RAPID: Quantifying the Great Marsh Sedimentation Event 2018

RAPID：量化 2018 年大沼泽沉积事件

• 批准号: 1832177
• 负责人: Zoe Hughes
• 金额: $ 0.8万
• 依托单位: Trustees of Boston University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-03-15 至 2018-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1832177&HistoricalAwards=false
• 关键词: RAPID; Quantifying; Great; Marsh; Sedimentation

An unprecedented event took place on New England marshes between late December 2017 and January 2018 wherein football field-sized patches of marsh were blanketed with 2 to 12 inches of ice-rafted silt and sand. This is hugely significant because these marshes normally gain elevation through sediment accumulation more slowly than local sea level is rising. Because marshes are essentially flat, even a small rise in water level will affect a large area. Ultimately, if the waters remain too deep, marshes will drown and switch to open water, with loss of all ecosystem services and coastal protections they afford. The large amount of sediment that was deposited in January, assuming the marsh grasses can grow through it, will give the areas a boost in elevation, making them more resilient to rising waters. The event occurred when two weeks of extreme cold were followed by higher than normal astronomic tides, combined with a powerful northeast winter storm. During the prolonged cold snap, tidal waters froze to the bottom of creeks and bays, incorporating bottom sediment into the ice, which thickened and trapped more sediment with each successive low tide. When the storm surge occurred, the ice rafts floated up and onto the marsh surface, where they became stranded, eventually melting and leaving the sediment on the marsh. In a normal year, ice-rafting would bring about 5% of the annual inorganic material deposited, but this single storm layer is estimated to be equivalent to 71 years of normal accretion. Such infrequent, high sedimentation ice rafting events may therefore be integral to sustaining marshes in New England (as hurricanes can be to wetlands in the southern US); yet are rarely documented. This project addresses this knowledge gap by mapping the area covered in sediment using aerial photography ground-truthed with measurements of thickness and coverage in the field, to fully quantify the amount of sediment transported onto the marsh and its impact on marsh elevation. In addition to quantifying the importance of winter storm processes to northern marshes, this study will provide insights into marsh restoration strategies, such as artificial elevation building through "thin layer deposition" whereby locally-dredged sediment is sprayed on top of the marsh surface to build new marshes or elevate existing marshes. The January 2018 event sedimentation provides a natural laboratory to study how a Spartina patens-dominated high marsh will respond to artificial sedimentation, and what thickness of sediment the marsh can withstand. The study broadens participation in geoscience research, as it is headed by an early-career woman researcher, and also forms an independent research project for an undergraduate student, who will analyze sediment samples, translate field data into maps, and combine these data with the aerial photography to determine the impact of this event. The primary goal of this study is to assess the extent and volume of the extreme sedimentation event which occurred in January 2018 on the Great Marsh, MA. The researchers will collect measurements of thickness and surface extent of the deposit before the spring grass begins to shoot and obscure its coverage. They will conduct aerial survey mapping to collect ortho-photographs capturing the entire marsh region (~130 km2) at a resolution of 10 cm2 and will use a combination of automated image analysis, manual digitization, and comparison to historical images to map the new ice rafted sediment. Field-based ground-truthing of aerial photos will involve a minimum of eight land to barrier island transects (by foot) across the marsh to map and measure the sediment deposits. Multiple sediment plugs of known volume will be extracted for grain size, organic content and bulk density analysis. Sediment thickness and lateral extent of the deposit, determined from the aerial photographs and field surveys, will allow the researchers to estimate the total volume of sediment deposited during this event, enabling a comparison to known accretion rates. The data collected for this study will provide a record and a baseline for future investigations, including analysis of storm conditions when the ice rafts inundated this marsh, and correlation of vectors of transport to wind direction, water depth and marsh elevation, as well as assessments of grass recovery following the sedimentation event. The work will inform the proposed marsh restoration practice of "thin layer deposition", by determining the thickness of introduced sediment layers that can be tolerated by the marsh ecosystem as aids to accretion under accelerating rates of sea-level rise.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.

