RAPID: Collaborative Research: Marsh Sedimentation due to Hurricanes Florence and Michael Flooding Event in SC

飓风对沙质海岸线的破坏以及对沿海城市和居民社区的破坏负有责任。然而，科学家们最近开始认识到，它们可以对一些沿海系统产生净积极影响。特别是，虽然风暴波可能会对沿海湿地造成地表开挖和沼泽边缘的侵蚀，但飓风也可能在风暴潮淹没期间造成净沉积：风暴波和洋流将沉积物从底部搅动到近岸和海湾内，并在伴随的风暴潮期间将这些悬浮泥沙沉积在沼泽上。尽管这一过程很少见，但一场飓风会给沼泽表面增加更多的无机沉积物，而不是几十年来每天的过程所造成的影响。它还表明，沼泽植物群落受益于风暴沉积，因为从新的泥沙中获得了额外的营养。因此，飓风可以显著提高沼泽地的复原力，并有助于它们在海平面加速上升的情况下的可持续性。今年9月，飓风佛罗伦萨被预测为4级飓风，并在北卡罗来纳州海岸造成严重破坏。然而，高空风切变显著降低了风速，在风暴登陆时将其降级为1级飓风。尽管如此，随着飓风向内陆移动，与风暴相关的降水淹没了北卡罗来纳州和南卡罗来纳州的沿海地区，降雨量为6至20英寸。三天的降雨量大大增加了当地河流的水位和流量，其中几条河流直接流入南卡罗来纳州中部的温亚湾。这些洪水将大量悬浮泥沙带到沿海海洋和近岸的沼泽。我们计划在罗曼角地区测量这些源自河流的沉积物，并将这次沉积事件与2017年伊尔玛飓风后绘制的风暴潮衍生沉积物进行比较。这项研究的主要目标是证明飓风佛罗伦萨和迈克尔导致的无机沉淀率增加对支持罗曼角尖端前陆的沼泽系统的潜在好处。与2017年的飓风伊尔玛不同，伊尔玛产生了高能海浪、强劲的陆上风和1米的风暴潮，并导致1-2厘米厚的泥层沉积，而飓风佛罗伦萨和迈克尔对南卡罗来纳州中部的沼泽几乎没有直接影响，因为它们的风力较弱，风暴路径的影响较小。然而，佛罗伦萨，以及程度较小的迈克尔，产生了强烈的降水，极大地增加了沿海河流的水位和流量一个数量级。其中几条河流将洪水和大量悬浮泥沙直接带入温亚湾和附近的沼泽系统。在飓风伊尔玛之后对先前采样的罗曼角沼泽进行重复取芯，将提供一个难得的机会，将高河流沉积事件与高能量飓风造成的沉积事件进行比较。我们将重新访问和重新采样我们的罗曼角地点(沿两个横断面的10个站点)，以及测量我们在浑水湾附近的十个长期地点(在罗曼角)。此外，横断面沿线至少还有四个地点将被占用，以确保与飓风相关的沉积物的广泛空间覆盖。将在每个站点进行沉积物岩芯、浅层沉积物样本和观测，以记录新的沉积，并确定我们是否可以区分每个飓风伊尔玛(由周围沉积物引起的海浪和风暴潮)和飓风佛罗伦萨(引入新的河流沉积物)提供的沉积物。后者将通过观测、短沉积物岩心的沉积学分析(粒度、结构、物理结构、颜色)和与每次飓风伊尔玛和飓风佛罗伦萨/迈克尔有关的地表样品的整体地球化学分析(TOC、TN、Delta13CTOC、Delta15NTN)来完成。此外，我们将检查现有的记录，以确定它们是否记录了过去与风暴相关的沉积峰值。来自NSF的资金支持现场采样和对20个表层沉积物样本(分别来自Irma和Florence/Michael的10个)的实验室处理和地球化学分析，以及将在新的采样站收集的我们10个岩芯的短芯分析。这一奖励反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Hurricanes are responsible for the destruction of sandy shorelines and devastation to coastal cities and residential communities. However, scientists have recently begun to recognize that they can have net positive influences on some coastal systems. In particular, although storm waves may cause surface excavation and erosion of the marsh edge in coastal wetlands, hurricanes may also cause net sedimentation during storm surge inundation: storm waves and currents stir sediment off the bottom in the nearshore and within bays and deposit this suspended sediment on marshes during the accompanying storm surge. Although this process is infrequent, a single hurricane can add more inorganic sediment to the marsh surface than is emplaced by daily processes acting over several decades. It has also been shown that marsh plant communities benefit from storm sedimentation due to the added nutrients derived from the new mud. Thus, hurricanes can significantly increase the resiliency of marshes and help their sustainability in a regime of accelerating sea-level rise. This past September, Hurricane Florence was predicted to be a category 4 hurricane and wreak havoc along the North Carolina coast. However, upper wind shear significantly reduced wind velocities, downgrading the storm to a category 1 hurricane by the time it made landfall. Still as the hurricane moved inland, precipitation associated with the storm deluged coastal regions of North and South Carolina with 6 to 20 inches of rain. This amount of rain over a three day period greatly increased the stage and discharge of local rivers, several of which empty directly into Winyah Bay in central South Carolina. These flood waters carried large plumes of suspended sediment to the coastal ocean and to the proximal marshes. We plan to measure this river-derived sediment in the Cape Romain region and compare this sedimentation event to storm-surge-derived deposits mapped in 2017 following Hurricane Irma.The major goal of this study is to document the potential benefits of increased rates of inorganic sedimentation caused by Hurricanes Florence and Michael to the marsh system backing the Cape Romain cuspate foreland. Unlike Hurricane Irma in 2017, which generated high-energy waves, strong onshore winds, and a 1 m storm surge, and resulted in the deposition of a 1-2 cm thick mud layer, Hurricanes Florence and Michael had little direct impact on the marshes of central South Carolina due to their weaker winds and less impactful storm tracks. However, Florence, and to a much lesser extent Michael, produced intense precipitation, which vastly increased the stage and discharge of coastal rivers by an order of magnitude. Several of these rivers carried floodwaters and substantial quantities of suspended sediment directly into Winyah Bay and nearby marsh systems. Repeat coring of the Cape Romain marshes at locations previously sampled following Hurricane Irma would provide a rare opportunity to compare a high riverine sedimentation event to that produced by an energetic hurricane. We will revisit and resample our Cape Romain locations (10 stations along two transects), as well as measure our ten long-term SET sites near Muddy Bay (in Cape Romain). In addition, at least four other sites along the transects will be occupied to ensure broad spatial coverage of the hurricane-related deposit. Sediment cores, shallow surface sediment samples, and observations would be taken at each station to document new sedimentation, and determine if we can differentiate sediment delivered by each Hurricane Irma (wave- and storm-surge- induced from ambient sediment) and Hurricane Florence (introduction of new riverine sediment). The latter will be done via observations, sedimentological analyses (grain size, texture, physical structures, color) of short sediment cores and bulk geochemical analyses (TOC, TN, delta13CTOC, delta15NTN) of surface samples associated with each Hurricanes Irma and Hurricanes Florence/Michael. In addition, our existing SET records would be examined to determine if they capture past storm-associated spikes in sedimentation. Funds from NSF support field sampling and the lab processing and geochemical analyses of 20 surface sediment samples (10 each from Irma and Florence/Michael), and short-core analysis from our 10 cores to be collected at new sampling stations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.