TRURO — The nor’easter that struck the Outer Cape on Oct. 10 and lingered for almost three days caused massive erosion at Ballston Beach. Sand dunes and bluffs were carved away, exposing the bare clay beneath.
The new scenery is unfamiliar — almost Martian in its hues and its bizarre ridged formations cut into the clay — and some observers were alarmed by the magnitude of the changes, worried that the pace of erosion has picked up dramatically.
National Seashore cartographer Mark Adams confirmed that Ballston Beach has been overwashed in storms repeatedly in recent years, and that those events may be happening more frequently. “But we don’t have excellent records before the 1990s of how often this happened,” he said. “It remains to be seen whether this is a dramatic break in history. If we start getting this every year, then yes, that will be something we haven’t seen in our lifetimes.”
The ‘newest land in the world’
Naturalists say that erosion at Ballston isn’t necessarily bad. Adams said that the sand eroded from the beach is not lost forever. It’s just offshore.
He explained that, as waves scour the bottom of the bluffs at high tide, eroded sediment becomes “the raw material for building coasts, or beaches and dunes.”
In Cape Cod Bay, winds tend to blow from the south. As waves wash sand from bluffs and carry it northward, sediment is transported up the coast towards Provincetown.
Safe Harbor Environmental Services director Gordon Peabody said that this sand has built the land that is now Provincetown, which wouldn’t exist without erosion in Wellfleet and Truro. “That’s some of the newest land in the world,” he said.
Adams explained that there’s a basic difference between a sand dune and a bluff: dunes are formed by material that has already been eroded from bluffs by wind or waves, whereas bluffs are made of original material deposited by glaciers.
“Everything in those bluffs has never been in the ocean before, at least not in several thousand years,” Adams said. On the other hand, “The sand on the beach has been mixed and polished by waves and tides. You can feel the difference between a closely-packed bluff and free sand.”
When sediment is eroded from bluffs and enters the ocean, only the finest grains are lost, about 10 to 20 percent of the total. The rest remains in the ocean, ready to build up new dunes and beaches.
The bluffs have been retreating at a steady rate of 3 to 5 feet per year for millennia, Adams said.
Changes in the sand dunes, on the other hand, are “highly variable,” he said. “There were several storms in 2015 and 2016 that took those dunes away.” A village, complete with a bowling alley and community center, even used to inhabit the Ballston dunes.
Once lost, dunes often rise again, built up by sand from the ocean. “I have no doubt that [the Ballston dune] will reform because there’s tons of sand in the nearshore waters that could be dropped off there in the quiet weather,” Adams said.
Breaches cause concern
The incursion of salt water into the Pamet estuary when the Ballston dunes are breached is cause for concern.
The Pamet River formed when a coastal pond emptied into Cape Cod Bay several thousand years ago, carving a channel into the glacial bluffs. Only the sand dunes of the barrier beach at Ballston protect the head of the river from the ocean.
Without those dunes, salt water could enter the river from the ocean more frequently. “How will that change the river or affect peoples’ wells?” Adams asked. “There’s been enough studies to say salinity isn’t really a risk, but it’s a good question to ask.”
He said that flooding in the Pamet “is actually a benefit to the river.” That’s because waves wash sediment into the marsh. “That’s a new base elevation for salt marshes, for habitats,” he said, “so the land can keep up with sea level rise.” The Army Corps of Engineers and the town have discussed restoring the Pamet River by removing a tide gate, allowing more salt water into the upper estuary.
Intense storms are becoming more frequent as the planet warms, and climate change may also lead to slower-moving storms. If a storm lingers through several high tides, erosion increases. “The more tides in a storm, the greater your risk” of extreme erosion, Peabody said.
The increased risk has to do with a relationship between sand bars offshore and wave action. Typically, a sand bar diffuses wave energy by about 25 percent, protecting the shore, Peabody said. But during a storm, wind pumps waves over the sand bar, loading waves into the water between the sand bar and shore.
“That results in a difference in elevation between the water next to the dunes and offshore,” he said. Eventually, the weight of the water near shore is great enough to rip a channel through the sand bar.
With a hole in the sand bar, full-strength waves can reach the shore, leading to “exponential erosion,” Peabody said.
Although he can’t be sure what happened at Ballston two weeks ago, Peabody suspects a rip channel formed. “Especially with so much clay removed, rip channels may have been implicated,” he said.
There are some ways to cope with and adapt to beach and bluff erosion.
First, it’s important to acknowledge that the landscape is dynamic. “The best thing to do is get out of the way” of sand dunes and coastal erosion, Peabody said.
Another strategy is ensuring that beach access paths don’t cut straight through bluffs or dunes. Instead, they should zig zag. Plants prevent erosion by holding sediment in place while also capturing wind-blown sand. “As soon as you kill plants by walking on them, the wind will take out the sand. Then the ocean comes through,” Peabody explained.
Other ideas like beach nourishment — that is, adding new sand — could be considered. But hard infrastructure like sea walls, revetments, or offshore jetties often fail, as they deflect wave energy instead of absorbing it. Erosion rates tend to increase in surrounding areas when sea walls are installed.
Although it’s hard to cope with a constantly changing landscape, Peabody thinks that the Outer Cape is suited to taking on the challenge. “On Cape Cod, everyone realizes we’re stewards of our natural resources,” he said. “It’s an intentional community. People pay a lot of attention.”