Sand variation, dredge site impacts concern ConCom
BY PETER B. BRACE INDEPENDENT WRITER
It may seem trivial to haggle over sand grain size, or what happens to shoals when sand is taken away from them when one ponders the volume of sand under the ocean.

But the Conservation Commission, and many Nantucketers, considers every grain of sand, and every shoal and species of sea life to be an integral part of a larger marine ecosystem.

It is little wonder that the Siasconset Beach Preservation Fund is having difficulty assuring the ConCom and island residents of the safety and negligible impact its project will have on this ecosystem.

At the third meeting held by the Conservation Commission on the Siasconset Beach Preservation Fund's (SBPF) proposal to use 2.6 million cubic yards of sand from the ocean bottom to rebuild three miles of beach in 'Sconset, the ConCom picked apart SBPF's dredge site plan, questioning how it is that this private group of bluff residents can ensure that all natural systems will return to normal over time.

The dredging area, which SBPF calls its borrow site, is a 195-acre portion of sea bottom situated 2.5 to 2.9 miles east of Nantucket in water 30 to 60 feet deep. Rick Spadoni of Coastal Planning & Engineering of Boca Raton, Fla., told the ConCom and the roughly 40 people in attendance at the meeting that SBPF would use a hopper dredge to suck sand off the ocean bottom. The 3,000 or 6,000-cubic-yard capacity vessel would then steam close to the beach, hook up to a submerged pipeline and pump the sand in slurry form onto the beach.

But it is not this aspect of the project that has the ConCom worried. With natural populations of surf clams, winter flounder, scup and several species of crabs living in the dredging area - coupled with the unknown impact on wave energy and wave height as the dredged area gradually refills with sand - the Conservation Commission peppered SBPF's engineers with impact and mitigation questions.

According to the engineers, SBPF cast 177 miles of transect lines from which they harvested 117 sand grain samples, finding what it believes is the dredge site with the most compatible grain size to the existing beach sand - that is, medium to coarse grain sand.

But ConCom member Bob Rudin pointed out that the borrow site sand is coarser than sand already on the beach.

"You're going to cover the current beach with a much coarser material and a much larger grain size," Rudin noted, who also questioned the impacts on shorebird survival. "Right now, you have a surface layer that does not have any gravel on it, and if you put an 11- percent gravel content on the beach, I'm wondering … will that deter nesting of piping plovers and least terns?"

Steve Barrett, Director of BlueWave Strategy, and SBPF's wildlife expert, said plovers and terns thrive on beaches with all types of grain sizes. Spadoni then said that after equilibration - when the dynamic of currents, waves, tides and wind returns the beach to its normal state, albeit with much more sand - the sand grain size differences will even out.

"After equilibration, the finer grain sands will head offshore and the heavier grain sand will end up in the surf/swash zone," said Spadoni.

Rudin, and other ConCom members were also concerned with how taking millions of cubic yards of sand from the sea bottom would affect waves hitting the island's eastern shores, but Spadoni said there would be little or no impact

"What we found in the model is very little change in wave height," said Spadoni. "Actually, closest to shore, we saw a reduction in wave height as a result of dredging the borrow area. Even the wave direction is virtually unchanged. Both wave height and wave direction due to excavating the borrow area are insignificant."

Rudin persisted, saying that he is worried more about how the shoal behind the dredging area will be affected once the sand is removed. Other ConCom members, including Sarah Oktay, were troubled by the engineers' lack of information on species of finfish and invertebrate fish species in the dredge area, and the consequences the fish will deal with during and after dredging.

"In the extreme case you would be losing a whole section of the shoal," said Rudin. "My concern is that we could end up with a half-mile long hole in the shoal."

But Spadoni said the dredge site would refill gradually as waves of sand flow in from the north, and fill in rapidly during 50-year storm events.

Concerned about marine life survival, tied to the restoration of natural conditions on the ocean floor, Oktay questioned the thoroughness of Barrett's fish sampling efforts and shared Rudin's apprehensions about the slow rate of the filling in of the dredge site.

"That's a very slow infilling rate," she said. "I've seen one to three years at best and in some cases, I've seen eight to 11 years."

Oktay also wanted more information about how the underwater sound waves of the project could affect fish and marine mammals such as whales, seals, dolphins and porpoises that all move through the area.

The SBPF presentation will continue over at least two more meetings scheduled for this afternoon and Monday afternoon at 4 p.m. in the Large Group Instruction room at the high school. SBPF promised to answer both the ConCom's and public's questions at a later time. Answers to questions raised during the Sept. 10 and Sept. 17 meetings are available in print form at the ConCom office at 37 Washington St. Call

228-7230 for details. I