In early October 2015, residents along the southern and mid-Atlantic East Coast of the US held their breath as reports on Hurricane Joaquin’s gathering strength—but uncertain landfall destination—created mounting anxiety.
Luckily, Joaquin moved back out to sea. But while hurricane landfall became a non-issue, other meteorological forces in its wake conspired to create a far worse scenario than anyone expected. For residents of South Carolina, October 2015 was the scene of unprecedented flooding.
Every Thousand Years—Or Maybe Next Year
Within days, officials and the media were dubbing it the “1,000-year storm,” which suggests that the last horrific flood took place during the Norman Conquest (1066) and that another of the same caliber is a safe 10 centuries away. Unfortunately, both are myth.
According to blog reports during the storm by University of Georgia professor Marshall Shepherd, who is also a Weather Channel host and past president of the American Meteorological Association, “Most people do not actually understand the concepts of what a 100- or 1,000-year rain event means. Many people literally assume it means this event can only happen every 1,000 years.”
But, he explains, the popular but erroneous “1,000-year” moniker has very little to do with actual years, and everything to do with statistical probability. Which means a similar disastrous event could occur next week, next year, or hundreds of years from now.
Although statistical probability helps in the predictive value, the US Geological Survey describes the 1,000-year concept as based on data gathered over a period of at least 10 years to create what it terms a frequency analysis tool. Experts look at magnitude and duration of rainfall events, streamflow measurements, and other data as the basis for calculating recurrence intervals. But for people who were watching the water rise by the hour, this was hardly important; as Marshall says, “It was just unbelievable.”
The Sequence of Events Leading to Disaster
According to Derrec Becker, public information coordinator for the South Carolina Emergency Management Division, “The nature of flooding is unlike any other disaster. It doesn’t get a lot of attention until it actually happens and kills people.”
Becker says this was not a one- or two-day event, but an emergency of 20-days’ duration from October 4 to 23. Rainfall, flooding, evacuations, heavy infrastructure and property damages, and water supply emergencies created an around-the-clock crisis for the entire state, but especially in the hardest-hit midlands region. However, the agency had already been on high watch and preparing for Joaquin, monitoring the hurricane’s path, and on September 30, 2015, urging residents and agencies to be “ready to respond if the need arises.”
By October 1, officials were predicting heavy rain and flooding even if the hurricane headed out to sea. Before the dinner hour that same day, the governor had declared a state of emergency, saying “Heavy rain from a separate weather system will create flash flood conditions throughout the entire state.” On Friday, October 2, one flood-related death had been reported and the worst was still to come, with flood warnings being issued for 14 counties.
Becker says the state was able to quickly get a presidential declaration of a federal emergency, which helped to mobilize agencies, and to request assistance. On Saturday, October 3, when President Obama declared a federal emergency, nearly 9,000 people were already without power. By Sunday, October 4, there were more than 300 reports of flooded roads, the eastern shore Edisto Beach sea wall had been breached, sanitary sewers were overflowing in North Charleston, and five weather-related deaths had been reported. By Tuesday, October 6, the death toll had risen to 17 people, primarily from auto-related drownings as people attempted to drive on flooded roadways.
weather system caused heavy flooding.
Becker says devastating rainfall in the presence of already high tides set the scene for devastation. “We were already fighting the effects of King Tides, and four days of rain—reported from the various rain gauge stations as between 22 and 27 inches—were funneling straight in from the hurricane.”
Preparing in the Historic Districts
East of the midlands, the region of the state most significantly affected, architect Jerry English of Charleston architecture firm Cummings & McCrady describes the concerns on the coastal side when storm events are approaching. “Over here on the coast, we had the highest rainfall in this storm event, even more than the midlands who got 17 inches; we got 27 inches of rain over a three- or four-day period. But because our area is so low, you don’t get a lot of quick runoff, so much [of that water] goes into the city of Charleston Harbor.”
English says all of the area’s stormwater is redirected to natural water bodies. “The city has put in some major storm drainage improvement with deep excavation tunneling and pumping.”
He was a principal on the Charleston team that had recently finished a sea wall restoration, replacing a wall built just prior to World War I. The new sea wall portion protects the historic peninsula of the High Battery promenade that faces Charleston Harbor and looks out to Fort Sumter. The promenade is a popular walkway for tourists enjoying the city’s historic cobblestone streets, elegant pastel color homes, and rich Colonial and Civil War history. English says, “Our most important concern we have about any storm is trying to determine when landfall will occur and how this is happening relative to the state of the tides.”
The tides, he says, “are central to any view of whether there could be flooding or damage.” In October 2015, weather experts reported that the already high “King Tides” of late September precipitated the highest tides recorded in Charleston Harbor—greater than 8 feet—since Hurricane Hugo set a record of 12.56 feet in 1989.
English cites wave action as another serious concern. “Wave action has greater effect than wind. In designing the new High Battery, we were using new tough standards based on better knowledge and years of weather data analysis. Everything from tides, wave action, and seismic considerations are used, and then we also have to adhere to our strict historic preservation policies in the materials and building process.”
While floodwaters covered the new High Battery and roadways beyond during October, the new sea wall performed perfectly. The city is continuing the renovation of the remaining mile-long adjoining Low Battery sea wall, which English anticipates will take many more years for completion.
The King of Tides, and a “Lunar Standstill”
Becker explains that “King Tides,” a term that originated in Australia, denotes the phenomena of higher-than-normal tides …