Fifth-year PhD student at Tulane University, Molly Keogh, shot this photo near Bohemia, looking northeast over the marshes of Breton Sound in southeastern Louisiana. Photo: Molly Keogh

By Barri Bronston
30 January 2019

(Tulane University) – A new Tulane University study questions the reliability of how sea-level rise in low-lying coastal areas such as southern Louisiana is measured and suggests that the current method underestimates the severity of the problem. The research is the focus of a news article published this week in the journal Science.

Relative sea-level rise, which is a combination of rising water level and subsiding land, is traditionally measured using tide gauges. But researchers Molly Keogh and Torbjörn Törnqvist argue that in coastal Louisiana, tide gauges tell only a part of the story.

Tide gauges in such areas are anchored an average of 20 meters into the earth rather than at the ground surface. “As a result, tide gauges do not record subsidence occurring in the shallow subsurface and thus underestimate rates of relative sea-level rise,” said Keogh, a fifth-year PhD student and lead author of the study.

“This study shows that we need to completely rethink how we measure sea-level rise in rapidly subsiding coastal lowlands” said Törnqvist, Vokes Geology Professor in the Tulane School of Science and Engineering.

The study, published in the open-access journal Ocean Science, says that while tide gauges can accurately measure subsidence that occurs below their foundations, they miss out on the shallow subsidence component. With at least 60 percent of subsidence occurring in the top 5 meters of the sediment column, tide gauges are not capturing the primary contributor to relative sea-level rise.

An alternative approach is to measure shallow subsidence using surface-elevation tables, inexpensive mechanical instruments that record surface elevation change in wetlands. Coastal Louisiana already has a network of more than 300 of these instruments in place. These data can then be combined with measurements of deep subsidence from GPS data and satellite measurements of sea-level rise, Keogh said.

Rates of relative sea-level rise obtained from this approach are substantially higher than rates as inferred from tide-gauge data. “We therefore conclude that low-elevation coastal zones may be at higher risk of flooding, and within a shorter time horizon, than previously assumed,” Keogh said.

She said the research has implications for coastal communities across the globe.

“Around the world, communities in low-lying coastal areas may be more vulnerable to flooding than we realized. This has implications for coastal management, city planners and emergency planners. They are planning based on a certain timeline, and if sea level is rising faster than what they are planning on, that’s going to be a problem.”

The research was funded by the National Science Foundation.

Tulane study says seas may be rising faster than thought

Schematic dip-oriented cross section comparing the depth of tide-gauge benchmarks and GNSS station foundations to the local depth to the Pleistocene surface. Sites are arranged by increasing depth of the Pleistocene surface. Note that two GNSS stations have minimum foundation depths, indicated here by small, downward-pointing arrows. Graphic: Keogh and Törnqvist, 2019 / Ocean Science

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 usefulness 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 m of the sediment column in this area, tide gauges in coastal Louisiana do not capture the primary contributor to RSLR. Similarly, global navigation satellite system (GNSS) stations (n=10) are anchored an average of > 14.3 m below the land surface and therefore also do not capture shallow subsidence. As a result, tide gauges and GNSS 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 in this context. Shallow subsidence is measured by rod surface-elevation table–marker horizons (RSET-MHs) and added to measurements of deep subsidence from GNSS data, plus sea-level rise from satellite altimetry. We show that for an LECZ the size of coastal Louisiana (25 000–30 000 km2), 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 within a shorter time horizon than previously assumed.

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



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