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

虽然已经有定性和定量的观察海洋表面重力波能量传播的快速衰减在泥泞的海底在许多地方，并提出了几种理论来解释这些现象，一直没有研究，直接表明什么机制是负责观察到的耗散。在2007年、2008年和2010年，WHOI的研究人员Traykovski特罗布里奇沿着其他大学的合作者一起对路易斯安那州大陆架进行了实地观察，其中包含必要的测量，以直接确定负责衰减表面波能量的过程。在5米和9米等深线的波能通量的差异表明，在波强迫事件后形成的高浓度泥层的衰减急剧增加。在7米等深线上，一组向下瞄准的脉冲相干多普勒和声学反向散射剖面仪测量了通过上覆水和泥层的湍流分辨速度和反向散射剖面。这些观测结果表明，在最大衰减期间，泥层中的湍流波动停止，波边界厚度增加到与泥层厚度相似。这与两层粘性理论预测的耗散峰值一致，其中泥浆层的粘度比清水的粘度增加了四个数量级。本研究将研究泥浆层的流变特性，作为泥浆层内的时间和深度的函数，因为它从具有相对流动泥沙浓度的完全湍流过渡到静止的弹性泥浆。初步分析还确定了两个不同的波模式的泥浆-水界面。当泥浆是移动的时，通过检查水平速度结构，可以在泥浆层中识别出与表面波具有相同波长和频率的外模波。在强强迫和较低的泥沙浓度的内部模式波，具有类似的频率的表面波，但更短的波长为2至3米的剖面仪阵列也测量。分析将耦合这两种波模式的动力学研究与泥浆流变特性的逆解，以确定波能耗散的机制。该分析将从局部和区域强迫的角度研究形成泥层的过程。同样在2008年进行的其他测量表明，浅水中的衰减率比粘性双层理论预测的要高。因此，正反馈机制的作用，即增加衰减增加沉积的潜力将在深度和跨岸dependencyofattenuation. Intellectualmerit的背景下进行检查：识别机制，通过直接原位观测耗散波能在一个泥泞的海底是最重要的，但尚未实现的一步，在了解这些系统的行为。可用于此分析的测量提供了一个独特的机会来测试各种拟议的理论机制的数据，足以解决与不同processes.Broader影响的变化动态：拟议的分析探讨泥浆引起的波衰减和沉积物沉积增加之间的正反馈机制的潜力。相关机制的准确描述是必不可少的数值模拟这些过程中，数值模拟可以指导管理问题的细沉积物输入到浅海岸系统，如密西西比/Atchafalaya分流。

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.