Coastal Modeling

August 1, 2007

Inland retreat: Coastal erosion forced the National Park Service to move the Cape Hatteras lighthouse away from the advancing edge of the ocean

 

© Karen Kasmauski/Corbis

Sea-level rise is one of the most dramatic consequences of a warming climate. Carbon dioxide (CO2) and other greenhouse gases trap solar radiation that re-radiates from the Earth's surface, warming the atmosphere. As average air and sea temperatures rise, ice masses melt, raising the sea level in the world's oceans. Sea-level rise and intensifying storm activity—another demonstrable, but less predictable, result of global warming—will have a profound effect on the shape of coastlines around the world.

Sea level has been on the rise since the end of the last ice age, around 18,000 years ago. The rate of that sea-level rise, which had been highly variable for much of this interglacial period, slowed markedly about 6,000 years ago and stayed that way until the coming of industrialization. According to Brad Murray, associate professor of geomorphology and coastal processes, the current rate of sea-level rise is about double what it was a century ago, and we can expect it to double again by the end of this century.

Murray is a geologist and a modeler who specializes in coastal processes—erosion, accretion, and other causes of changes in shorelines. He leads a five-person team that is developing a model of "large-scale coastal behavior" (changes on scales greater than kilometers or years) on the Carolina coasts.

The larger model integrates natural physical processes and human behavior. One important component is a dynamic economic model of the way in which decisions about coastal management are made, which is where Duke resource economist Marty Smith and marine policy specialist Michael Orbach come in. Joseph Ramus, a coastal ecologist at Duke, and Thomas Crowley, a modeler of past climates, round out the interdisciplinary powerhouse.

Coastal communities have tried all sorts of things to stabilize shorelines. In the face of rising sea level and intensifying storm activity, efforts to fortify our coasts are sure to increase. Murray's models of sandy shorelines like those found along the coasts of North and South Carolina suggest that human activities do as much to shape the shoreline as natural drivers like storms and sea-level change.

"Heavily developed coastlines are a new kind of system in that they're driven by both natural and human [factors]," says Murray. Seawalls, the most drastic means of armoring a shoreline, have been prohibited on North Carolina beaches since the late 1970s, thanks largely to the work of iconic Duke geologist Orrin Pilkey. Beach renourishment—the expensive process of bringing in sand from remote sources like offshore bars or inland pits—is the principal form of shoreline manipulation here.

Beach renourishment has become such a fundamental part of shoreline management in the Carolinas that Murray and his colleagues treat it as an intrinsic part of the model. "Actual physical changes affect decisions to nourish, and nourishment projects affect the physical coastline," says Murray. "This coupling creates the possibility for nonlinear feedback loops that involve both human and natural dimensions."

The team's preliminary work suggests direct interactions between widely separated parts of the coast: As repeated renourishment changes the shoreline orientation in one location, adjacent stretches of coastline are affected. Changes in these adjacent shorelines in turn influence the shape of regions further removed from the original project. Murray and his colleagues hope that by broadening the scales over which coastal-management decisions are considered, their work will help policy makers avoid surprises.