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测到海洋表面重力波能量在许多地点通过泥泞海底传播的快速衰减，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所观察到的耗散是什么机制造成的。2007年、2008年和2010年，WHOI的调查者Traykovski&Amp；Trowbridge与其他大学的一组合作者在路易斯安那州的大陆架上进行了实地观测，其中包含了直接确定导致表面波能量衰减的过程所需的测量结果。波能通量在5m和9m等深线的差异表明，由于在波浪强迫事件后形成了高浓度泥层，因此衰减显著增加。在7m等深线上，一组向下瞄准的脉冲相干多普勒仪和声波后向散射剖面仪测量了上覆水和泥层中的湍流分辨速度和后向散射剖面。这些观测表明，在最大衰减期，泥层中的湍流波动停止，波边界厚度增加，接近泥层厚度。这与两层粘性理论预测的耗散峰值是一致的，泥层的粘度比清水的粘度增加四个数量级。这项研究将考察泥层从相对流动泥沙浓度的完全湍流过渡到静止的弹性泥浆时，泥层的流变特性随时间和泥层内深度的变化。初步分析还确定了泥水界面上的两种截然不同的波动模式。当泥浆运动时，通过检测水平速度结构，可以在泥浆层中识别出与面波具有相同波长和频率的外模波。在强强迫和低泥沙浓度期间，剖面仪阵列也测量到了频率与面波相似但波长短得多的内模波。分析将把对这两种波动模式的动力学研究与泥浆流变特性的反解结合起来，以确定波浪能量耗散的机制。分析将从当地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，浅水中的衰减增加的速度比粘性两层理论预测的要大。因此，正反馈机制的作用，即增加衰减增加沉积的潜力，将在衰减的深度和跨岸相关性的背景下进行审查。智力优势：确定通过直接现场观测在泥泞的海底上空消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提供了一个独特的机会，可以用足以解决与不同过程相关的变化动力学的数据来测试各种提出的理论机制。广泛的影响：拟议的分析考察了泥浆诱导的波衰减和泥沙沉积增加之间的正反馈机制的可能性。对相关机制的准确描述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将细颗粒沉积物输入到诸如密西西比河/阿查法拉亚分流等浅海系统的管理问题。

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.