Tide-surge historical assessment of extreme water levels for the St. Johns River: 1928–2017

Abstract

An historical storm population is developed for the St. Johns River, located in northeast Florida—US east coast, via extreme value assessment of an 89-year-long record of hourly water-level data. Storm surge extrema and the corresponding (independent) storm systems are extracted from the historical record as well as the linear and nonlinear trends of mean sea level. Peaks-over-threshold analysis reveals the top 16 most-impactful (storm surge) systems in the general return-period range of 1–100years. Hurricane Matthew (2016) broke the record with a new absolute maximum water level of 1.56m, although the peak surge occurred during slack tide level (0.00m). Hurricanes and tropical systems contribute to return periods of 10–100years with water levels in the approximate range of 1.3–1.55m. Extratropical systems and nor’easters contribute to the historical storm population (in the general return-period range of 1–10years) and are capable of producing extreme storm surges (in the approximate range of 1.15–1.3m) on par with those generated by hurricanes and tropical systems. The highest astronomical tide is 1.02m, which by evaluation of the historical record can contribute as much as 94% to the total storm-tide water level. Statically, a hypothetical scenario of Hurricane Matthew’s peak surge coinciding with the highest astronomical tide would yield an overall storm-tide water level of 2.58m, corresponding to an approximate 1000-year return period by historical comparison. Sea-level trends (linear and nonlinear) impact water-level return periods and constitute additional risk hazard for coastal engineering designs.