Sacrificial Sands, by Tim Kana

I suspect the Montauk sandbag seawall provided some protection to properties before it was damaged during Hermine. If the reduced damages exceeded the cost of the seawall, it was probably worth the expense. But as a long-term fix for beaches, seawalls of any type are problematic. They do nothing to restore or hold a sandy beach in place.

Nourishment is also problematic if it entails only sand placement on the dune or on the dry sand beach. And therein lies the problem with many beach protection efforts. Some of the post-Sandy projects are small scale and only place sandbags, nourishment, or vegetation in the dune area. That helps a little and may provide some immediate protection to vulnerable buildings, but it doesn’t fix the whole beach — particularly the underwater part.

My firm helped the town of Nags Head, N.C., on a post-storm dune project after Hurricane Isabel a dozen years ago. The Federal Emergency Management Agency offered enough money to rebuild a dune 6 feet high and 15 feet wide at the crest. What did the town get for $7 million? Only about 400,000 cubic yards, or 25,000 truckloads, of sand, at nearly $18 per cubic yard. And what happened to the emergency dune? Most of it washed out within a year, and the sand became mixed thinly with the beach or underwater bars. That’s what happens when you do small projects and place all the sand — or sandbags — on the dune.

Think of the beach-dune system as a house. At minimum the house has a roof, first floor, and foundation. So, by analogy, the roof is the foredune and the foundation is the underwater bar. That leaves the first floor to represent the recreational beach — where we like to spend most of our time.

When storms come along, we first worry about the roof collapsing because that can ruin our living quarters. Similarly, if dunes wash out and leave buildings open to the sea, the natural reaction is to rebuild the dune A.S.A.P. But such repairs won’t be adequate for long if the foundation has been undermined. And that’s the simplest way I know to explain why beach nourishment works best when it places copious amounts of sand underwater as well as in the dune or on the recreational beach.

Back at Nags Head, the town followed the Isabel dune with a 4.6-million-cubic-yard nourishment project costing about $30 million for construction in 2011. That was over 10 times more sand than the Isabel dune. We recommended that all the sand be placed on the recreational beach and underwater. No sand in the dunes. Economies of scale, offshore borrow areas, and large dredges brought the unit costs down to less than $7 per cubic yard. 

Five years later, 90 percent of the nourishment volume remains within the 10-mile placement area, and almost one million cubic yards — 20 percent of the sand — has shifted into the dunes, building them up at a rapid rate. 

Many experts predicted the same outcome for the Nags Head nourishment as the Isabel dune — rapid losses and failure of the project in the first big storm. So far, Nags Head has weathered Hurricanes Irene and Sandy with negligible damage to property and only small losses of sand each year. And the dunes continue to grow.

The 2014 nourishment project at Bridgehampton and Sagaponack appears to be performing like Nags Head. Sand is shifting to the dunes as well as the underwater zone, enhancing the entire beach profile while staying more or less in place. When there is ample sand underwater, the visible beach has a foundation to hold it in place. And if the dry sand beach stays wide through the winter, onshore winds will shift some sand into the dunes.

Before giving up on nourishment, or opting for sandbags and seawalls, think how a beach adjusts through the seasons. Sand moves offshore in winter because bigger waves have to dissipate over a broader surf zone. Almost as sure as the sun rises, some sand will come back each summer just when we need more area for beach blankets. The shape and height of offshore bars are a signal to alert us where the beach is most likely to cut back from year to year — usually where there are gaps in the bar. If we track these changes carefully, we can determine whether the beach needs more sand and, if so, how long the added sand is likely to stay where we place it.

I’m encouraged to see that more nourishment will be accomplished along Long Island as a result of Sandy than any storm response in recent memory. Some of the efforts, like Montauk’s, are clearly just a first step. But the cumulative effect of all the added sand will be positive for the beach system and work its way downcoast over time, helping offset many years of erosion. 

The Montauk beach doesn’t have a huge supply of sand upcoast, so it will take a larger-scale nourishment effort to maintain it. If scaled up enough, there are probably some opportunities for economies of scale. But before looking into that, there needs to be a consensus within the community for a long-term solution to erosion. Whether consensus is possible is likely more uncertain than how the Montauk beach will react in the next storm.

Tim Kana, Ph.D., is president of Coastal Science and Engineering, based in Columbia, S.C. His firm designed the Bridgehampton and Sagaponack beach nourishment project, and he is on the board of the American Shore and Beach Preservation Association.