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

圣约翰，维尔京群岛是一个热带岛屿，主要由火山喷发产生的火山灰和碎片组成。它暴露在飓风和海啸等重大地质灾害中。近岸海岸的水深测量对这类事件的爬升和涌浪的严重程度有很大的影响。圣约翰和其他类似的火山岛不同于碳酸盐/石灰岩岛屿，它们是平坦的，没有明显的地形。圣约翰有陡峭的斜坡和靠近海岸的岩石类型，很容易被风化。这使得这些岛屿很容易受到陆地沉积物的侵蚀。2017年8月，在几个近海海湾进行了多波束测深调查。由于飓风伊尔玛在飓风过后经过维尔京群岛，因此有一个宝贵的机会来了解飓风在这一近海环境中的影响。该项目将完成第二次多波束水深测量并收集沉积物岩心，以评估该岛的侵蚀程度、流入海湾的沉积物数量以及近海海湾的海底形状如何变化。了解这些影响对于创建更具弹性的沿海社区非常重要。两名研究生将参与实地工作和数据分析。结果将提供给当地的地球科学和环境组织。该项目的主要科学目标是量化飓风伊尔玛在美属维尔京群岛圣约翰造成的泥沙流入的数量、分布和类型。分离这次飓风事件的影响将使人们更好地理解如何解释在沉积物岩心中观察到的古风暴沉积飓风伊尔玛飓风后的多波束测深和后向散射调查计划与圣约翰绘制的伊尔玛飓风前地区(珊瑚港、约翰逊湾和桑德斯湾)完全重叠，并填补以前调查的数据空白。从珊瑚港内部向海移动的岩芯横断面将使用袖珍可控震源岩芯系统从测量船上取下。该项目的结果将限制风暴与非风暴沉积向海洋环境输送的沉积物通量。除了了解飓风事件之外，这些新数据还将对一次性风暴事件与每年至十年一次的非风暴沉积的相对贡献产生重要的洞察。

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 ofsediment influx that occurred as a result of Hurricane Irma on St. John, U.S. Virgin Islands. Isolating theeffects 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.