The Wrack

The Wrack is the Wells Reserve blog, our collective logbook on the web.

Sea level rise redux: Using what we know

Posted by Wells Reserve Contributor | July 1, 2002

It is probably a rare coastal beachfront property owner who is not aware that beaches are dynamic systems that erode and accrete in response to storms, sediment supply, rising sea level, and the proximity of sea walls, jetties, and other forms of coastal "armor." Many beachfront owners are also aware that "natural" barrier beaches and their dune systems are able to persist in the face of sea level rise by transgressing, or migrating shoreward.

Of course, awareness does not protect property threatened by an eroding barrier beach. Much coastal property was developed years ago, when beach sand was plentiful, before the term "global warming" had been coined, and when only paleogeologists gave much thought to the phenomenon of sea level rise. In recent years, however, much time and energy has been devoted to discussing ways to protect property threatened by beach erosion.

But erosion problems on the beach are only the front line—the same problems can and will occur within and around the salt marshes, known as back-barrier marshes, that form behind beaches. In fact, barrier beaches and back-barrier marshes have been migrating landward together for more than 3,000 years in the Gulf of Maine, in response to a gradually rising sea.

In addition to migrating inland in response to sea level rise, salt marshes also grow upward (in the vertical dimension) through the process of peat formation. The grasses that form the marsh, known commonly as salt marsh hay (Spartina patens) and smooth cordgrass (Spartina alterniflora), die at the end of the growing season, break down into fragments and mix with sediment brought in from the estuary with the flood tides and deposited on the ebb. Over the centuries, this process forms a deepening layer of peat.

In the oldest sections of our marshes, Reserve scientists and visiting investigators from the University of Maine and the University of New Hampshire have cored down through 24 feet of peat, providing a virtual time line of sea level rise over the past 4,000 years. We have found that as sea level has risen over the past several millennia, the Reserve's marshes have accreted peat apace, so the marshes persist at the proper elevation with respect to the tides. Now, though, geologists are seriously concerned that some marshes in the Gulf of Maine are no longer keeping up with sea level rise.

The Reserve is collaborating with many other sites along the east coast in the fine scale, long-term measurement of marsh elevation using devices known as sediment elevation tables (SETs). Over the past several years, we have installed 13 SETs throughout the Webhannet marsh, with funds from NOAA's Cooperative Institute for Coastal and Estuarine Environmental Technology. We hope within the next several years to have enough data to determine trends (either positive or negative) in marsh elevation in response to rising sea level.

Meanwhile, if we continue to develop the perimeter of our marshes, and continue to restrict tidal flow onto the marshes through road crossings, tide gates, and undersized culverts, we will deprive these breathtaking and extremely productive ecosystems of their ability to respond to sea level rise, both horizontally and vertically.

Instead, we recommend establishing and maintaining a 250-foot shoreland buffer of trees, shrubs, or tall grass (not lawn) between developed property and the marsh-upland border. We believe tides and watersheds should freely deliver the sediments critical to marsh accretion. Without these measures, property owners at the marsh perimeter will experience the same erosion problems that pose such a challenge to many of our beachfront neighborhoods today.

This article originally appeared in Watermark 19(2)

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