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.
