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

更好地理解细颗粒沉积物悬浮、迁移和沉降的物理过程是推进底栖生物生态学、生物地球化学和地质学领域的关键。对海底可蚀性和悬浮颗粒性质的控制是限制对细颗粒泥沙运动的准确预测的两个最大未知数。最近的发现表明，在浑浊、生物活跃的沿海环境中，水流与两种主要类型的泥质颗粒包装的相互作用明显不同。尽管它们是由几乎相同的泥粒形成的，而且往往是同时存在的，但松散结合的絮体和生物压实的颗粒对循环的侵蚀、沉降和反应截然不同。目前，这两种常见类型的颗粒包对流动的不同物理响应没有得到充分的观测记录，也没有得到充分的分析，也没有在最新的模型中得到令人满意的表示，特别是当它们同时发生时。这项研究中需要解决的关键问题包括：(I)湍流和颗粒包装如何影响泥沙沉降的性质？(Ii)床面压力和颗粒包装如何影响床面的可蚀性？(Iii)流体动力学和颗粒包装之间的反馈如何控制局部优势颗粒分布？相关的假设包括：(I)更强的湍流使较大的颗粒悬浮，但撕裂絮体，因此湍流增加了颗粒的沉降速度，但降低了絮体的沉降速度。(Ii)随着应力的增加，颗粒的侵蚀质量取决于它们各自的大小和密度，而絮体的侵蚀质量更多地取决于沉积以来的时间。(Iii)与较重的颗粒相比，汇聚的近床流动和/或减少的湍流更有效地捕获缓慢沉降的絮体，而发散的近床流动和/或增加的湍流分散絮体，留下剩余颗粒的滞后沉积。以上假设将通过观察、分析和模拟三管齐下的方法进行验证。对湍流速度和泥沙浓度时间序列的观测，结合侵蚀微观世界和颗粒成像相机，将能够在时空变化的浑浊环境中连续估计WS和床的可蚀性。细颗粒泥沙动力学的分析方法将纳入正式的数据同化技术，从而能够对侵蚀速率参数和悬浮颗粒的尺寸分布等关键变量进行持续的客观推断。虽然同时考虑了颗粒和絮体的存在，但将使用三维建模来扩展整个水动力系统的局部观察和分析，评估收敛和发散环流的作用，并执行更系统的敏感性分析。广泛影响：加深对细粒沉积物传输和沉积的了解，将有助于改善对沿海地区人类影响的管理，应用包括污染物传输(污染物通常与细微沉积物结合并随其一起移动)、生态学(泥浆提供关键的栖息地，并承载关键的生物地球化学循环)以及河口、港口和疏浚废物的工程。数据同化尚未被广泛应用于泥沙输运；因此，拟议的研究具有变革工作的潜力。PI积极参与了几个主要的社区建模工作，为研究结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，他们将接受观察、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指导与此项目相关的本科生研究？和/或威廉和玛丽？S的高级论文，就像他们过去做的那样。研究所教授的研究生班将包括数值方法、泥沙输运和河口动力学。研究结果将通过VIMS的活动向公众传播，包括一系列公开讲座，以及每年吸引数千名参观者的年度海洋科学日。

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.