Hurricane Irma affected the Florida peninsula in September 2017. The east coast of the peninsula was hit particularly hard: the city of Jacksonville flooded around the St. Johns River estuary with non-tidal water levels that exceeded 1.5 m and precipitation that surpassed 20 cm in 24 hours. This study used observations such as water and wind velocities, river discharge, and conductivity data to determine whether compounding forcings influenced flood levels. Results show that flooding was initiated by a pulse from the ocean and then exacerbated by high river discharge. The 1-2 punch from the ocean and then the river caused record flooding, with impacts that lasted through the rest of September. Peak water levels occurred while hurricane winds were receding, and river discharge was increasing. Compound flood models should consider the phase lag between driving processes, as the individual peaks may not occur simultaneously, yet exacerbate flooding.
