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Present Day Relative Sea Level Rising Faster Than Expected in Coastal Louisiana


Researchers: Molly E. Keogh and Torbjörn E. Törnqvist, Tulane University

Written by Linda Rowan
12 April 2020


Summary

A new method to measure relative sea-level rise in low-elevation coastal zones combines global navigation satellite system (GNSS) data with measurements from rod surface-elevation table–marker horizons (RSET-MHs) and satellite altimetry. The RSET-MHs measure shallow subsidence, the GNSS measures deep subsidence, and satellite altimetry provides rates of oceanic sea-level rise. Combining these measurements shows that the sediments in coastal Louisiana are subsiding faster than previously recognized and thus relative sea level is rising at a higher rate and flooding is more likely sooner rather than later.


Observations

Tide gauges have been utilized for tens to hundreds of years to measure relative sea level. The gauges measure changes in water surface elevation with respect to the depth of their associated benchmarks. Thus, tide gauges installed in sediments cannot measure subsidence that occurs above the base of the benchmarks. In coastal Louisiana, most of the subsidence occurs within 5 meters of the surface and a majority of the gauges cannot measure this subsidence because their benchmark foundations are deeper.

As part of the Coastwide Reference Monitoring System, the United States Geological Survey (USGS) operates nearly 350 RSET-MH sites in coastal Louisiana, covering an area of over 30,000 square kilometers. Previous work with RSET-MH data has shown that shallow subsidence occurs in the uppermost 5 meters of sediment in coastal Louisiana and has a mean rate of 6.8 millimeters per year. The RSET-MHs measure shallow subsidence and this information is combined with the GNSS that measures deep subsidence and satellite altimetry data that measures the oceanic component of sea-level rise to get a complete picture of relative sea-level rise.

Results

Ten GNSS sites in coastal Louisiana with known foundation depths ranging from 1 to 36.5 meters depth (median of 14.9 meters) were used to determine the deep subsidence. A Monte Carlo simulation of the RSET-MH data shows that a minimum of 40 RSET-MH sites per study area with a spacing of about two RSET-MH sites per 1,000 square kilometers could provide robust measurements of shallow subsidence and improve relative sea-level rise estimates with respect to the land surface. Because this new method includes measurements of shallow subsidence, the rates of relative sea-level rise from this combination of RSET-MH, GNSS, and satellite altimetry data are higher than those inferred from tide gauges. Thus coastal Louisiana may be at a higher risk of flooding in the near future. The method can be applied around the world in other low elevation coastal zones to determine if sea level is changing at rates not appreciated by previous analyses; there could be higher risks of flooding in other coastal zones around the world.

Related Links


References

Measuring rates of present-day relative sea-level rise in low-elevation coastal zones: a critical evaluation, Molly E. Keogh and Torbjörn E. Törnqvist, Ocean Sci., 15, 61–73, 2019, doi: 10.5194/os-15-61-2019.

Keywords

relative sea-level rise, subsidence, low-elevation coastal zone


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Last modified: 2020-04-15  15:52:17  America/Denver