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Discussion papers | Copyright
https://doi.org/10.5194/os-2018-75
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 23 Jul 2018

Research article | 23 Jul 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

Measuring rates of present-day relative sea-level rise in low-elevation coastal zones: A critical evaluation

Molly E. Keogh and Torbjörn E. Törnqvist Molly E. Keogh and Torbjörn E. Törnqvist
  • Department of Earth and Environmental Sciences, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118-5698, USA

Abstract. Although tide gauges are the primary source of data used to calculate multi-decadal to century-scale rates of relative sea-level change, we question the reliability of tide-gauge data in rapidly subsiding low-elevation coastal zones (LECZs). Tide gauges measure relative sea-level rise (RSLR) with respect to the base of associated benchmarks. Focusing on coastal Louisiana, the largest LECZ in the United States, we find that these benchmarks (n=35) are anchored an average of 21.5 m below the land surface. Because at least 60% of subsidence occurs in the top 5–10m of the sediment column in this area, tide gauges in coastal Louisiana do not capture the primary contributor to RSLR. Similarly, GPS stations (n=10) are anchored an average of >14.3m below the land surface and therefore also do not capture shallow subsidence. As a result, tide gauges and GPS stations in coastal Louisiana, and likely in LECZs worldwide, systematically underestimate rates of RSLR as experienced at the land surface. We present an alternative approach that explicitly measures RSLR in LECZs with respect to the land surface and eliminates the need for tide-gauge data. Shallow subsidence is measured by rod surface-elevation table‒marker horizons (RSET-MHs) and added to measurements of deep subsidence from GPS data, plus sea-level rise from satellite altimetry. We show that for a LECZ the size of coastal Louisiana (25,000–30,000km2), about 40 RSET-MH instruments suffice to collect useful data. Rates of RSLR obtained from this approach are substantially higher than rates as inferred from tide-gauge data. We therefore conclude that LECZs may be at higher risk of flooding, and within a shorter time horizon, than previously assumed.

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Molly E. Keogh and Torbjörn E. Törnqvist
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Molly E. Keogh and Torbjörn E. Törnqvist
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Short summary
Relative sea-level rise is traditionally measured using tide gauges, but we question the reliability of tide-gauge data in low-elevation coastal zones. Examining benchmarks, we find that tide gauges typically do not record subsidence occurring in the shallow subsurface and thus underestimate rates of relative sea-level rise. We present an alternate method of measuring relative sea-level rise and conclude that low-elevation coastal zones may be at higher risk of flooding than previously assumed.
Relative sea-level rise is traditionally measured using tide gauges, but we question the...
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