When homes began to be threatened on Taylor's Island on the coast of Delaware Bay, it became clear something had to be done about the shore erosion that had been occuring for decades. A narrow stretch at the tip of Taylor's Island had lost an estimated 200 ft. to erosion since the 1980s, threatening a handful of homes.
Plans had been made to stabilize the shoreline since 2000, with federal funding finally approved in 2007. The Army Corps of Engineers financed 75% of the project, with Dorchester County funding the remaining 25%.
Shoreline Design, Edgewater, MD, was awarded the bid on the $1.5 million project and began work in July 2007. Company owner Wes Matheu previously worked as a project manager for the Army Corps, as well as with several marine contractors, before starting his own company in 2003. He was already familiar with the situation on Taylor's Island.
"I actually looked at this project when I worked for the Corps about 12 years ago and the people out there were trying to get something done," Matheu notes. "The Corps will spend federal dollars on a project as long as it's protecting a federal roadway or public infrastructure."
Over the years, Matheu had seen the erosion the Delaware Bay tides were causing in the area. He also witnessed the efforts to mitigate the erosion while funding was pending. The existing shoreline stabilization material "was kind of haphazardly dumped and it was maybe a couple of feet over the high tide at best," he recalls. "The tide ranges about 2 to 3 ft. in this location, and it's not just the tides. The wind kicks up and the fetch - which is the distance the wind blows across the water to form a wave - is very wide here."
The new revetment wall was designed to have a height of about 3 to 4 ft. higher than the existing material. The toe of the slope at the bottom of the bay is located 36 ft. from the shoreline and is itself 4 ft. wide. The revetment would be a total of about 9 ft. high with a 2:1 slope.
Tricky conditions require accuracy
The first task was to take away the existing rock along the shoreline. "The process was to strip the shoreline and sort the debris," says Matheu. "They wanted us to save [the bigger rock] that was 400 lbs. on up. Everything smaller was supposed to be stockpiled in the yard. We used some of the small stuff to rebuild the subgrade, and we would excavate the toe, then cover everything with a layer of No. 2 stone."
Toe construction required excavation of about 2 ft. of soil at the bottom of the bay and placement of armor rock in the excavation. "We never really saw the really low tides," says Matheu. "We always had at least 4 ft. of water we were working with, so you could never see what you were doing when you put the toe in."
For surveying, Shoreline Design brought in its Topcon HiPer Lite+ base station and rover "fourth sensor" utilizing Global Navigation Satellite System (GNSS) technology. The excavator bucket - which held the GNSS rover antenna - was lowered to the surface of the water in order to mark the depth of the subgrade and, later, the elevation of the toe stone.
As a quality control measure, the GNSS was periodically recalibrated to the bucket. "We probably handled about 15,000 to 20,000 tons of rock on this project, so the teeth on the bucket actually wear [down]. The teeth are the measuring point, so occasionally we would recalibrate the bucket and check it against the hand-held [field monitor] just to make sure we were as accurate as we could be," says Matheu.
In addition to surveying, the GNSS was used to verify the elevation of the revetment for inspectors. "Normally, when you do this type of work, you have to put a lot of stakes in the ground, measure up to your 2:1 slope and figure out where the crest of your structure is going to be, which is an imaginary point," Matheu says. "Some agencies require contractors building revetments to build a 2x4 template so that the inspector can look up and down the line to see if you're hitting the right elevation. It's a lot of field work to build one of these. We saved a lot of time that way."