Nantucket Sound Stabilization Methods Examined

By: Tim Wood

Chatham's Nantucket Sound coast, looking east from Red River Beach in Harwich. The town is examining ways to stabilize the shifting shoreline. SPENCER KENNARD PHOTO

CHATHAM – For years, the town has taken every available opportunity to add sand to Nantucket Sound beaches, especially in the Cockle Cove area. Erosion nonetheless remains a major problem along the south shore, and addressing it in a comprehensive way will take more than just beach nourishment.

“While that's very valuable, it's not sufficient to really maintain the beaches as we'd like,” said Coastal Resources Director Ted Keon.

Options for keeping sand on the beaches could include shoreline structures, but will certainly continue to require sand replenishment, said John Ramsey of Applied Coastal Research and Engineering, hired by the town to assess alternatives to address erosion along the south coast.

“Nourishment is going to be a critical component to making sure you have long-term viability,” he said while presenting shoreline options to members of the south coastal harbor plan committee recently.

A chief reason Chatham's Nantucket Sound beaches are starved of sand is that the large breakwater at the Red River inlet blocks west-to-east flow of sand along the south coast. It's one of a series of structures built along Nantucket Sound during a 30-year period following the Hurricane of 1938 that has essentially interfered with the natural transport of sediment, impacting the stability of the shoreline. The situation has segmented the shore into three distinct areas, each with its own unique problems, and potentially its own solution.

“This system, maybe not as dramatically as the east side of Chatham, certainly has some serious changes that have happened over the last several decades,” Ramsey said.

The westernmost segment is from Red River to Mill Creek. It contains 14 rock groins that trap sand and have been “somewhat effective” at reducing erosion, according to Ramsey's report. It has, however led to “chronic long-term erosion” of the shore immediately to the east. That section, the second segment along the south coast, runs from Cockle Cove to Ridgevale Beach, where there has been significant erosion at least since 1998, when the town first began adding sand. The final segment is Harding's Beach, which receives sand flowing from west to east, but is also subject to higher wave action. The most eastward section is most in equilibrium, Ramsey said.

Potential alternatives to help stabilize the shoreline include continuing the current beach nourishment program; increasing the scale of beach nourishment; installation of sand-trapping structures like groins; offshore wave attenuation structures such as artificial reefs or submerged breakwaters; revetments or seawalls; or “soft” shoreline protection such as sand filled bags.

Each segment may require its own solution, Ramsey said, and a combination of alternatives will likely be most effective. A submerged breakwater, for instance, may help attenuate waves in the Harding's Beach area to cut down on erosion, while groins could help hold sand in place in locations to the west. Continued addition of sand will likely be required in all three areas.

“There's never a place where one of these solutions will work all on its own,” he said.

Ramsey evaluated a number of alternatives, looking at construction cost ranges from low (less than $500 per linear foot), medium ($500 to $1,500 per foot) and high (more than $1,500 per foot). In the low range were continued beach nourishment efforts, but that also had a design life on the low side, from 20 to 50 years. Also in the low range was building wood groins in the Cockle Cove/Ridgevale Beach areas. Medium cost alternatives included reconstruction of a wood bulkhead extension on the east Mill Creek jetty and submerged breakwaters off Harding's Beach. Alternatives judged to have high costs included rehabilitating the existing Mill Creek west jetty, stone groins between Cockle Cove Beach and Ridgevale Beach; and above-water breakwaters at Harding's Beach. Those solutions also had the highest design life, from 30 to 50-plus years.

Removal of the existing groins, as some have suggested as a solution to the erosion problem, will likely only provide temporary relief, with the sand released by the groins likely depleted within five years, according to Ramsey's report. The groins also help protect upland infrastructure by holding beaches in place.

“Without these structures, natural coastal erosion processes would alter the form of the beach and impact existing public and private waterfront development in South Chatham,” the report reads.

The efficacy of permitting is also a factor, Ramsey said. Groins, for instance, are a “super challenge to permit in this state.” Wood structures, which could be removed if necessary, may be easier to permit, he said.

Even if the town can get permits to build structures like these, continuous beach nourishment is likely to be a permitting requirement, Keon said. “That's a big deal,” he said. The amount of sand necessary could be “a pretty big number.”

“That's a condition you probably won't be able to get away from,” he said.

Another factor that's emerged is the continuing shoaling of the Stage Harbor entrance channel, mainly from the east due to the washing away of South Beach. This is creating a “sediment nightmare” that is pushing tons of sand west toward the channel as well as eroding Crescent Beach, said Keon. The town secured a state coastal resiliency grant to look at the situation.

“We're trying to get our hands around that as fast as we can,” he said. “We're focused on really a potential crisis scenario for this area,” while the situation along the Nantucket Sound shore to the west is ongoing.

No matter what alternatives are chosen, it will take decades for the south shoreline to stabilize, Ramsey said. “I don't want anybody to think there's some magic panacea here that can give stability to the shoreline, especially if we get the higher levels of sea level rise people are projecting,” he said.

Ramsey will now evaluate the alternatives, their cost and permitting viability. He projected having a draft ready by the end of January.