Recent subsidence rates for Barataria Basin, Louisiana

Abstract

Subsidence in south Louisiana is a fundamental process and major source of uncertainty influencing restoration planning and design and wetland habitat resilience. The primary goal of this study was to conduct an analysis of recent subsidence rates within the Barataria Basin. High-resolution geodetic GPS elevation measurements at 19 benchmarks were used to determine recent subsidence rates for the Barataria Basin. Net elevation changes at all stations equaled or exceeded predicted measurement uncertainties based on session duration. Water elevation change from two gauges in the southern part of the basin supplemented survey data, resulting in a range of subsidence velocities at 21 locations from about 2 to 7 mm/year. These data record short-term subsidence trends (6- to 16-year time series) that are expected to be representative of conditions at proposed restoration sites over the next 20 to 50 years. Foundation/rod depths for benchmarks ranged from near surface to about 30-m deep. None of these foundations/rods were sleeved or anchored and thus were not isolated from surrounding consolidating sediment and associated downdrag forces. Therefore, subsidence rates at these sites were considered representative for the entire sediment column. Subsidence rate comparisons for nearby deep and shallow rod benchmarks support this conclusion. Spatial variability in subsidence indicates a compelling relationship between subsidence rates and the age, composition, and thickness of Holocene deltaic deposits. The Coastal Protection and Restoration Authority of Louisiana intends to use this same approach to continue refining subsidence rates throughout coastal Louisiana to support coastal restoration planning and design.