Analysis of Field Measurements of Viscous Damping of Ocean Surface Waves by Fluid Mud

流体泥浆对海面波浪粘性阻尼的现场测量分析

• 批准号: 1059914
• 负责人: Peter Traykovski
• 金额: $ 56.45万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-04-15 至 2016-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1059914&HistoricalAwards=false
• 关键词: Analysis; Field; Measurements; Viscous; Damping

Although there have been both qualitative and quantitative observations of rapid attenuation of ocean surface gravity wave energy propagating over muddy seafloors in numerous locations, and several theories have been proposed to explain these phenomena, there have been no studies that directly show what mechanisms are responsible for the observed dissipation. In 2007, 2008 and 2010 WHOI investigators Traykovski & Trowbridge along with a group of collaborators from other universities conducted field observations on the Louisiana shelf that contain the necessary measurements to directly identify the processes responsible for attenuating the surface wave energy. The difference in wave energy flux at the 5 and 9 m isobath showed a dramatic increase in attenuation as high concentration mud layers formed after wave forcing events. On the 7 m isobath an array of downward aimed pulse coherent Dopplers and acoustic backscatter profilers measured turbulence resolving velocity and backscatter profiles through the overlying water and mud layer. These observations showed that during the period of maximum attenuation, turbulent fluctuations in the mud layer cease and the wave boundary thickness increases to be similar to the mud layer thickness. This is consistent with the peak of dissipation predicted by two-layer viscous theory with an increase in viscosity of the mud layer of four orders of magnitude over that of clear water. This study will examine the rheological characteristics of the mud layer as a function of time and depth within the mud layer as it transitions from a fully turbulent flow with relatively flow sediment concentration to a stationary elastic mud. The preliminary analysis has also identified two distinct wave modes on the mud water interface. When the mud is mobile, external mode waves with the same wavelength and frequency as the surface waves can be identified in the mud layer by examining the horizontal velocity structure. During strong forcing and lower sediment concentrations internal mode waves, with similar frequencies to the surface waves, but with much shorter wavelengths of 2 to 3 m were also measured by the profiler array. The analysis will couple studies of the dynamics of these two wave modes with inverse solutions for the rheological characteristics of the mud to determine the mechanisms of wave energy dissipation. The analysis will examine the processes that form the mud layers in terms of both local and regional forcing. Other measurements, also taken in 2008, show that attenuation in shallow water increases at a rate that is greater than that predicted by viscous two layer theory. Thus the role of positive feedback mechanisms, whereby increased attenuation increases the potential for deposition will be examined in the context of the depth and cross-shore dependence of attenuation.Intellectual merit: Identifying the mechanisms which dissipate wave energy over a muddy seafloor via direct in-situ observations is the most important and yet unachieved step in understanding the behavior of these systems. The measurements available for this analysis provide a unique opportunity to test a variety of proposed theoretical mechanisms with data that is sufficient to resolve the varying dynamics associated with the different processes.Broader impacts: The proposed analysis examines the potential for positive feedback mechanisms between mud induced wave attenuation and increased sediment deposition. Accurate description of the relevant mechanisms is essential for numerical modeling of these processes, and numerical modeling can guide management issues regarding the input of fine sediment into shallow coastal systems such as the Mississippi/Atchafalaya distributaries.

尽管对在许多地点泥泞的海底传播的海洋表面重力波能量的快速衰减进行了定性和定量观测，并提出了几种理论来解释这些现象，但还没有研究直接表明造成观测到的耗散的机制。在2007年、2008年和2010年，WHOI的调查员Traykovski & Trowbridge和一组来自其他大学的合作者在路易斯安那州大陆架进行了实地观察，其中包含了直接确定导致表面波能量衰减的过程的必要测量。波浪强迫事件后形成高浓度泥层，5 m和9 m等深线处波浪能通量差值衰减急剧增加。在7米等深线上，一组向下瞄准的脉冲相干多普勒和声波后向散射剖面仪测量了湍流分辨速度和通过上覆水和泥浆层的后向散射剖面。这些观测结果表明，在最大衰减期间，泥层湍流波动停止，波边界厚度增加到与泥层厚度相近。这与两层粘性理论预测的耗散峰值一致，泥层的黏度比清水的黏度增加了4个数量级。本研究将研究泥浆层的流变特性，作为时间和深度在泥浆层内的函数，因为它从具有相对流动沉积物浓度的完全湍流过渡到静止的弹性泥浆。初步分析还确定了泥水界面上两种不同的波浪模式。当泥浆处于活动状态时，通过检测泥浆层的水平速度结构，可以识别出与表面波波长和频率相同的外模态波。在强强迫和较低沉积物浓度时，剖面仪阵列还测量了与表面波频率相似的内模态波，但波长要短得多，为2 ~ 3 m。该分析将结合这两种波动模式的动力学研究和泥浆流变特性的反解，以确定波浪能量耗散的机制。该分析将从局部和区域强迫两方面考察形成泥层的过程。同样在2008年进行的其他测量表明，浅水中的衰减速度比粘性两层理论预测的要快。因此，正反馈机制的作用，即衰减的增加增加了沉积的可能性，将在衰减的深度和跨岸依赖性的背景下进行研究。智力优势：通过直接的现场观测确定在泥泞的海底上耗散波浪能量的机制是理解这些系统行为的最重要但尚未实现的步骤。可用于此分析的测量为测试各种提出的理论机制提供了独特的机会，这些理论机制的数据足以解决与不同过程相关的变化动力学。更广泛的影响：拟议的分析考察了泥浆引起的波浪衰减和增加的沉积物沉积之间的正反馈机制的可能性。准确描述相关机制对于这些过程的数值模拟至关重要，数值模拟可以指导有关细粒沉积物输入浅海岸系统（如密西西比河/阿查法拉亚河）的管理问题。