Seasonal tides + sea level rise + Hurricane Nicole swells = major coastal flooding in the Southeastern US
The highest normally occurring tides of the year, known as king tides, combined with sea level rise and a boost from offshore currents associated with Hurricane Nicole brought coastal flooding along the Southeastern US in mid-October.
King tides are a seasonal event occurring a few times a year when the orbits and alignment of the Earth, moon, and sun combine to produce the greatest tidal effect.
On top of a baseline global sea level rise increase of 8 inches since 1880, Hurricane Nicole's offshore currents are further elevating the seasonal tides. Days before the expected peak water levels in South Florida (October 16 and 17), Nicole contributed to higher than predicted tides in Virginia Key Beach in Miami, which saw tides one foot higher than predicted, and Fort Lauderdale, which reported tides 15 inches above predicted levels.
October 16 saw the worst flooding for much of South Florida, turning streets into rivers in Miami Beach, Fort Lauderdale.
At their peak, the king tides reached levels approximately 3 to 4 feet above normal. In this context of 3-4 feet, the sea level rise to date of 8 inches makes for a significant increase in tide height which in turn reaches much further inland in low-lying areas. NOAA reports that nuisance flooding has already increased 300 to 925 percent due to sea level rise to date.
Sea level rise and high tides
Climate change has boosted king tides—the highest of high tides—even higher. Due to the melting of land ice and the expansion of sea water as it warms, global sea level has risen 8 inches since 1880.
Sea level rise boosts high tides so that they reach higher and extend further inland than in the past, increasing the risk of coastal flooding during high tide events. High tides are a problem even on storm free days—a phenomenon known as “nuisance flooding.”
Since 2006, flooding in Miami Beach has soared—400 percent from high tides and 33 percent from rain. The increased flooding, calculated from insurance claims, media reports and tidal gauges, stems from a regional rise in sea levels well above global increases. From 1998 to 2006, tidal gauges in South Florida show sea level rise keeping up with global averages of about .04 to .20 inches a year. But in 2006, local sea level rise suddenly underwent a rapid acceleration, averaging about .20 inches to a half inch per year.
Across the US, NOAA reports that nuisance flooding has already increased 300 to 925 percent due to sea level rise to date. NOAA water level gauges document an increasing frequency of tidal flooding around much of the US, which is driven primarily by local relative sea level rise. Since 1950, about two-thirds of US coastal flood days in 27 different locations were linked to human-caused sea level rise.
A small vertical increase in sea level can translate into a large increase in horizontal reach by storm surge depending upon local topography. In Florida alone, more than 30,000 people live on land for each vertical inch above the high tide line, averaged across the first six feet. About 5 million people in the United States live less than 4 feet above high tide. Nearly half of them live in Florida. Sea levels are now 8 inches higher due to sea level rise, an increase that drives flooding much further inland along low-lying areas such as much of the southeastern US coastline.
Offshore storms, like Hurricane Nicole during this event, can heighten the risk of coastal flooding further by generating swells, or wind-generated waves, that elevate coastal waters. The greatest risk to human life and infrastructure, however, occurs when coastal storms make landfall, bringing major storm surges that ride atop higher sea levels due to climate change.
While sea level rise may be modest relative to the total height of high tides, it can be responsible for a disproportionate amount of damage. This is because human infrastructure and natural systems have developed to cope with a range of historical extremes, and climate change signals like accelerated sea level rise can overwhelm existing human systems and structures.