fork also was on site and was used to pick up and haul the 40-ft. (12 m) sections of pipe.
“It is continuous work and extremely hard on the machinery,” Morrison said. Ideally, however, the machinery is underutilized because the gritty material is flowing into place without much assistance from tracked equipment.
“We tried to let the dredge do most of the work,” Morrison said. “We’re not too unhappy when machinery is just sitting there.”
The process of replenishing beach areas hasn’t changed much from the earliest days of the restorative work. Schmitt said the offshore equipment gets larger, the pumps more powerful and dredging more precise.
“But we’re still talking big boats and sand and pipe,” she said.