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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Chesapeake Bay Region, Not Just D.C., Is Sinking

By Catherine A. Cardno, Ph.D.

Researchers have established why the Chesapeake Bay is seeing a rise in sea level that is double the global average. 

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Researchers drilled 70 boreholes up to 100 ft deep in the Blackwater National Wildlife Refuge on Maryland’s Eastern Shore to help determine the age of the layers of sand, rock, and organic matter there. Combining this data with data from lidar technology and GPS receivers helped create a three-dimensional portrait of the Chesapeake Bay that extends back millions of years. Wikimedia Commons/Jcantroot

August 11, 2015—While a small minority of the public has vocally argued with the majority of scientists about the existence of climate change, melting polar ice, and rising sea levels, tidal recordings in the Chesapeake Bay have quietly shown a sea level rise double that of the global average for the last 60 years. Known as the relative sea-level rise, the increases currently measure 3.4 mm/yr versus the global average of 1.7 mm/yr. Researchers have now established why the region is seeing the fastest increases in sea level along the United States's East Coast: the entire Chesapeake Bay region is sinking.

While this might sound like the next generation of Hollywood blockbusters—the film industry having exhausted asteroids, volcanoes, tsunamis, and earthquakes—the reason the region is sinking is yet another global change that is related to melting ice sheets. (Read " Report: Rising Arctic Temperatures Have Global Implications" by Catherine A. Cardno, Ph.D., on Civil Engineering online.)

The hypothesis—confirmed by the current research—has been that the crushing weight of a colossal sheet of ice in northern North America that built up during the last ice age had caused a bulge to form under the Chesapeake Bay region. Known as a "forebulge," the land is now subsiding as the disappearance of the ice sheet 10,000 years ago reduced the pressure on the northern portion of the continent, causing the bulge under the Chesapeake Bay area to collapse. (The Laurentide Ice Sheet extended as far south as Long Island, New York, at one point and began retreating approximately 27,000 years ago.)

The findings are published in the article " Pleistocene Relative Sea Levels in the Chesapeake Bay Region and Their Implications for the Next Century," which is published in this month's GSA Today, a publication of the Geological Society of America. The paper is the work of seven authors. Lead author Benjamin D. DeJong, Ph.D., completed the research—partially funded by the U.S. Geological Survey (USGS)—as part of his doctoral work at the University of Vermont while simultaneously employed by the USGS. DeJong, who wrote in response to questions posed by Civil Engineering online, is currently a project geologist with the Johnson Company, an environmental science and engineering firm based in Montpelier, Vermont.

For the study, DeJong drilled 70 boreholes of up to 100 ft in depth in the Blackwater National Wildlife Refuge on Maryland's Eastern Shore. He then calculated the age of the layers of sand, rock, and organic matter in the cores and combined the information with the remotely sensed topographical data from lidar technology, and using GPS map data created a three-dimensional portrait of the Chesapeake Bay that extends back millions of years. The results confirmed the hypothesis that the area is still experiencing forebulge collapse—and it will continue for centuries. Indeed, within the next century the region is expected to lower by approximately 15 cm, according to the paper.

Lidar-based mapping and lab advances that allowed the gathered sediments to be dated enabled the completion of the research, according to Paul Bierman, Ph.D, a geomorphologist and professor of geology and natural resources at the University of Vermont and the senior author on the paper, who wrote in response to questions posed by Civil Engineering online. Bierman's lab has collaborated with the USGS for two decades, providing data that have been used to determine the dates of geological features and trace erosion across the United States. "The triumph of technology allowed this work," Bierman noted. "None [of these methods] were in wide use 20 years ago."

DeJong said, "The lidar data were crucial for recognizing the geomorphology of the landscape—prior to the acquisition of lidar data in 2003, geologists relied on relatively coarse topographic maps ([with] 5 to 10 ft contours), but with lidar came 30 cm resolution topographic data that allowed us to see all of the subtle features interpreted as subaqueous and near-shore morphologic features."

Blackwater was an ideal location from which to take the core samples because it is one of the lowest-elevation and most rapidly subsiding areas bordering the Chesapeake Bay, according to the paper. "Blackwater was a great place to do the work—accessible public land but also an ecosystem dramatically affected by sea level rise as the area is so [low and] flat," Bierman said. Small changes in sea level resulted in big changes in the landscape's response, he noted.

But while the coring was completed at Blackwater, the results have implications for the entire Mid-Atlantic region extending from Delaware to northern North Carolina—not just on Washington, D.C., as has been widely reported in many media outlets. "We know [the forebulge's] boundaries from geologic mapping of where shorelines and deposits are found—the idea being that such deposits are warped by changing land levels," Bierman explained. "They started flat but are flat no longer."

"The effects of the forebulge lie on a gradient," DeJong explained. "Baltimore will 'feel' it slightly less if not comparably to Washington, D.C., and it technically continues to the southern tip of the Atlantic Coast of the United States and beyond. But the magnitude drops off significantly south of northern North Carolina."

And what about the ground surface behavior as the land subsides, and its potential impact on the built environment? "The core of this process operates so deeply below the crust that the effects should not be seen on the surface," DeJong explained. "But, there are active fault zones in the D.C. region, as well as a few recently recognized surface-cutting faults that have been hypothesized as being related to forebulge dynamics," he noted. "These features are currently under study and, while they cannot yet be confidently linked to glacioisostatic adjustment, we also cannot rule out this possibility or their potential to affect [existing built] infrastructure."

"The changes occur at depth but the surface ﴾the topography﴿ is supported by the material at depth—when the material at depth moves, the surface goes with it," Bierman added. "The built environment will be affected because land levels in general have been and will be sinking slowly but certainly at a rate of 6 to 8 inches per century."

Now that the reason behind the six decades of rising sea level data in the Chesapeake has been established, the next step in the geological research is to establish the boundaries of the forebulge in a north-south direction, according to DeJong. 

The engineering and public response, however, needs to be one of action. "With the land going down, sea level will go up and the built environment will slowly but surely flood, unless we take action with engineering approaches or we move things out of the way," Bierman noted.

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