2 June 2015
By Nanci Bompey
Waves in the northern Gulf of Mexico are higher than they were 30 years ago, contributing to a greater risk of coastal erosion and flooding in Florida, Alabama, Mississippi and Louisiana, according to a new study.
The significant wave height, or the average height of the largest one-third of waves, increased in the northern Gulf of Mexico by nearly 20 centimeters (eight inches) over the past three decades, according to the study’s authors. This growth, combined with increases in sea level, boosted the risk of coastal erosion and flooding in the region by 30 percent since the 1980s, according to the new research.
If wave heights and sea levels continue to rise, the erosion and flooding risk in the region could increase by up to 300 percent over the next 30 years, according to the new study published in Geophysical Research Letters, a journal of the American Geophysical Union.
Erosion is caused when waves attack, overtop and submerge beaches and dunes that protect coastal communities. Scientists use information about beach elevation, waves and water levels to calculate the probability that these events will occur and lead to erosion.
“If we have, nowadays, 10 hours per year when the water level is high enough to attack the dune, then it may be 40 hours per year in 30 years,” said Thomas Wahl, a postdoctoral researcher at the University of South Florida in St. Petersburg and lead author of the study. The new study could help agencies like the U.S. Geological Survey and the Army Corps of Engineers figure out when and how to maintain the dunes and berms on barrier islands that protect the coastal mainland, he added.
Beach erosion and coastal flooding are major concerns for communities along the northern Gulf of Mexico that stretches from Florida to Texas. While mounting risk of both has been anticipated for years from long-term sea level rise, the new study looks beyond that general trend to other lasting but seasonally or sporadically induced changes in sea level and wave height. Previous studies have largely ignored the coastal effects of those changes, which are primarily induced by wind patterns, according to the study’s authors.
Currently, scientists are only able to predict whether a particular storm will cause damage. The additional information in the new study could help them evaluate the likelihood that a particular area could experience erosion and flooding in the future, without knowing the particulars of an individual storm, according to Nathaniel Plant, a scientist at the USGS St. Petersburg Coastal and Marine Science Center, and co-author of the new study. This could inform communities about their vulnerability to flooding and erosion over the next 30 years, including how much of a role sea level or wave height will play, and help them decide how to respond, he added.
The new study combined observations of water level and wave height from tide gauges and wave buoys with computer models to generate a continuous record of sea level and wave information for Dauphin Island, a barrier island off the coast of Alabama, from 1980 to 2013. The study’s authors combined this information with wind, topographical and other information about Dauphin Island, which they used as a proxy for many other barrier islands along the northern Gulf Coast.
The new study finds that significant wave height increased by about six millimeters per year over the past 30 years in the region. It finds that significant wave height also varies seasonally, with lower waves in the summer and higher waves in the winter.
The new findings build on a previous study that Wahl published in Geophysical Research Letters in 2014. That study found that short-term fluctuations in sea level – when sea levels rise a little in summer and fall again in winter — have been intensifying over the past 20 years in parts of the Gulf Coast. The study found that, from the Florida Keys to southern Alabama, seasonal sea levels have been getting higher in the summer and lower in the winter over the past two decades.
Seasonal variations in wave-height, however, as revealed by the new study, show an opposite trend, with the extremes of the cycle abating over time, meaning that average summer wave heights are not going as low — and winter wave heights not as high — as they were.
The combination of these wave-height- and sea-level changes leads to a higher erosion risk in the summer and a smaller erosion risk in the winter for areas along the Gulf Coast, the new study finds. For a particular beach, the effect of these changes will depend on how steep or flat it is.
Plant said understanding what causes erosion during different times could be useful for helping different communities prepare for the future. For example, he noted, groups working to help shorebirds, which forage on the barrier islands in the winter, will have different concerns than the summer tourism industry.
— Nanci Bompey is a public information specialist/writer at AGU.