Faulting, Subsidence, Land Loss
Subsidence and land loss affect directly coastal areas and wetlands. Approximately 40% of the coastal wetlands of the lower forty-eight states of the United States are located in Louisiana, and this critical ecosystem is disappearing at an alarming rate: Louisiana has lost up to 40 square miles of marsh per year for several decades, which represents 80% of the nation’s annual coastal wetland loss. Coastal Louisiana has lost ~5000 km2 of formerly emergent wetlands to open water since the 1930s. If the current rate of loss is not slowed by the year 2040, an additional 800,000 acres of wetlands will disappear, and the Louisiana shoreline will advance inland as much as 33 miles in some areas. The projected cost for Louisiana alone is $3.6 billion in direct financial losses to homes, businesses, and infrastructure over the next 50 years. Yet, our understanding of subsidence and its controlling parameters remains limited.
Along the Gulf Coast, and Louisiana in particular, subsidence is controlled by the following processes: (1) tectonics (fault processes and salt movement), (2) compaction of Pleistocene and Holocene sediments, (3) sediment loading, (4) glacial isostatic adjustment, (5) anthropogenic causes such as fluid withdrawal and management of surface water. Assessing how coastal subsidence evolves is complex because subsidence rates are highly variable both spatially and temporally: subsidence rates vary in space, potentially over very short distances (for example, tens of meters); subsidence rates vary as a function of depth (rates are always highest near the surface); subsidence rates can vary through time, depending on the driving processes. The strong interdependence of subsidence forcings hinders the unequivocal identification of its causes. Most subsidence-related research in the United States has been focused on the evolution of the Mississippi River Delta, where subsidence is dominated by sediment compaction. However, little is known about how the various processes operate in other areas of the Gulf Coast outside of the influence of the Mississippi River Delta. |