Horizontal shear flows and lateral mixing in estuaries and the coastal ocean: Sediment transport in the shoal-channel estuary

智力优势：这是一项观察性研究，旨在解决河口和沿海海洋流动和运输分析中的两个基本问题。第一部分阐述了在水平剪切存在的情况下造成横向混合的机制。在许多河口和海岸流动中，假定横向输送主要是横向环流与垂直混合相互作用。假设横向剪切不稳定可能在横向剪切层间交换的建立中起主要作用。虽然是间歇性的，但这些不稳定性产生了连贯的横向涡流，产生了广泛的横向交换，特别是当它们沿着垂直分层的通道形成时。第二个重点领域集中在潮汐和风引起的泥沙动力学变化在浅滩河道河口。检查这些过程的方法依赖于基于现场的水动力学和悬浮沉积物动力学观察，以及床层样品分析和床层声学成像的结合。该策略是追求两个不同但互补的仪器阵列：一个将横向定向穿过海峡和浅滩，另一个将纵向定向沿着浅滩-海峡界面。对这些观测结果的分析将利用纵向动量、盐度和悬浮沉积物的地方预算和整体预算，以便建立负责横向交换以及交换和再悬浮的相关变异性的机制。对这两个基本过程的考察将在浅滩-河道河口泥沙运输的背景下结合起来。更广泛的影响：拟议的工作将对科学界和旧金山河口的管理产生广泛的影响。首先，横向混合的研究，特别是横向剪切不稳定性的作用，将对河口和海岸流动的建模产生重要影响。在粗分辨率模型中，必须指定水平扩散系数。时变扩散系数几乎从不用于此目的，但时变扩散系数对更大规模输运的影响将是深远的。随着计算能力的提高，包括大涡模拟（LES）在内的高分辨率模型越来越容易实现。在这种情况下，亚网格尺度的参数化成为建立未解析过程交换和混合的关键组成部分。如果横向剪切不稳定性实际上是横向混合的一个重要组成部分，那么LES方法可能能够解决它们，但是在分层潮汐流中建立横向剪切不稳定性性质的现场尺度数据集是不可用的。与科学影响同样重要的是，这项工作对旧金山湾河口管理的潜在影响。在南旧金山湾周边潮汐沼泽的恢复中，沉积物再分配的动态是高度不确定的，但对恢复的成功至关重要。由于待修复土地的沉降，大量的泥沙必须积累，使这些地区达到沼泽平原的高度，这将显著改变整个河口的泥沙动态。这里提出的工作结果将直接告知关于恢复活动的模型开发工作和管理决策。

Intellectual Merit: This is an observational study to address two fundamental issues in the analysis of estuarine and the coastal ocean flows and transport. The first addresses the mechanisms responsible for lateral mixing in the presence of horizontal shears. In many estuarine and coastal flows, lateral transport is assumed to be dominated by transverse circulation interacting with vertical mixing. It is hypothesize that lateral shear instabilities could play a major role in establishing exchange across horizontal shear layers. Although intermittent, these instabilities create coherent lateral eddies that produce extensive lateral exchange, particularly when they are formed along a vertically stratified channel. The second area of emphasis focuses on the tidal- and wind-induced variability of sediment dynamics in a shoal-channel estuary. The approach to examining these processes relies on field-based observations of the hydrodynamics and suspended sediment dynamics, as well as a combination of bed sample analysis and acoustic imaging of the bed. The strategy is to pursue two distinct, but complementary, instrument arrays: one will be oriented laterally across the channel and shoals, the other will be oriented longitudinally along the shoal-channel interface. Analysis of these observations will utilize budgets, both local and volume integrated, for longitudinal momentum, salinity and suspended sediment in order to establish the mechanisms responsible for lateral exchange and the associated variability of exchange and re-suspension. The examination of these two fundamental processes will be brought together in the context of sediment transport in a shoal-channel estuary. Broader Impacts: The proposed work will have broad impacts both in the scientific community and in the management of the San Francisco Estuary. First, the examination of lateral mixing, particularly the role of lateral shear instabilities, will have important implications for the modeling of estuarine and coastal flows. In coarse resolution models, horizontal diffusion coefficients must be specified. Time varying diffusion coefficients are almost never chosen for this purpose, but the implications of a temporally varying diffusion coefficient on larger-scale transport would be profound. As computational capability increases, higher resolution models, including large-eddy simulation (LES), are becoming more realizable. In these cases, the subgrid- scale parameterization becomes the critical component in establishing exchange and mixing by unresolved processes. If lateral shear instabilities are, in fact, an important component of lateral mixing, LES approaches would likely be able to resolve them, but field-scale data sets to establish the nature of lateral shear instabilities in stratified tidal flows are not available. Just as significant as the scientific impacts, if not more so, are the potential impacts of this work on the management of the San Francisco Bay Estuary. In the restoration of tidal marshes around the perimeter of South San Francisco Bay, the dynamics of sediment redistribution are highly uncertain, but also critical to the success of the restoration. Due to the subsidence of the lands to be restored, a large quantity of sediment must accumulate to bring these areas to marsh plain elevation, which will significantly alter the sediment dynamics throughout the estuary.The results of the work proposed here will directly inform that model development effort and management decision-making regarding restoration activities.