Shoring Up Cliffs Called Harmful to Bay- Geologist says silt would create underwater desert
News Article: Baltimore Sun
March 23, 2010|By Frank D. Roylance
Efforts to shore up the base of the cliffs on the Western Shore of the Chesapeake Bay could trigger problems for the area's freshest fossil beds - and turn the bay floor into a silty "desert," an expert on the region's geology says. Lauck Ward, a geologist at the Virginia Museum of Natural History who has studied the Maryland and Virginia cliffs for more than 30 years, says the best fossils, laid down as much as 18 million years ago, would be buried by riprap and slumping sand. "Scientifically it would be a wipeout," he said. "But ... that's just one of the problems."
Since the end of the last Ice Age, 10,000 years ago, sea levels have been rising, flooding the lower Susquehanna River valley and creating the Chesapeake Bay. As the water rose, it cut cliffs into the ancient marine sediments exposed at high points along the old riverbanks.
Nor'easters gnaw at the base of the cliffs. The bottom layers have more clay, more water, and more tensile strength to resist erosion than the sandier layers above.
The erosion is also buffered by the narrow beach formed by the sand that falls from above. Riprap boulders are stronger, but they bring other problems, Ward said.
Wave action in front of the boulders would scour away the beach, and the rocks would cut off the flow of cliff sand into the bay, Ward said. The cliff sand is "the only sand that's getting out into the bay. Everything else that's coming down the rivers ... is superfine silt and clay in suspension."
Sealing off that sand would make the bay bottom increasingly silty and inhospitable to oysters, crabs and other life, he said.
"It's going to wipe out the bottom, make the bottom a barren desert," he said. "It's obvious to paleontologists." Fossil species are few in the silty sediment layers in the cliffs. In the sandier layers, "there's an explosion of diversity. The only way to get it [today] is to leave the ... cliffs alone."
Riprap does not stop erosion at the top, either, Ward said. Over time, water seeps into the top sediments. Freeze-thaw cycles and drying cause slides and slumps from the cliff face. Trees help to draw that water out. But much of that cover has been cut.
A U.S. Geological Survey report in 2003 noted that when left alone, with tree cover at the top, the cliffs retain an angle of 70 to 80 degrees, while the cliff face retreats at a rate of 2.3 feet to 4.3 feet a year.
A decade earlier, the USGS reported that where the "toe" of the cliffs are stabilized, the cliff tops keep retreating until the angle from the base to the top decreases to a stable, vegetated slope of 30 or 35 degrees. The "cliffs" are destroyed.
"Other protective structures aimed to slow down erosion of slump material ... are generally ineffective," the USGS said.
In a study summary, authors Curt Larsen and Inga Clark said, "We can tell you confidently that if you are foolish enough to build or buy a house on a cliff edge with a beautiful view of the Chesapeake Bay, simple toe protection of that cliff is not going to save your bacon."