Print this pageComparison of Methods Used to Measure
Modern Subsidence in Southeastern Louisiana
Roy K. Dokka
Department of Civil & Environmental Engineering and Center of Geoinformatics,
Louisiana State University, Baton Rouge, Louisiana 70803
ABSTRACT
The current landscape of south Louisiana is largely the result of the interplay of subsidence, accretion, global sea level rise, and human activities. Annual flooding has built land by terrigenous sediment deposition and by stimulation of wetland biologic processes to produce organic materials. Although deltas generally do not grow much above sea level, the surface of the Mississippi River Delta has accreted sufficiently over the past several thousand yr to maintain its position with respect to a slowly rising sea level over a sinking land. Unfortunately, this functioning natural system was disrupted by humans seeking relief from river flooding. While congressionally-mandated and local flood control measures have effectively stopped flooding and maintained river navigation for commerce, these measures have had the unintended consequence of severely curtailing the sediment replenishment that once balanced the effects of subsidence and eustatic rise. Today, as subsidence and slow global sea level rise continue, the Louisiana coast is slowly being inundated by the Gulf of Mexico. Although the past history and driving forces behind global sea level is reasonably understood, the nature of subsidence along the Louisiana coast has recently become unsettled due to new highly precise and accurate geodetic measurements that are at odds with previous estimates based on chronostratigraphy of coastal peat deposits. This paper intends to bring clarity to this controversy by focusing on the measurement characteristics of each method and their suitability to support on-going efforts to protect this low-lying land against future coastal flooding and to restore the ecosystem.
Our analysis of measurement characteristics demonstrates that only geodetic and tide gauge-based methods are capable of providing
direct measurements of 20th and 21st century subsidence. Geologic methods such as peat chronostratigraphy can only provide extrapolations of data
points 100s to 1000s of yr old. Geodetic data also show that subsidence over the 20th century was not slow, constant, and restricted to coastal wetlands
as previous chronostratigraphic measurements suggest. Instead, geodetic measurements show that areas lying well beyond the coast have subsided, that
20th century subsidence has been variable in both time and space, and that the rate and amount of subsidence have been several times higher than recorded
by previous measurements. While only measurement methods such as peat chronostratigraphy methods can address issues of pre-1900 Quaternary geological
history reconstruction, only tide gauge and geodetic methods can directly provide measurements that are relevant to documenting surface change and understanding
the processes that have caused 20th and 21st century subsidence.
Dokka, R. K., 2009, Comparison of methods used to measure modern subsidence in southeastern Louisiana: Gulf Coast Association
of Geological Societies Transactions, v. 59, p. 225-242.
AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana