Improved Observation, Analysis and Modeling of Fine Sediment Dynamics in Turbid, Biologically Active Coastal Environments

深入了解细沙悬浮、运移和沉降的物理过程是推动底栖生态学、生物地球化学和地质学等领域发展的关键。对海底可蚀性和悬浮颗粒性质的控制是限制精确预测细沉积物运动的两个最大的未知因素。最近的研究结果表明，在浑浊的、生物活跃的沿海环境中，流动与两种主要类型的泥浆颗粒包装的相互作用是明显不同的。虽然它们是由几乎相同的泥浆颗粒形成的，而且经常同时存在，但松散结合的絮凝体和生物压实的颗粒对循环的侵蚀、沉降和反应非常不同。目前，这两种常见类型的粒子包对流动的不同物理响应没有得到充分的观测记录，没有得到充分的分析，也没有在最先进的模型中得到令人满意的表示，特别是当它们同时发生时。本研究要解决的关键问题包括：(i)湍流和颗粒包装如何影响沉积物沉降的性质？（ii）床应力和颗粒包裹如何影响床的可蚀性？（iii）流体力学和粒子封装之间的反馈如何控制局部优势粒子分布？与此相关的假设包括：(1)较强的湍流使较大的颗粒悬浮，但撕裂絮凝体，因此湍流增加了颗粒的沉降速度（ws），但降低了絮凝体的沉降速度（ws）。随着应力的增加，颗粒的侵蚀质量取决于其个体的大小和密度，而絮凝体的侵蚀质量更多地取决于沉积后经过的时间。（iii）收敛的近床流和/或减少的湍流相对于较重的球团更有效地捕获缓慢沉降的絮凝体，而发散的近床流和/或增加的湍流分散絮凝体，留下剩余球团的滞后沉积。以上假设将采用观察、分析和建模三管齐下的方法进行检验。湍流速度和沉积物浓度时间序列的观测，与侵蚀微观世界和粒子成像相机相结合，将能够在空间和时间变化的浑浊环境中持续估计ws和河床的可蚀性。细泥沙动力学分析方法将纳入正式的数据同化技术，允许对诸如侵蚀速率参数和悬浮颗粒大小分布等关键变量进行持续客观的推断。在考虑颗粒和絮凝体存在的同时，三维建模将用于扩展整个水动力系统的局部观察和分析，评估会聚和发散循环的作用，并进行更系统的敏感性分析。更广泛的影响：增加对细颗粒沉积物运输和沉积的了解将有助于改善人类对海岸带影响的管理，包括污染物运输（污染物通常与细颗粒沉积物结合并一起移动），生态学（泥浆提供重要的栖息地，并承载重要的生物地球化学循环），以及河口，港口和疏浚废物的工程。资料同化尚未广泛应用于泥沙输运；出于这个原因，拟议的研究具有变革性工作的潜力。pi活跃在几个主要的社区建模工作中，为研究成果和产品的知识转移提供了现成的场所。该项目将支持两名研究生，他们将接受观测、数据分析和同化以及数值模拟方面的培训。pi可能会通过VIMS指导与该项目相关的本科生研究。REU和/或William & & Mary？就像他们过去所做的那样。研究将被纳入由pi教授的研究生课程，包括数值方法，泥沙运输和河口动力学。这项研究的结果将透过VIMS举办的活动，包括一系列公开讲座，以及每年吸引数千人次参观的年度海洋科学日，向公众传播。

A better understanding of the physics of fine sediment suspension, transport and settling is key to advancing the fields of benthic ecology, biogeochemistry and geology. Controls on seabed erodibility and suspended particle properties are two of the largest unknowns limiting accurate prediction of fine sediment movement. Recent findings suggest that within turbid, biologically active coastal environments, the interactions of flow with the two dominant types of muddy particle packaging are notably distinct. Although they are formed from nearly identical mud grains and are often simultaneously present, loosely bound flocs and biologically compacted pellets erode, settle and respond to circulation very differently. At present, the distinct physical responses to flow of these two common types of particle packages are not adequately documented by observations, have not been sufficiently analyzed, and are not satisfactorily represented within state- of-the art models, especially for cases when they occur together.Key questions to be addressed in this study include: (i) How do turbulence and particle packaging affect the nature of sediment settling? (ii) How do bed stress and particle packaging affect bed erodibility? (iii) How do feedbacks between hydrodynamics and particle packaging control the locally dominant particle distribution? The associated hypotheses include: (i) Stronger turbulence suspends larger pellets but rips apart flocs, so turbulence increases settling velocity (ws) for pellets but decreases ws for flocs. (ii) With increased stress, the eroded mass of pellets depends on their individual size and density, whereas the eroded mass of flocs depends more on the time that has passed since deposition. (iii) Convergent near-bed flow and/or reduced turbulence more efficiently traps slowly settling flocs relative to heavier pellets, while divergent near-bed flow and/or increased turbulence disperses flocs, leaving a lag deposit of the remaining pellets.The above hypotheses will be tested using a three-pronged approach of observations, analysis, and modeling. Observations of turbulent velocity and sediment concentration time-series, in concert with erosion microcosms and a particle imaging camera, will enable continual estimation of both ws and bed erodibility in a spatially and temporally varying turbid environment. Methods for analysis of fine sediment dynamics will incorporate formal data assimilation techniques, allowing continual objective inference of such key variables as the erosion rate parameter and the size distribution of suspended particles. While accounting for the presence of both pellets and flocs, three-dimensional modeling will be used to extend localized observations and analysis over an entire hydrodynamic system, evaluate the roles of convergent and divergent circulation, and perform more systematic sensitivity analysis.Broader Impacts: Increased understanding of fine grained sediment transport and deposition will help improve management of human impacts on the coastal zone with applications including contaminant transport (contaminants often are bound to and travel with fine sediment), ecology (muds provide crucial habitat, and host crucial biogeochemical cycles), and engineering of estuaries, harbors, and dredge spoils. Data assimilation has not been widely applied to sediment transport; for this reason, the proposed research has the potential for transformative work. The PIs are active in several major community modeling efforts, providing ready venues for knowledge transfer of research results and products. The project will support two graduate students who will be trained in observations, data analysis and assimilation, and numerical modeling. The PIs will likely mentor undergraduate research pertaining to this project through VIMS? REU and/or William & Mary?s Senior Thesis, as they have done in the past. Research will be incorporated into graduate classes taught by the PIs including numerical methods, sediment transport, and estuarine dynamics. Results of the study will be disseminated to the general public through events at VIMS including a series of public lectures, as well as an annual Marine Science Day that attracts thousands of visitors per year.