Studies find that river diversions can overcome Louisiana's rapid sinking

Studies led by researchers at Tulane University and other institutions show the likely benefits of land building by river diversions, despite these deposits initially causing rapid subsidence -- the gradual sinking of land -- in coastal Louisiana.

Published in the Journal of Geophysical Research: Earth Surface, the papers focus on subsidence in coastal wetlands and in shallow bays. Using different methods, both show that subsidence rates increase due to sediment deposition. But deposition rates typically outpace this accelerated subsidence, with new land creation and higher land elevations as a result.

The lead authors of the two papers are Molly Keogh of the University of Oregon and Elizabeth Chamberlain of Wageningen University in the Netherlands.

"We found that both wetlands and bays subside rapidly due to loading with fresh sediment," said co-author Torbjörn Törnqvist of Tulane. "The wetland study shows that the majority of this subsidence happens in the shallowest 10 feet and within the first centuries after deposition. The shallow bay study shows that up to half the elevation gain that is possible by deposition is lost to subsidence."

Much has been made of processes deeper in the Earth's crust like faults, he said, "but the available research that attaches hard numbers to deep processes shows that those are secondary factors. Shallow processes are not only much more rapid, often by a factor of 10, but also occur almost everywhere along our coast, and they are ongoing today."

Both studies are based on new borehole data collected throughout coastal Louisiana. The cores were analyzed for numerous properties, including sediment density and age. Computer modeling of deposition and land growth by means of river diversions suggests that the subsidence driven by accumulating sediment has a limited impact on reducing the rate of land growth, both horizontally and vertically.

Other co-authors are at Coastal Carolina University, the Louisiana Universities Marine Consortium, Deltares in the Netherlands and Yonsei University in South Korea. Funding was provided in part by the U.S. National Science Foundation.