5 August 2014
Sea-level spikes can harm beaches worse than hurricane
Posted by abranscombe
By Alexandra Branscombe
WASHINGTON, DC – Unforeseen, short-term increases in sea level caused by strong winds, pressure changes and fluctuating ocean currents can cause more damage to beaches on the East Coast over the course of a year than a powerful hurricane making landfall, according to a new study. The new research suggests that these sea-level anomalies could be more of a threat to coastal homes and businesses than previously thought, and could become higher and more frequent as a result of climate change.
The new study found that unexpected increases in water level of a few centimeters (inches) to a half a meter (almost two feet) above the predicted high tide correlated with the loss of more than half a meter (almost two feet) of beach height on a North Carolina barrier island during 2009 and 2010. This was similar to the amount of erosion in 2010 to 2011 when Hurricane Irene – a category one hurricane with a storm surge of two meters (almost seven feet) high – swept away about a third of a meter (just over a foot) of sediment from the same beaches, according to a new study published last week in Geophysical Research Letters, a journal of the American Geophysical Union.
The new findings suggest that short-term increases in sea level that can last for weeks or months are a more significant factor in beach erosion than previously thought. Scientists call these upward or downward spikes of water level, which occur globally and can stretch along an entire coastline, sea-level anomalies.
Sea-level anomalies are caused by atmospheric and ocean processes, like prevailing winds or changes in water temperature, rather than storms. Unlike hurricanes, which can be detected before they make landfall, sea-level anomalies are mostly undetected until after they arrive, when tide gauges pick up periods of aberrant water levels. Although temporary, sea-level anomalies can last much longer than hurricanes. This constant beating of the shoreline with waves above the point where water usually hits the beach can cause more loss of sand to the ocean than a hurricane storm surge lasting only a few hours, according to the study.
“We see that these anomalies resulted in as much, and in some cases more, erosion than a year with a hurricane,” said Ethan Theuerkauf, a research assistant in marine sciences at the University of North Carolina at Chapel Hill’s Institute of Marine Sciences in Morehead City and lead author of the new paper.
Sea-level anomalies, which on the East Coast are largely influenced by the Gulf Stream, are likely to increase in size and number as climate change warms the ocean and alters ocean currents, the new research says. These short-term increases in sea level can threaten coastal homes and businesses, although they are not usually included in shoreline management plans, according to the new study.
“Steep gradient beaches are the most vulnerable to sea-level anomalies, because the increased water level will push waves higher on the beach and dunes,” Theuerkauf said. If the dunes are low, then water could wash into marshes or beachfront homes that are usually protected by the dunes, he said.
To study the impact of sea-level anomalies on beach erosion, the study’s authors collected sediment samples and measured the depth of erosion at six different beaches at Onslow Beach, a barrier island in North Carolina, each February from 2009 to 2012. By measuring these changes, the scientists were able to tell how much sediment eroded from the beach during this time. Each of these sites also vary in width, dune height and beach gradient, making the island an ideal location for studying how sea-level anomalies impact the many different types of beaches found along the East Coast, Theuerkauf said.
The study’s authors also gathered hourly water-level data from the National Oceanic and Atmospheric Administration’s tide gauge in Wrightsville Beach, N.C., over the four years of the study to identify whether sea-level anomalies or storm surges occurred on the shoreline. Anomalies and hurricanes typically produce different tide gauge patterns of how quickly the coastal sea level rises and falls. The scientists measured the amount of erosion that occurred across each year for the length of the study to conclude the amount of damage caused by each of these ocean events.
The researchers find that, at most of the sites, the beach erosion from sea-level anomalies that occurred between the February 2009 and February 2010 data collecting – a year with frequent anomalies but no large storms—was almost the same or greater than the amount of beach erosion from 2011 to 2012, during which Hurricane Irene hit the shoreline. In 2009 and 2010, the tide gauge records showed frequent sea-level anomalies with waves reaching three meters (10 feet) high.
A year and a half later, Hurricane Irene hit the shoreline producing a storm surge approximately two meters (about 7 feet) high and waves over 4 meters (13 feet). The sea-level anomalies from 2009 to 2010 caused an average of 55 centimeters (22 inches) of beach loss in the zone that falls between the high tide and low tide lines. From 2011 to 2012, the year Hurricane Irene struck, there was an average of 40 centimeters (16 inches) of erosion in this same zone.
The scientists also found that anomalies caused significant erosion of the backshore, the area above the high tide line that is normally only reached by high waves during extreme events. During the 2009 to 2010 study period, erosion from sea-level anomalies in the backshore zone was around 25 centimeters (10 inches), which was similar to the 27 centimeters (11 inches) eroded in the year with Hurricane Irene.
The new research shows that anomalies are a driver of coastal change, said John Jaeger, an associate professor in Geological Science at the University of Florida in Gainesville who was not involved in the new study. He said sea-level anomalies are detrimental to coastal communities, especially if they coincide with other events that erode the shoreline, like a hurricane, or on top of global sea-level rise.
While anomalies along the East Coast can be caused by unusual changes in water temperatures or strong cyclical winds, most anomalies in the region are likely caused by the Gulf Stream, a powerful and fast-moving ocean current that flows from the tip of Florida up the coast to Newfoundland, Canada, according to the study. When the Gulf Stream, which is usually a swift current, undergoes some natural warming and therefore slows down, water will pile up onto the shoreline, he said.
Other researchers have noted that as climate change heats the ocean, the Gulf Stream is also expected to slow down. The result could be an increase in the frequency and intensity of sea-level anomalies, the new study suggests. The ocean will also expand as the planet warms, raising global sea levels, which will mean anomalies and storm surges could reach even further inland, the study says.
“It all goes back to coastal vulnerability to hazards; [the anomalies examined in this study] illustrate that shorelines are already vulnerable,” Jaeger said. “These sea-level anomalies exacerbate the change and retreat of the shoreline.”
– Alexandra Branscombe is a science writing intern in AGU’s Public Information department