A Comparison of Geologic and Geodetic Measurement Methods Used to Quantify 20th Century Subsidence of the Louisiana Coast

Engineering design of hurricane storm surge protection structures and coastal wetlands restoration efforts requires accurate predictions of subsidence over the next 50-100 years. Accordingly, rigorous quantitative assessment of the precision and accuracy of each of the methods used to measure subsidence is warranted. Here, we compare the differences in geologic and geodetic strategies to infer subsidence, precision, accuracy, and the amount of time that is averaged in computing subsidence rates. Geologic methods that track the position of peat and fossiliferous strata through time cannot resolve subsidence at a spatial resolution of less than a meter and most critically, a temporal resolution of less than 33-270 years. Geologic subsidence rates of coastal Louisiana for the late 20th century are based on unwarranted linear extrapolations of data points hundreds and thousands of years old. In contrast, geodetic methods such as leveling, GNSS, and InSAR employ highly precise technologies and rigorous analysis techniques that can measure mm to cm vertical change that occurs over time intervals as short as a month. In contrast to geological methods, geodetic methods have the ability to detect and quantify processes operating at annual to decadal timescales such as compaction, faulting and anthropogenic activities. Geodetic methods provide the most detailed and reliable measurements of 20th century subsidence and are also the only reliable means to obtain detailed kinematics data regarding specific processes causing subsidence. The so-called “controversy” regarding subsidence measurements of the 20th century along the Louisiana coast stems from: a) misperceptions about the inherent measurement uncertainties and resultant spatial and temporal resolution provided by various measurement systems; and b) the manner in which time-average data are handled and reported. In setting design and performance requirements for hurricane protection structures and coastal restoration efforts, only geodetic techniques match the requisite temporal scale and measurement precision and accuracy requirements.