A Sinking State and a Rising Sea: Salt Marshes Provide the Answer

Rhode Island’s historic coastal cities, such as Newport and Providence, were built on ground that is slowly sinking and has been for thousands of years.

Newport’s historic waterfront may be overwhelmed with seawater in the not-so-distant future.

“We’re standing on what we’d like to think is solid ground, but anyone [who’s ever] been in an earthquake knows that it isn’t as solid as we might like to think. It moves. It shakes. It’s active,” said Simon Engelhart, a geoscientist at the University of Rhode Island, who spoke at the Coastal State Discussion Series event on February 14. “It turns out that the ground you’re standing on is sinking … and that’s what’s going to change your idea about what sea level is doing.”

Rhode Island, he said, has been sinking about 1 millimeter annually, or 1 inch every 25 years. This is a small number when compared to predictions of sea level rise that could reach nearly 10 feet in Rhode Island by the end of the century. The cumulative effects, however, have played a substantial role in present-day flooding despite the fact that melting glaciers on land, expanding warming waters, and the disruption in the speed of the Gulf Stream will play a much larger role in the future of sea level.

“It seems like such a tiny amount of sea level rise … but it’s not just that inch every 25 years that we’re thinking about now. It’s the fact that when Newport or Providence was established in the early to mid-17th century, there’s been 400 years of that inch every 25 years,” he said, pointing out that these cities have since experienced about 16 inches of sea level rise just from the land sinking. “So, when you’re dealing with a tidal range between 36 to 40 inches, that 16 inches is a pretty big shift to deal with.”

 

 

 

                                STORMTOOLS shows where Rhode Island communities
                                can expect
1′, 3′, 5′, and 7′ of sea level rise.

Today’s high tides and storm surges are now higher, meaning more flooding of roads, septic systems, and storm drains. This leads to all sorts of ramifications from impairing road access and damaging people’s homes to saltwater intrusion on freshwater reservoirs and water pollution from overwhelmed septic systems or wastewater treatment facilities.

These challenges aren’t faced by all communities equally because sea levels do not rise equally around the world. How much sea level one area experiences versus another is largely dependent on the shape of the coastline, wind directions, the speed of ocean currents, the gravitational pull on melting glaciers, and thermal expansion of warming waters.

“A large amount of the sea level rise that we’ve seen [in New England] over the 20th century and what continues into the 21st century comes from warming of the water already in the oceans,” said Engelhart.

But to fully understand how sea levels are impacting Rhode Island, he added, there needed to be a better understanding of how the land was changing on a local level.

“When we talk about sea level rise, it’s a function of what the oceans are doing and what the land is doing. We can’t think of those two as separate,” he said.

Weight of the Past

Rhode Island, and other areas along the East Coast, has been sinking since the last glacier that covered North America disappeared over 7,000 years ago. In its glory days, this three mile-thick ice sheet reached all the way from Canada to Block Island and Long Island 26,000 years ago. It was so massive that when it did finally melt the seas rose over 200 feet, said Engelhart. It also left the land underneath deformed.

The weight of the glacier, he explained, pushed the ground down the same way a loaded cargo ship would sink into the water. The ground was pushed down into the more fluid mantle layer below, which was pushed outward and raised the surrounding land, creating a bulge. Rhode Island and other areas on that bulge were elevated as a result.

Once the ice sheet melted, the land began to sink back to its original elevation as the mantle returned to where it belonged in a similar way a memory foam mattress returns to its original shape, said Engelhart.

Answers in the Salt Marsh

What was previously known about sea level in Rhode Island was based on information derived from the Providence and Newport tide gauges, whose data only go back to the 1930s, explained Engelhart.

And because the tide gauges are fixed on land, it was hard to know whether sea level changes were the result of what the land was doing or what the ocean was doing. In addition, this data only accounted for a time period when human influence on climate and sea levels had already begun.