2017年12月下旬至2018年1月期间，新英格兰沼泽地发生了前所未有的事件，其中足球场大小的沼泽地被2至12英寸的冰筏淤泥和沙子覆盖。这是非常重要的，因为这些沼泽通常通过沉积物积累获得海拔比当地海平面上升的速度要慢。由于沼泽地基本上是平坦的，即使水位上升很小，也会影响很大的区域。最终，如果沃茨仍然太深，沼泽将淹没并转向开放水域，失去所有生态系统服务和沿海保护，他们提供。1月份沉积的大量沉积物，假设沼泽草可以在其中生长，将使这些地区的海拔上升，使它们更能适应上升的沃茨。这一事件发生在两周的极端寒冷之后，伴随着比正常天文潮汐更高的潮汐，以及一场强大的东北冬季风暴。在持续的寒流期间，潮汐沃茨冻结在小溪和海湾的底部，将底部沉积物融入冰中，随着每次连续的低潮，冰变厚并捕获更多的沉积物。当风暴潮发生时，冰筏漂浮到沼泽表面，在那里它们搁浅，最终融化并在沼泽上留下沉积物。在正常年份，冰漂流将带来约5%的年度无机物质沉积，但这一单一风暴层估计相当于71年的正常增长。因此，这种罕见的高沉积冰漂流事件可能是维持新英格兰沼泽不可或缺的（飓风可能是美国南部的湿地），但很少有记录。该项目解决了这一知识差距，利用航空摄影对沉积物覆盖的地区进行测绘，并在实地测量沉积物的厚度和覆盖范围，以充分量化沉积物转移到沼泽地的数量及其对沼泽地海拔的影响。除了量化冬季风暴过程的重要性，以北方沼泽，这项研究将提供深入了解沼泽恢复策略，如人工海拔建设通过“薄层沉积”，即当地疏浚沉积物喷洒在沼泽表面的顶部，以建立新的沼泽或提高现有的沼泽。2018年1月的事件沉积提供了一个天然实验室，以研究米草属占主导地位的高沼泽将如何应对人工沉积，以及沼泽可以承受的沉积物厚度。这项研究扩大了地球科学研究的参与，因为它是由一位早期职业女性研究人员领导的，也为本科生形成了一个独立的研究项目，他将分析沉积物样本，将现场数据转化为地图，并将这些数据与航空摄影相结合，以确定这一事件的影响。这项研究的主要目标是评估2018年1月发生在马萨诸塞州大沼泽的极端沉积事件的范围和数量。研究人员将在春草开始发芽并掩盖其覆盖范围之前收集存款厚度和表面范围的测量结果。他们将进行航空测绘，以收集以10平方厘米的分辨率捕获整个沼泽地区（约130平方公里）的正射照片，并将使用自动图像分析，手动数字化和与历史图像比较的组合来绘制新的冰筏沉积物。基于实地的航空照片地面实况调查将涉及至少八个陆地到屏障岛屿的横断面（步行），横跨沼泽地，以绘制和测量沉积物沉积。将提取已知体积的多个沉积物塞，以进行粒度、有机物含量和体积密度分析。根据航空照片和实地调查确定的沉积物厚度和存款的横向范围，将使研究人员能够估计这一事件期间沉积的沉积物总量，从而能够与已知的沉积速率进行比较。为这项研究收集的数据将提供一个记录和未来调查的基线，包括分析风暴条件时，冰筏淹没这片沼泽，和相关的矢量的运输风向，水深和沼泽海拔，以及评估后的沉积事件草恢复。这项工作将通知拟议的沼泽恢复实践的“薄层沉积”，通过确定的厚度引入沉积层，可以容忍的沼泽生态系统作为艾滋病在加速速度的海平面上升。这一奖项反映了NSF的法定使命，并已被认为是值得通过评估使用基金会的智力价值和更广泛的影响审查标准的支持。