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

知识价值：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横向混合的机理。在许多河口和海岸水流中，横向输送被认为是由横向环流与垂直混合相互作用所主导的。假设横向剪切不稳定性可能在建立水平剪切层之间的交换中起主要作用。虽然这些不稳定是间歇性的，但它们会产生连贯的横向涡旋，从而产生广泛的横向交换，特别是当它们沿着垂直分层的水道形成时。第二个重点领域侧重于浅滩河口沉积物动力学的潮汐和风致变异性。检查这些过程的方法依赖于对水动力学和悬浮泥沙动力学的实地观测，以及河床样品分析和河床声学成像的结合。战略是追求两个截然不同但相互补充的仪器阵列：一个将横向定位于海峡和浅滩，另一个将沿浅滩-河道界面纵向定位。对这些观测结果的分析将利用当地和总量综合的纵向动量、盐度和悬浮泥沙预算，以确定横向交换和相关的交换和再悬浮变化的机制。这两个基本过程的研究将在浅滩河口泥沙输送的背景下进行。更广泛的影响：拟议的工作将在科学界和旧金山河口的管理方面产生广泛影响。首先，横向混合的检验，特别是横向切变不稳定的作用，将对河口和海岸水流的模拟产生重要的影响。在粗分辨率模式中，必须指定水平扩散系数。为了这个目的，几乎从来没有选择过随时间变化的扩散系数，但随时间变化的扩散系数对更大范围的输运的影响将是深远的。随着计算能力的提高，更高分辨率的模式，包括大涡模拟(LES)，正变得更容易实现。在这些情况下，次网格尺度的参数化成为通过未分解过程建立交换和混合的关键组成部分。如果横向切变不稳定实际上是横向混合的一个重要组成部分，大涡模拟方法很可能能够解决它们，但没有现场尺度的数据集来确定分层潮流中横向切变不稳定的性质。这项工作对旧金山湾河口管理的潜在影响，即使不比科学影响更大，也与科学影响一样重要。在南旧金山湾周边潮汐沼泽的恢复中，沉积物再分布的动力学是高度不确定的，但也是恢复成功的关键。由于要恢复的土地的下沉，必须积累大量的泥沙使这些地区达到沼泽平原的高程，这将显著改变整个河口的泥沙动态，本文提出的工作结果将直接为模型开发工作和恢复活动的管理决策提供参考。

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.