Tropical Storm Debby (June 24-26, 2012) was a 1 in 15 year event for Bald Point, a coastal barrier in Apalachee Bay, FL. With as much as 1.8 m of surge and 0.5 m of rainfall during the storm, significant impacts on the landscape due to flooding and overwash are likely. This Rapid Response project will characterize the geomorphic and sedimentological signature of TS Debby at Bald Point, leveraging substantial work already conducted in this area by the principal investigator, a scientist at the Woods Hole Oceanographic Institution. LiDAR collected before the storm will be compared to post-storm total station surveys to quantify net erosion/accretion. Sediment samples from washover surfaces and shallow, backbarrier cores will be collected for textural and microfossil analyses. This work will help to calibrate and verify previously generated records of tropical storm frequency based on Bald Point's backbarrier stratigraphy.
The project will lead to a better understanding of the impacts and historical frequency of severe storms in the northeastern Gulf Coast region. A post-doctoral researcher and an undergraduate student from WHOI's REU program will be supported. Results of the project will be communicated to coastal management groups and the general public. Data will be made broadly available through the WHOI Coastal Systems Group website.
This worked aimed to test the sensitivity of coastal sink hole ponds on Apalachee Bay, Florida to overwash deposition by examining the possible impacts of Tropical Storm Debby in 2012. These sink hole ponds provide very long (multi-millennia) records of event beds deposited by hurricanes and thus provide important information on how hurricane climate may have varied. But determining the sensitivity of the site to storm-induced deposition (in other words, what are the characteristics of the storms that are capable of producing event beds?) is key to interpreting the long term record derived from these sediment archives. Field evidence indicated that Tropical Storm Debby did not produce enough coastal inundation to produce an event bed at the sites. Using climatological-hydrodynamic modeling of inundation and theevent bed-based long-term reconstructions we show that Apalachee Bay is far more vulnerable to tropical cyclone surge than historically observed. While most extreme surges were generated by the uppermost storm intensities, large surges resulted from a wider range of intensities. Medium intensity storms (e.g., category 3) shoulder a surprising proportion of surge-related risk as they outnumber extremely intense storms and tend to have larger inner wind fields, which can produce higher and more extensive surges than the more compact wind fields of more intense storms. Thus, in addition to storm intensity and track, storm size plays an important role in determining the surge magnitude. This finding implies that it may be difficult to infer the specific paleohurricane intensity from the sedimentary record by constraining the magnitude of the storm surge that produced an overwash deposit. However, the approach presented here provides a means of assessing the population of storms of a variety of intensities, sizes, and tracks that are capable of producing surge levels required to transport coarse-grained barrier and nearshore sediment to coastal ponds and wetlands that preserve a record of their occurrence. This analysis also suggests that in addition to storm intensity, track, and size, local environmental effects such as coastally trapped Kelvin waves in this case must be considered when assessing risk and potential mitigation strategies. The return period for historically unprecedented overwash event beds determined from the 4500-year paleo-record at Mullet Pond (42-year event) is similar to the return period for inundation regime storms derived from the 5175-year modern synthetic hurricane climatology (37-year event). However this does not imply that risk of extreme hurricane inundation in Apalachee Bay has been constant over time. Statistically significant clustering of high threshold events in the Mullet Pond record suggests that changes in global or regional climatic boundary conditions likely played an important role in driving the temporal variation in extreme hurricane inundation over the last several millennia. Thus significant temporal variability in the probability of extreme hurricane-induced inundation has occurred over the last 4500 years. In comparison to the last several millennia, the historical interval of the last few hundred years has been anomalously quiescent with respect to the most extreme hurricane-induced inundation events. Applying the climatological-hydrodynamic numerical method to various non-stationary climate conditions, including reconstructed paleoclimates, may shed light on the temporal variations of paleohurricane activity.
