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Second Story – Fall 2014
 
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Water Cities: Can We Climate-Proof the Coast?
VOLUME 28, NUMBER 4, FALL 2014             

By John H. Tibbetts                                                                       back to main story

New tools spotlight state's sheltered shorelines

Think you’ve seen all of the South Carolina coast? Well, think again. There are nooks and crannies of shoreline that you’ll probably never find. The state has 187 miles of sandy beachfronts but many thousands of miles of tidal sheltered shorelines.

To view them all, you’d need to travel along the state’s many bays and sounds, trace coastal rivers and tributaries as far as saltwater will go, and follow scores of brackish creeks that wind through the archipelago from the Georgia state line past the sea islands named after kings, Johns and James, and then on to Bulls and Capers north of Charleston, and finally past the Grand Strand to Little River Inlet near the North Carolina state line.

Chester Jackson, a coastal geologist at Georgia Southern University, says that his new study offers evidence of more than 4,200 miles of estuarine shoreline just within the southern third of the South Carolina coast from the Savannah River to Edisto Island. This shoreline nearly equals the distance from Charleston to Anchorage, Alaska.

Previous mapping showed less than 3,000 miles of estuarine coast, but new digital technologies allow scientists to see finer-grained and more accurate details of shoreline meanderings.

The entire estuarine shoreline of South Carolina is expected to be completely mapped and analyzed by the end of 2015 through a project supported by the U.S. Army Corps of Engineers and the S.C. Department of Health and Environmental Control’s Office of Ocean and Coastal Resource Management (SCDHEC-OCRM).

An April 2010 report by the S.C. Shoreline Change Advisory Committee encouraged SCDHEC-OCRM to develop baseline data for estuarine shorelines. Jackson’s study is an important part of that ongoing effort.

One striking finding of the Jackson study is that almost two-thirds of estuarine shoreline in the study area experienced net erosion since the 1880s, with some shorelines eroding very quickly. Boating activities, dredging, and shoreline armoring drive some erosion along the coast, but it seems clear that tidal-creek currents are become faster, affecting the location and intensity of erosion.

Sea-level rise is pushing more seawater through inlets into estuaries and filling creeks higher. Increased water volume and stronger currents in estuaries are creating new meandering patterns for creeks and shoreline erosion.

“Tidal creeks are taking on more water and speed,” says Jackson, “so they are encroaching more and more” into salt marshes and upland shorelines. “The tidal streams are a lot more dynamic than we thought.”

But it’s difficult to determine how much erosion, velocity changes, and movement of tidal streams are directly attributed to sea-level rise. Storms and human activities are important contributors to estuarine changes, too.

If the state loses salt marshes to erosion, it will lose crucial storm-
buffering habitat and important nursery habitat for fisheries.

Salt marshes can reduce the destructive energy of many coastal storms, according to a 2014 report by the National Research Council. But the marshes have to be relatively wide to provide a significant defense to upland shorelines where people have built homes and businesses.

The Charleston peninsula was once bordered by extensive salt marshes where the Battery stands today. Narrow, scattered fringes of salt marsh along the peninsula provide little storm protection. So, scientists say that it’s important to protect South Carolina’s 500,000 acres of remaining salt marsh.

As sea level rises, many salt marshes will migrate inland, but some will be blocked by hard erosion-control structures. Bulkheads are vertical wooden structures intended to control erosion along tidal creeks and bayfronts. Revetments are large rocks or boulders that similarly hold the tidal shoreline in place. Salt marshes, over time, will be squeezed between such hard structures and rising sea level until many marshes drown.

Since 2001, more than 1,300 property owners in South Carolina have acquired state permits to install hard, erosion-control structures along sheltered shorelines.

South Carolina officials consider issuing a permit for a bulkhead or other hard erosion-control structure along an estuarine (non-beachfront) shoreline when a waterway is eroding a property’s high land. A landowner, however, would not receive a state permit if salt-marsh vegetation is migrating into an upland area and there is no evidence of erosion from a waterway.

But a South Carolina landowner with a migrating tidal shoreline can build a bulkhead farther upland on his property, outside of state jurisdiction, in anticipation of further erosion.

Where exactly are the most rapidly changing sheltered shorelines? The state has lacked coast-wide information about them. Soon, for the first time, the state will have the new digital map and statistical analysis of shoreline change.

“This mapping effort is particularly valuable because it significantly enhances our understanding of how the estuarine environment fluctuates and changes over time, and how human activities may influence that change,” says Dan Burger, director of the Coastal Services Division of SCDHEC-OCRM.

“With these new maps and analytical tools,” he says, “we’ll be better able to identify where particularly vulnerable areas of erosion are. These data may also help inform policies and interventions to guide development away from hazardous areas and to explore alternative shoreline stabilization techniques, such as living shorelines or oyster restoration, in suitable areas.”

SCDHEC-OCRM is sharing information from the new maps with coastal municipalities to inform their hazard-planning efforts.


Last updated: 12/17/2014 8:25:04 AM
Second Story – Fall 2014

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