Frequency of coastal floods increasing in South Carolina because of sea level rise | McClatchy Washington Bureau

Most of the coastal floods in South Carolina during the past 66 years caused problems for residents only because of sea level rise, a new study indicates.

The report, titled “Unnatural Coastal Flooding,” puts the blame on human activity, the same notion behind the debate in the United States over climate change. The report isn’t referring to major storms, such as Hurricane Matthew, which produced sea surges and storm tides that would have washed over large areas of land with or without the recorded 8 inches of rise in the Atlantic waters. Instead, it looks at what is called “nuisance flooding.”

The report says: “Just as sea levels have been rising, so too has the frequency of coastal floods, in particular recurrent minor or ‘nuisance’ floods, often tidal in nature. Nuisance floods do not cause major damage, but do cause material harm, inconvenience and economic drag.”

One of the study’s authors said it would be a mistake to dismiss sea level rise from any coastal-flooding conversation.

“Every coastal flood today is deeper and more damaging because of sea level rise caused by humans through climate change,” Ben Strauss, of Climate Central – an organization for climate scientists that is based in Princeton, New Jersey, and committed to public education on the issue – wrote in an email answer to questions. “The same thing absolutely applies for Hurricane Matthew.”

The study is far broader in scale than just South Carolina, referring to 27 places around the U.S. coast. It relies on a basic fact: During the past 116 years, ocean measurements have shown an average of an 8-inch rise in the water level.

The report may undersell the impact of sea level change for South Carolina. Measurements at the tidal gauge in Charleston at the mouth of the Cooper River show the sea is 12.5 inches higher today than it was in 1920.

This study follows up a full state report from the same group on the effects of sea level rise on South Carolina. That report, written two years before Matthew, warned “floods exceeding today’s historic records are likely to take place within the next 20-30 years in the Hilton Head and Myrtle Beach areas.”

Todd Ehret, a National Oceanic and Atmospheric Administration oceanographer who specializes in tides, said the readings in Charleston came from water level gauge 8665530, just as they had for the past century. The findings deliver a simple message: The sea is higher than it used to be. Ehret was not associated with the study.

“Storms that 100 years ago would have stopped just short of the top of the Battery wall today are flowing over it,” he said. “Now, this is relative sea level, compared to land. You build a structure on land, and that’s a constant of a reference point. But land can be subsiding, and land can be rising. Our readings show the level of the sea relative to the land.”

The study focuses on only 8 inches, noting that this is the amount scientists attribute to global warming – and one of the authors added that about 6 inches of that are thought to be directly attributable to human activity, with the other 2 thought to be attributed to natural warming.

An introduction to the report notes that even those 8 inches have meant floods that can “block traffic, degrade infrastructure and cause a multitude of problems in daily life, even on sunny, storm-free days.”

The report takes a simple approach. Study the days of flooding at tidal gauges around the country, find the days that exceeded the National Weather Service’s “nuisance flood” threshold, then subtract sea level rise.

Reduce South Carolina’s recent coastal floods by 8 inches, and of the 216 flood days from 2005 to 2014, only 40 days of flood would still have reached the nuisance level.

In other words, without sea level rise, there were 176 days when sea water would not have covered streets and backed up drainage systems.

The study doesn’t include 2015, but NOAA’s statistics indicate that Charleston had a record number of 38 days of nuisance flooding that year. Charleston is used because of the tidal gauge there, though for 2015 NOAA reported record numbers of coastal flooding days for cities throughout the region.

The report, put together by Climate Central, shows that the number of nuisance-flooding days overall is increasing. From 1995 to 2004, there were 175 flood days, 130 of which would not have reached the level of nuisance without sea level rise at that date.

From 1985 to 1994, there were 102 flood days, with 72 attributed to sea level rise.

Strauss said local variations in sea level weren’t surprising. He said one reason for local variations was water temperature: Where it it rising most quickly, the water is expanding. Another reason is that ocean currents are slowing: “Whether that’s due to climate change, we don’t know.”

“People expect the oceans to be like peanut butter spread smoothly and evenly over bread,” he said in an interview. “Sea levels are more like chunky peanut butter.”