RAPID: Post-Hurricane Irma Erosion and Deposition in Coastal Embayments of Eastern St. John, U.S. Virgin Islands

Ohio State University, The, Columbus OH

Investigators

Abstract

St. John, Virgin Islands is a tropical island made up primarily of ash and debris from volcanic eruptions. It is exposed to major geohazards such as hurricanes and tsunamis. The bathymetry of the nearshore coast has a strong influence on the severity of runup and surge from such events. St. John and other similar volcanic islands are unlike carbonate/limestone islands that are flat and lack significant topography. St. John has steep slopes and the type of rocks immediately near the coast that are prone to weathering. This makes such islands vulnerable to land-based sediment erosion. A multibeam bathymetric survey was conducted in August 2017 in several offshore bays. Because Hurricane Irma passed over the Virgin Islands after the hurricane, there is a valuable opportunity to understand the impact of the hurricane in this offshore environment. This project will complete a second multibeam bathymetric survey and collect sediment cores to assess the extent of the erosion of the island, the quantity of sediments that flowed into the bays, and how the shape of the floor of the offshore bays changed. Understanding these impacts are important for creating more resilient coastal communities. Two graduate students will be involved in the field work and data analysis. The results will be provided to local geoscience and environmental groups. The major scientific objective of this project is to quantify the volume, distribution, and type of sediment influx that occurred as a result of Hurricane Irma on St. John, U.S. Virgin Islands. Isolating the effects of this hurricane event will yield a better scientific understanding of how to interpret paleo-storm deposits observed in sediment cores A post-hurricane Irma multibeam bathymetry and backscatter survey is planned to exactly overlap the regions of St. John mapped pre-Irma (Coral Harbor, Johnson Bay, and Saunders Bay) and also, to fill in data gaps from previous surveys. A transect of cores from the interior of Coral Harbor moving seaward will be taken from the survey vessel using a pocket vibra core system. Results of the project will constrain the sediment flux delivered to the marine environment by storms versus non-storm sedimentation. In addition to understanding the hurricane event, these new data together will yield important insights into the relative contribution of a one-off storm event versus that of annual-to-decadal non-storm sedimentation.