Determining Recent Subsidence Rates for Barataria Basin, Louisiana: Implications for Engineering and Design of Coastal Restoration Projects

Subsidence in south Louisiana is a primary factor influencing restoration design and wetland habitat resilience. Subsidence rates vary widely due to the age and complexity of deltaic deposits, underlying geologic controls, and human alterations to wetland habitat. Understanding the causes and rates of subsidence is critical to successful planning and implementation for State Master Plan projects. The primary goal of this project was to conduct an assessment of recent subsidence rates within the Barataria Basin. In particular, we determine campaign-style geodetic GPS elevation measurements for CPRA/NGS secondary benchmarks and CORS primary benchmarks. These data record short-term subsidence trends (5- to 15-year time series) from direct survey measurements that are expected to be indicative of conditions at proposed restoration sites over the next 20 to 50 years.  Further, water-level gauge measurements were evaluated for documenting subsidence relative to eustatic sea-level rise estimates for the northern Gulf of Mexico.

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High-resolution geodetic GPS elevation measurements at 19 benchmarks were used to determine short-term subsidence velocities for Barataria Basin. Net elevation changes at all stations equaled or exceeded predicted measurement uncertainties based on session duration (>1 cm). 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 2 to 7 mm/yr. Foundation/rod depths for benchmarks ranged from near surface to about 30-m deep. However, none of these foundations/rods were isolated from surrounding consolidating sediment and associated downdrag forces. For unsleeved benchmark rods, elevation differences between the rod cap and marsh/ground surface were not noted in any of the survey records. As such, subsidence velocities at these sites were considered representative for the entire sediment column. Subsidence velocity comparisons for deep and shallow rod benchmarks within a few kilometers of each other support this conclusion (CMS-BM-01 rod depth = 1 m and velocity = 5.4 mm/yr; H359 rod depth = 24.4 m and velocity = 5.7 mm/yr).

Greatest subsidence rates were recorded in the southern portion of the basin where Holocene sediment thickness is greatest, deltaic sediment is youngest, and subsurface sediment composition is primarily fine-grained. Velocities ranged from 5 to 7 mm/yr in this region. Mid-basin subsidence rates ranged from 4 to 5 mm/yr, and those to the north where Holocene sediment is relatively thin and overlapping delta lobes are common, velocities ranged from <2 to 4 mm/yr. These data indicate that subsidence rate ranges used in the 2017 Master Plan for Barataria Basin can be refined from 2 to 20 mm/yr to approximately 2 to 7 mm/yr.