JACKSONVILLE, Fla. – A perfect weather combination resulted in the historical surge of water along the St. Johns River banks. Hurricane force gusts at the river mouth drove torrents of water into the river at speeds close to the fastest ocean currents in the world.
The fact that Hurricane Irma, was the most intense hurricane observed in the Atlantic in the last decade had little to do with why streets flooded in San Marco and Downtown.
Record water levels in the St. Johns river reached 5 feet seven inches sending water into homes and streets around downtown and many miles down the river.
Tributaries flowing into the largest river in Florida backed up flooding homes in inland locations like Flagler Estates.
A local nor'easter began a couple days before Irma tracked up the state sending currents of water flowing into the river. As the storm pushed up into north Florida hurricane gusts at the mouth of the St. Johns river reached 87 mph.
Sensors measured the current rushing through the river at speeds up to 3.4 mph nearing the average speed of the fastest ocean current in the world, the Gulf stream which flows 60 miles offshore Jacksonville around 4 mph.
The timing was bad for businesses and homeowners along the river. The peak of winds corresponded over two high tide cycles* days after a full moon and the river had no chance to discharge the water into the Atlantic.
High river water clogged sewer routes leaving the eight to eleven inches of rain from Mayport to Ortega with no place to escape.
Most roads were blocked or impassable beyond the typical low spots that flood due to routine nuisance flooding.
These nuisance-flooding events occur multiple times a year. But as sea level continues to rise, the nuisance flooding will get more and more frequent and will become even more of a problem.
Sea level rise will make these results more common in the years to come even without ocean currents pushing water inland.
The sea level trend in Mayport has increased 10 inches in the last 100 years with a 95% confidence level.
*(The river is tidal down to Lake George. Tides typically raise the river level about 1.2 feet at Jacksonville, decreasing some to 0.7 feet at Orange Park where the river widens, and increasing back to 1.2 feet at Palatka as it narrows again.)