CONSTRAINING HURRICANE WIND SPEED AT LANDFALL USING STORM SURGE OVERWASH DEPOSITS FROM A SINKHOLE IN ST MARKS, FL

Instrumental records of hurricane strikes in the United States are relatively short, extending back only ~160 years. Sediment deposits created by hurricane-induced coastal inundation serve as valuable proxies of storm activity and can extend these records back several millennia. Spring Creek Pond, a ~15 m deep coastal sinkhole located within the Florida Panhandle, has remained relatively undisturbed over the last 2500 years allowing it to accumulate fine grained, organic-rich sediments punctuated by well-preserved quartz sand layers most likely associated with hurricane surges. Here, we provide a record of hurricane inundation and, for the first time, an approximate Saffir-Simpson storm category for each event-deposit.

Grain size analyses are performed on each event-deposit to help constrain flow conditions required for erosion and transport. Age constraints on the deposits are developed by using carbon-14, cesium-137, and lead-210 radiometric dating. A series of 2-D coastal inundation simulations of 152 synthetic storms using the Regional Ocean Modeling System (ROMS) establishes the relationship between hurricane wind speed and maximum grain size mobilized by the storm surge. This relationship is used to assess the most likely Saffir-Simpson category which produced each event-deposit.

It was found that 1) simulated wind speeds for modern deposits are consistent with the intensities for historical hurricanes affecting the site, 2) all deposits throughout the 2500 yr record are capable of being produced by hurricanes, 3) a period of increased numbers of intense hurricanes was observed between ~1700 and ~700 yrs ago which is consistent with past reconstructions from nearby sites, and 4) no major (> category 3) hurricanes have struck the site in the past ~500 years, suggesting that the current reoccurrence rate of 1 every 46 years for this area is high.