By the numbers, sands and dams
30 - Major river systems along the East Coast and Gulf of Mexico.
144 - Dammed reservoirs.
10 - Estimated percentage of soils eroded along Atlantic river basins that make it to the coast.
3 - meters per year, average increase in erosion rates along the Southeast coast since the time of dam construction.
Sources: National Park Service, various studies courtesy of the S.C. Geological Survey.
More than 100 million cubic yards of sand have been pumped onto beaches along the Carolinas and Georgia so far to stave off erosion. It hasn't done much good. The barrier islands often lose that sand faster than periodic renourishments can keep up.
There's always a culprit. On Folly Beach, where a renourishment project is now underway, the blame falls on the Charleston jetties stopping the north-south flow of sand in shore currents. Storms also wreak havoc. Seas inexorably are climbing a few millimeters higher each year.
But there's another, little regarded factor: dams. The islands were formed by a combination of rising seas and the sweep of sand from delta inlets, at least some of it historically deposited from large, free-flowing river basins. Today, nearly every major river system in the East is dammed or channelled.
No study has been done to show exactly how much that has deprived the coast of sand, and it's disputed how big a difference dams make in the coastal plain. But geologists generally agree that coastal erosion worsened after the dams went up.
In Louisiana, where Mississippi Delta barrier islands were shredded by Hurricane Katrina, scientists are looking to restore sand diverted upstream to bolster the depleting islands. But it's not likely to work as well here and less likely to be tried, even as the barrier islands shrink. In the coastal plain, much more sand tends to sink in the flats than carry downstream to the coast.
"Most lower river valleys are flooded lands. They wouldn't be delivering a lot of sand to the coast. In the Carolinas and Georgia you have 'drowned estuaries.' It would be hard to blame oceanfront erosion on dams," said Rob Young, a coastal geology professor at Western Carolina University, where the study was done gauging how much sand has been used to renourish beaches.
The Santee River, when it flowed freely, used to plume fresh water a mile into the ocean during spring floods. The basin is considered one of the largest on the East Coast. Not so coincidentally, the chain of Lowcountry barrier islands begins in its delta near McClellanville. The basin today has a series of dams that runs nearly from the North Carolina mountains along the Catawba River on down. Santee Cooper utility lakes managers say little sediment builds up behind the Lake Marion dam near St. Stephen.
When free flowing, the river used to swamp a basin miles wide through the Lowcountry in spring floods, and geologists suspect most of its sand was deposited there. Sediment from rivers like the Santee helped build up coastal marshes, but the ebb and flow of tidal inlets largely feeds sediment offshore that distributes along the beaches, said Kerry Castle, South Carolina Geological Survey senior research geologist. Rivers play a relatively small role.
In the coastal plain, dams probably aren't that big of an issue in beach erosion, said William Graf, University of South Carolina geography professor emeritus, who has studied the phenomenon. But barrier islands probably were partly constructed by the deposits, he said. And erosion is occurring faster since the dams were built, both Castle and Graf said.
American Rivers, an environmental advocate, sought a study of how the Lake Marion dam affected beach erosion, as part of a challenge to Santee Cooper's ongoing federal re-licensing for the dam. The study was one of a number of requests made as the group pushes for more water release for the downstream ecosystem. Federal regulators didn't require the study to be made.
"We know that dams block flow and exacerbate erosion below the dam," said Gerrit Jobsis, Southeast regional director for the group. Having better data on sand transport "is an important thing that should be addressed."
The S.C. Senate is considering a bill that would make removable storm wave-busting walls legal on the beach, based on a wall designed by a Lowcountry carpenter to protect his parents' beachfront home on Sullivan's Island. It's one more patchwork attempt to stave off encroaching seas along the developed barrier islands. But where that wall is now being tested, on the highly erosive beach along Dewees Inlet at Isle of Palms, something more significant is taking place. A long-sought offshore sandbar "reattachment" is finally beginning to take place, providing at least a short term supply of sand to the beach. This is the tidal inlets at work.
If more sand were flowing from the river deltas, the coastal sand flow would somewhat improve, virtually immediately, Graf said -that's the idea behind the Mississippi delta project. But from his office window in Columbia, Graf can look out at hills that are remnants of Pleistocene epoch sand dunes, back when the ocean reached the South Carolina fall line. One way or another, with seas rising again, parts of the coast are going under, grain by grain.
"From the standpoint of coastal management, you have to find ways to live with this," he said.
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More than a mile of Cape Island beach was lost to damage from Hurricane Irene in 2011. U.S. Fish and Wildlife managers are looking to buy more mainland property as the islands disappear.×
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