Hard-to-detect spikes in sea level -- driven by ocean currents and wind patterns -- may be worse for beach erosion than some hurricanes, and climate change could make the spikes worse on swaths of the East Coast, according to a new study.
The research in Geophysical Research Letters is worrisome for coastal planners, as sea-level spikes or "anomalies" -- which are different from sea-level rise driven by climate change -- are not currently under consideration as a risk factor, even though they may pack as much erosive power as strong storms, the study authors say. In some cases, anomalies can raise sea level by more than a foot beyond levels from tides and warming-induced sea-level rise.
Additionally, planners need to be contemplating a scenario of a hurricane occurring on top of a sea-level spike, which would be much more damaging than a storm without such an event, the authors note.
"Coastal managers commonly have beaches surveyed after large storms to measure how much beach was eroded away. They should also be on the lookout for years with frequent sea-level anomalies and include those in their beach-preservation plans," said Ethan Theuerkauf, a doctoral student at the University of North Carolina, Chapel Hill, and lead author of the study, which included other scientists at the university's Institute of Marine Sciences.
"Assessment of risk to coastal property owners is commonly based on storm-recurrence intervals, rates of shoreline movement, and recently in some areas, projections of sea-level rise. Sea-level rise anomalies should be included," he said.
He said the research was the first to thoroughly assess the erosion impacts of sea-level anomalies.
Anomalies are non-storm-related increases in sea level "forced by meteorological processes" lasting longer than two weeks, although they may occur for several months and stretch along almost entire continents. They can be caused by factors such as strong winds, although scientists are also pinpointing fluctuations in the speed of the Gulf Stream as a probable cause.
The researchers studied six North Carolina beaches between 2009 and 2012 on Onslow Beach, a barrier island 175 kilometers south of Cape Hatteras. The location was ideal, according to Theuerkauf, as the beaches there are very different in their topography.
Ender's Island Hurricane and Storm Damage Reduction project. (Credit: Flickr/CorpsNewEngland)
Anomalies: a steady, powerful beat
Each February, the researchers pulled cores from the beaches before and after low tide and measured erosion directly from the sediments via a new technique. They retrieved data on water levels from a National Oceanic and Atmospheric Administration tide gauge off nearby Wrightsville Beach.
The timing proved fortuitous, as samples were taken from the beaches in years with three types of distinct conditions. There were a few anomaly events, then large anomaly events and then a period after Hurricane Irene, which produced waves larger than 12 feet on the island.
Between February 2009 and February 2010, there were frequent, high-magnitude anomalies raising sea level from 10 centimeters to half a meter, even though there were no large storms. During that period, 40 percent of the water-level observations were anomalies, compared to the following year of few events, when there were 8 percent, according to the paper. Hurricane Irene, a Category 1 hurricane, hit a year and a half later in August 2011.
The cores revealed that erosion from the steady beat of waves from anomalies equaled or went beyond the impact from Hurricane Irene. The average erosion on the mid-intertidal zone -- the area exposed daily during the tidal cycle -- was 55 centimeters during the year with the frequent, strong sea anomalies. In the Hurricane Irene year, by contrast, erosion was 40 centimeters.
Overall, average erosion for all the studied zones were 17 inches in the anomaly year and 15 inches for the Hurricane Irene year.
Steep gradient beaches, or beaches with a steep slope, were especially vulnerable, as waves can expend energy over a broader area at those locations, said Theuerkauf. "Anomalies can weaken a beach's natural defense to hurricanes, dunes and backshore," he said.
Prior research suggests that climate change may worsen the occurrence and magnitude of anomalies by reducing the speed of the Gulf Stream. This factor could be especially relevant in areas south of Cape Hatteras, because of the influence there of the Florida Current, which feeds the Gulf Stream, said Theuerkauf.
"There is a balance between the speed of the current and the slope of the water surface in the current. When the speed is reduced, this balance is altered, resulting in increases in coastal water levels along the East Coast," he said. However, any coastal area can be affected by anomalies, he said.
In June and July of 2009, an anomaly was documented stretching from Massachusetts to Florida, he added.
The work follows other research suggesting wide regional variations with sea level. In February, for example, scientists reported that the storm surge risk in some seasons of the year may be worse in the eastern Gulf of Mexico than the western Gulf (ClimateWire, Feb. 4).
One of the authors of earlier research on anomalies said that the new work is a useful addition to an active -- and relatively new -- research area. While there is a hypothesis that climate change may worsen anomalies and alter ocean currents generally, there needs to be much more data as to the exact mechanism driving the phenomena, as well as the impact on the coast, said Tal Ezer, a professor of ocean, earth and atmospheric sciences at Old Dominion University.
Ezer said he would soon be releasing a paper showing that minor flooding in Norfolk, Va., from sea-level anomalies may be predicted by Gulf Stream measurements off the coast of Florida.
We need to "better understand why there are periods of more frequent anomalies ... and examine responses of other barrier islands," Theuerkauf said.
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.
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