“It’s just not long enough to put anything into context,” said Engelhart of the tide gauge data. “We need that long-term context to place the 20th-century sea level rise to understand how the coastline responded during that time … before there was significant human alteration on climate and sea level.”

Engelhart and his students turned to salt marshes for answers.

The plants and microscopic animals that live in this boundary between the land and the sea are organized by their proximity to the water. Those that like a lot of salty water are found closer to it, and those that don’t are found at higher elevations. As they die, their remains are absorbed to form a new layer of the salt marsh ‘peat’ that gradually builds over time.

Engelhart’s team dug up core samples dating back 3,000 years from some of Rhode Island’s remaining salt marshes. The types of animals and plants found in the peat layers tell how long an area has been a salt marsh and where sea level was at a certain time, “because we know from modern-day where they live relative to sea level,” he said. Measuring how much radioactive carbon is left in these plant and animal remains gives researchers an estimate on when these sea levels occurred.

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                        Over 50% of salt marshes have disappeared from Rhode Island.

                        Restoration efforts aim to protect remaining areas from pressures

                         of rising seas.

 

                           

“Salt marshes are amazing geological archives of how our coastlines have changed over time,” he said, explaining how the remains of plants and microscopic animals found in the samples reveal where sea level was at a given time period.

And what this information is showing is that sea levels are rising faster today than at any point in the last 3,300 years. Salt marshes, he explained, kept up with about a 1-millimeter change annually, but are showing signs of collapse with the 3 to 4-millimeter change currently happening.

Salt marshes are not only valuable nurseries for important species in Narragansett Bay, but they filter pollutants running off the land and minimize flooding by absorbing tidal and storm surge water.

“The key for marshes is that they have to maintain pace if they’re going to stay where they are,” he said, noting that this means giving them the space to move. “If we want them to persist, we have to let them migrate inland. If we trap them, we’re going to lose them.”

To protect them, he said, we need to determine which salt marshes are resilient and which ones will need help. Rhode Island began this process four years ago by developing the Sea Level Affecting Marshes Model (SLAMM) to identify existing marsh conditions and vulnerable areas. The model can predict where marshes will go, or how they will likely migrate, said Jim Boyd from the Coastal Resources Management Council. But there are limitations, he added, such as the model only accounts for sea level rise up to 5 feet.

“It was a first attempt by the state to get a handle on using a model and project how salt marshes would respond with future sea level rise,” said Boyd, noting that the model needs to be updated with new sea and land-level changes. “But it told us our marshes are in trouble.”

Some restoration efforts that followed included adding material to salt marshes to help prevent them from drowning. However, the process by which marshes will compact and build up when new material is added isn’t fully understood, said Engelhart, who expressed concern about these efforts.

“It’s strictly an experiment to see if it was a feasible alternative,” explained Boyd. “But it’s not the answer to our salt marshes.”

An answer may lie within hard conversations about coastal development that blocks salt marshes from migrating and coastal retreat in certain areas.

“Generally, adaptation to climate change and sea level rise is going to be very expensive, so we’re probably going to have to make difficult decisions about what we prioritize,” Engelhart said.

Dr. Engelhart’s work looking into past sea level changes in Rhode Island is supported in part by Rhode Island Sea Grant. Results from this work will provide a baseline for coastal managers to build a better understanding of future sea level rise and coastal hazards.

 

RELATED STORIES

+ GOLOCAL LIVE: URI Researcher Engelhart On Impacts Of A Sinking State & Rising Sea Levels

+ WPRI12: Rhode Island is sinking and sea levels continue to rise
+ ecoRI: Rhode Island Losing Ground in Battle Against Sea-Level Rise
+ Scientific Reports: Nuisance Flooding and Relative Sea-Level Rise; the Importance of Present-Day Land Motion

The Coastal State Discussion Series is sponsored by Rhode Island Sea Grant with the support of the Coastal Institute, the College of the Environment and Life Sciences, and the Graduate School of Oceanography at the University of Rhode Island.


– Meredith Haas | Rhode Island Sea Grant