A new study by Tulane University casts doubt on the reliability of sea level rise in low-lying coastal areas such as southern Louisiana and suggests that the current method underestimates the severity of the problem.
The relative increase in sea level, which is a combination of rising water and soil levels, is traditionally measured by tidal gauges. But researchers Molly Keogh and Torbjörn Törnqvist argue that in coastal Louisiana tidal gauges only say part of the story.
Tide gauges in these areas are anchored on an average of 20 meters to the ground than to the surface of the earth. "As a result, tidal gauges do not record declines in shallow subsoil and thus underestimate the relative increase in sea level," said Keogh, a fifth PhD study student and lead author of the study.
"This study shows that we have to completely rethink how we measure the rise in sea levels in fast-growing coastal lowlands," said Törnqvist, Professor Vokes Geology at Tulane School of Science and Engineering.
A study published in an open-access journal Oceánská věda, says that while tidal meters can accurately measure the drop that occurs beneath their foundations, they lack the component of a shallow decline. With at least 60% of the decrease occurring in the upper 5 meters of the sediment column, the tidal flow meters do not capture the primary contributor to the relative increase in sea level.
An alternative approach is to measure a shallow drop using surface elevation tables, cheap mechanical tools that record altitude change in wetlands. Coastal Louisiana already has a network of more than 300 of these tools. The data can then be related to measuring a deep drop in GPS data and satellite measurements of sea level rise, Keogh said.
The relative sea level ratios obtained from this approach are substantially higher than the rates derived from the tide and strain dates. "We will therefore conclude that coastal zones with low altitudes may be exposed to a higher flood risk and in a shorter timeframe than previously expected," Keogh said.
She said research has implications for coastal communities around the world.
"Throughout the world, communities in low-lying coastal areas can be more at risk of flooding than we have realized, which has implications for coastal zone management, town planners and emergency plans, are planned on a timetable and if sea levels rise faster than that what they plan to do, it will be a problem. "
The research was funded by the National Science Foundation.
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