Pleasant Bay and the Nauset Barrier Beach system will survive sea level rise, but not without potentially significant impacts.
With a projected sea level rise of between 1.2 and 2.9 feet, Pleasant Bay could lose one quarter to one half of its 392 acres of landside intertidal resource area by 2100, according to a new report from the Pleasant Bay Alliance. Revetments along the inner shore will exacerbate the loss by preventing beaches, salt marshes and tidal flats from moving inland. It's possible that this could impact public access to the shore as well as infrastructure and private property, the report states.
While the Nauset/North Beach barrier system that protects the bay from the Atlantic will remain intact, its configuration and rate of inlet formation and evolution will be different than it has in the past under sea level increases examined by scientists. What has for centuries been a 140- to 150-year cycle of breaching and regrowth could be shortened by half under the highest rate of sea level rise, and the beach could move westward at a quicker pace, according to the report.
Revetments are also likely to have an impact on the barrier beach process, said Coastal Geologist Mark Borrelli of the Center for Coastal Studies in Provincetown, which prepared the report for the Alliance. This is already seen with “unnaturally deep” water off Holway Street in Chatham. Revetments will prevent sediment from entering the system, increase the rate of erosion in other areas and shrink intertidal resource area, which are important for storm protection, pollution filtration and habitat, he said.
The implication of sea level rise for the 9,000-acre Pleasant Bay estuary – shared by Chatham, Harwich and Orleans – are significant, said Alliance Coordinator Carole Ridley. The bay is “really sort of ground zero for the aquatic food chain,” and its habitats depend on the proper functioning of a complex coastal system that includes erosion of beaches and landforms. In commissioning the report, the Alliance believed that it is important for future management of the bay's resources to understand how sea level rise will impact the system, she said.
“We felt it was important to get to the bottom of it in terms of the best available science,” she said.
The study looked at both global and regional sea level data, Borrelli said. Sea level increased about one foot over the last century, and the study looked at three scenarios for sea level rise through the year 2100: a low estimate of 1.2 feet, a mid-range of 2.1 feet, and a high rate of 2.9 feet.
“A lot of people think that's conservative,” he told an audience of about 100 people at the Chatham Town Hall Annex last Thursday.
Those numbers were plugged into computer models using existing data on shoreline and barrier beach changes. There is “really good” historical data on the movement of the barrier beach going back to the 1770s which shows the 140- to 150-year cycle as well as the gradual migration, or rollover, of the beach westward.
“This is what you want a barrier to do,” Borrelli said, calling the barrier beach “literally the first line of defense” for the bay.
What's critical is for that barrier beach process to keep pace with sea level rise, he said. Under the low estimate, the cycle continues unchanged, but under the mid and high ranges, the barrier beach process begins to accelerate. With a 2.9-foot increase in sea level, the cycle would be shortened to 75 years. Under that scenario, the 1987 inlet opposite Lighthouse Beach will close – the April 1 break in South Beach appears to be part of that process, as the sand spit begins to erode and move westward – and the 2007 inlet will become dominant; the beach will then grow south and a new inlet will form in about 2070. Low areas will experience more overwashes and wider areas of the beach will lose ocean-side beachfront.
“This would represent an increasing dynamic system and the uncertainty associated with future predictions on coastal evolution would in turn increase accordingly,” the report states.
“This is going to get more complicated,” Borrelli said at the presentation. “This is going to get more tricky.”
That is “obviously a major concern” for Chatham, said Coastal Resources Director Ted Keon, “because it's happening faster than in the past, and because of human intervention areas are not able to migrate” as they would naturally.
The good news, he added, is that the report indicates the process will continue and the barrier beach won't “drown in place” or completely disappear beneath rising ocean levels.
A combination of sea level rise and coastal engineering structures will lead to the loss of one quarter to one half of the inner shoreline's 392 acres of intertidal area under the scenarios. As water levels rise, seawalls will prevent the area between low and high tide, the intertidal zone, from moving inland, and will in turn cause erosion of beaches immediately seaward as well as on either end of revetments, according to the report. Intertidal areas fronting revetments are “the most vulnerable to increases in sea level,” it states, and are already thinning in some areas of the bay.
Intertidal area also include the marshes and wetlands that are the bay's most valuable resources, Borrelli said, serving as a nursery and feeding ground for dozens of species of birds, fish and shellfish.
Not factored into the report is an increasing intensity of coastal storms, which some experts say is already happening. In fact, said Borrelli, in the short term coastal storm will be a bigger factor in shaping the barrier beach and inner shoreline than sea level rise, but sea level changes will make it less likely beaches can respond to storms as they are designed to do.
“Large coastal systems such as Pleasant Bay have a vast capacity to evolve and adjust to a range of changing conditions,” he said. But “measures such as coastal armoring, while intended to prevent short-term erosion, actually reduce the system's ability to respond to dynamic changes by starving it of sediment.”
Resident Robert Zaremba questioned whether the seawalls along Chatham's inner shoreline will keep the barrier beach from reforming from the fish pier south by preventing erosion from freeing up sand to feed the system.
“I think that has huge implications for what the Old Village will be confronted with, basically open ocean,” he said.
Borrelli said the Cape Cod National Seashore's hands-off policy ensures that the outer beach dunes in Truro and Wellfleet will continue to supply sediment to the system. If seawalls were built in those areas, “then you'd be out of luck, you'd have major problems,” he said.
The report, added Keon, lays out the case for the importance of erosion in the creation and maintenance of beaches, marshes and tidal flats. While it doesn't say nothing should be done to protect property and resources, it points out the need to “not jump to the most biggest, the most robust [shoreline protection] because of the potential impacts projects like that can have.”
The Alliance also released draft guidelines for managing coastal erosion in Pleasant Bay. Ridley said the recommendations will help towns along the bay make policy decision and protect public resources and infrastructure.
“We view these as a tool for folks who live on the coast, for conservation commission and conservation commission members, for anyone interested,” she said. “With this information the Pleasant Bay communities are in a better position to assess potential impacts to resources and infrastructure and evaluate management strategies and policies available to address them.” The Alliances is accepting public comment on the guidelines through June 30.
Consideration of revetment requests and other management practices that impact the sediment transport processes of the barrier beach and Pleasant Bay systems need to take the sea level scenarios into account, the report states, and that will require “preservation of natural resources with protection of public and private property, infrastructure and access points.”
The sea level rise report and erosion guidelines are available on the Alliance website, www.pleasantbay.org.