Billy Mack of First Coastal, a coastal engineering firm based in Westhampton Beach, went before the East Hampton Town Trustees on Sept. 13 to ask permission to buttress a dune on the ocean beach, just east of the southern end of Wainscott Pond, with a low wall made of sand cubes.
The trustees own and manage the beach there on behalf of the public. The house owned by First Coastal’s client sits behind the dune in question, with the pond at its back. Tropical Storm Irene has narrowed the ocean beach there, and Mr. Mack said there was danger of an ocean-to-pond breach during the winter months.
His concern, he said, was based on the current state of a “sand wave” whose trough corresponded to the beach in front of the property. The eroding trough was deepened by Irene. Mr. Mack took aerial photos to illustrate his prognostication.
Waves of sand are the source of this area’s most profound beauty, and of an increasing array of challenges. The brouhaha that developed when a resident of Georgica Beach installed fencing to mark the seaward boundary of her property (and claim the only remaining beach) was caused by the trough of another Irene-induced sand wave that swallowed the rest of the beach, less than a mile from First Coastal’s Wainscott client.
Elsewhere along the coast, the alternating buildup and disappearance of beach has prompted other private-versus-public-beach debates and lawsuits.
On Tuesday, Mr. Mack and Aram Terchunian, First Coastal’s principals, met a reporter at the Princess Diner in Southampton to shed light on the mystery of sand waves, their cause, and what they can and cannot predict.
Over coffee, with aerial photos, a computer, and a sketch pad, the coastal engineers showed how sand waves are formed when the shape of ocean waves, their oscillating design of peaks and troughs, is transferred, actually sculpted by wave energy, into the same physical pattern in beaches and near-shore bottom.
Seen from the air, the south-facing shoreline is a wavy line with peaks and troughs. Sometimes seen from the air is the mirror image of a beach’s pattern of peaks and troughs carved into offshore sand bars. On the beach, the trough of a sand wave appears as a shallow cove with an eroded, narrow beach at its most landward point. The peaks of sand waves extend seaward on either side of the trough. The peaks are areas of sand buildup, or accretion.
Mr. Terchunian summoned Google Earth on his computer to bring forth an aerial image of ocean waves caught in the act of breaking, with white water from the broken waves forming the same oscillating design, a line snaking along down the beach.
The pattern is not always visible. During the summer months, it is masked by our predominant southwesterly flow of wind and waves. Wave action is strong enough to transport sand and rebuild beaches, but not strong enough to spur the currents that make the cuts in offshore bars, which, in turn trigger the sand-wave machine.
Mr. Terchunian described how storms with their powerful waves, tidal surges, and currents act as X-rays, or M.R.I.s, that reveal “the bones, the soft skeleton” of the sand-wave phenomenon.
The more easterly, often storm-driven, waves and weather during the winter months are responsible for the east-to-west march of sand waves along the coast. Mr. Terchunian said he first realized the importance of sand waves during the stormy early ’90s.
“I had a client at Old Town Pond in Southampton. Their beach was heavily eroded, but Fowler Point nearby was fine. During the big storm, [the wider beach] moved 1,000 feet. I said, how did that happen?”
Mr. Terchunian said he sent before-and-after aerial photos of the event to the late Nicholas Kraus, a native Long Islander and renowned coastal engineer, who in turn passed them on to Michelle Thevenot, a doctoral candidate. Ms. Thevenot wrote her thesis on the sand- wave phenomenon. These days, Mr. Terchunian said, the Army Corps of Engineers charts sand-waves and includes them in their plans when nourishing eroded beaches with sand.
As for using the sand-wave pattern to predict beach vulnerability, Mr. Terchunian and Mr. Mack said it was possible up to a point.
For example, the sand cube project near Wainscott Pond seemed justified, they said, by the beach’s configuration. Mr. Mack pointed to an aerial photo that showed how the deep trough of a sand wave threatened the client’s property. What’s more, it revealed what was not visible underwater just offshore, that being the trough’s mirror image — an opening in the sand bar.
Such openings are caused by strong currents moving offshore, the result of waves and storm surge piling up close to the beach and seeking an escape route. The escape currents cut paths through the offshore bars. The openings are the offshore troughs of a sand wave. They permit waves to pass through without the slowing effect of the bar en route to the beach, thus increasing their eroding power.
A strong storm could cut through the already narrow beach and unite the ocean and Wainscott Pond, or at least add to serious flooding that occurred behind the house during Irene. This was the concern that took Mr. Mack before the trustees. He noted that the dune in front of Ronald Lauder’s house, just 300 feet away, suffered no loss with the help of sand cubes at the toe.
Mr. Terchunian said photos of the beach just prior to the infamous 1938 Hurricane showed troughs in the sand waves, and narrow beach where storm surge inundated the land during the big storm.
“Sand waves are more important than dune” in determining where breaches and over-topping will occur, Mr. Terchunian said. “Dunes are just levies, the beach is doing all the work.” Along the same line, he said downdrift scouring of sand, caused by hard structures such as jetties, was certainly a cause of erosion but was trumped by the grander scale of the sand-wave phenomenon.
He said the jetties at Georgica had, over time, disrupted the “natural balance” of the sand’s comings and goings. Even if the jetties were removed, it would take years to regain it, he added.
Both men agreed that predictions based on sand-wave patterns could not extend too far out in time.
They said that sand waves move down the beach year to year, but the speed of their progress depended on so many variables that long-range predictions as to exactly where and when serious erosion will take place were virtually impossible to make.
Mr. Terchunian pointed out that coastal ponds such as Wainscott, Georgica, Sagg, and Hook did, at times, join forces with the trough of sand waves. “There is a fluid connection from pond to ocean,” he said. Breaching occurred when pond water under pressure to flow seaward turned the permeable barrier of rock and sand into a fluid mass that met storm surge moving shoreward.
First Coastal has been hired by the Sagaponack Village erosion control district to study the makeup, depth, and back-and-forth flow of sand. “This is all temporary until they come up with a long-term solution,” Mr. Terchunian said, referring to the kind of spot-beach nourishing, sandbagging, or sand-cube buttressing of dune the company has been involved in.
Offshore deposits were likely the future source of Sagaponack beach sand, he said. Mining sand at least a mile from shore in water that was at least 30 feet deep would guarantee that it tapped deposits safely beyond the reach of the naturally replenishing sand wave, so as not to starve the system.
“Long Island has such wonderful beaches, sand riches, glacial outwash sand. We are passionate about beaches,” Mr. Terchunian concluded.