Mechanisms of Fluid-Mud Interactions Under Waves

he interaction between water waves and a muddy sea bottom is a complex process that plays an important role in wave evolution in coastal regions as the deformable bottom can absorb considerable wave energy. The resulting decrease in wave height over an unconsolidated seabed results in a virtual lack of a surf zone as has been observed in a number of locations including China, India, South Korea, Louisiana. Laboratory studies show that waves can lose 90% of their height after propagating just a few wavelengths over a viscous mud. Remotely-sensed water wave attenuation and diffractive changes in wave direction due to bottom dissipation can provide information about the presence of areas of bottom mud. The mud can lead to the burial of seabed sensors or mines and it can affect navigation in coastal waters by interfering with bottom detection. The remobilization of temporarily deposited mud can cause drastic degradation of water column visibility affecting divers.

The major objective of this study is to examine the various mechanisms of water wave dissipation over muds by: performing detailed field observations of a muddy sea bed under waves to determine the nature of the energy dissipation; carrying out large-scale laboratory experiments examining a variety of wave-mud interactions, to complement the field by spanning a greater range of parameters and mechanisms; and theoretically formulating the various types of mechanisms and implementing them into numerical models, including a large-scale phase-resolving wave simulation (SNOW), as well as the phase-averaged model (SWAN).

Three approaches are used to examine these various damping mechanisms. Careful field measurements off the coast of western Louisiana will examine in detail the behavior of a mud bottom during wave events, providing the means to determine the operative damping mechanisms there Secondly, we will carry out laboratory experiments of waves over mud in two large wave tanks to isolate the mechanisms in greater detail: one tank is at JHU and the other is the Super Tank at the Tainan Hydraulics Laboratory, Taiwan. Finally, theoretical analysis and numerical modeling will provide the input for determining dissipation algorithms to be added into a phase-resolved computational model based on high order spectral methods (SNOW) and spectral forecast models, such as SWAN, including those mechanisms not observed in the field.