By Alisha Renfro, Ph.D., National Wildlife Federation
Hurricanes are powerful storms that have the ability to alter coastal landscapes in a few hours. The wetlands of coastal Louisiana are similar to wetlands elsewhere, but they are often more susceptible to storm impacts due to their low elevation and the thick, organic-rich soil (and the lack of mineral sediment) that dominates much of the region.
In a new study, “Hurricane Impacts on Coastal Wetlands: A Half-Century Record of Storm-Generated Features from Southern Louisiana,” Robert Morton and John Barras of the U.S. Geological Society (USGS) use aerial photographs and satellite images to identify the patterns of land loss and types of features that develop as a result of hurricanes in coastal Louisiana. The study examines how a storm’s path, duration, magnitude and resulting impact to wetlands to differentiate wetland loss attributed to storms from wetland loss associated with sea level rise, subsidence, decreased sediment input and other longer-term processes that affect Louisiana’s coastal landscape.
The study determines that a storm’s speed, intensity, direction, the shape and topography of the coastline, and the timing between the maximum storm surge and the maximum wind speed all affect the magnitude of impact that a storm event has on a coastal region. The erosional features from the hurricanes studied in coastal Louisiana were typically found to be long-lasting, with some impacts that could still be identified nearly a half-century later.
The most substantive storm impact is newly-formed ponds and expanded ponds, which convert hundreds of square miles of wetland into open water. Back-to-back storms are particularly damaging to the region’s wetlands, with the combined impacts of Hurricanes Katrina and Rita (2005) expanding the open water area in coastal Louisiana by more than 300 square miles, and Hurricanes Gustav and Ike (2008) increasing open water by another nearly 200 square miles.
Prior to the 1930s, topographic maps show that ponds generated from storms were confined to the marshes closest to the shore. However, since that time, storm-impact features have migrated farther inland due to the cumulative impact of storm events coupled with high subsidence rates, sea-level rise and the absence of mineral sediment from the river that once fueled natural marsh-building processes.
Before the 20th century, the Mississippi River provided enough sediment, nutrients, and freshwater to repair some of the extreme-storm impacts to the coastal wetlands, but that capability no longer exists. If the natural land-building processes of the Mississippi River are not restored, the wetlands will continue to become more vulnerable and storm effects will continue to move inland, destroying ecologically and economically vital habitat.