  Water wave attenuation by grease ice is a key mechanism for the polar
regions, as waves in ice influence many phenomena such as ice drift, ice
breaking, and ice formation. However, the models presented so far in the
literature are limited in a number of regards, and more insights are required
from either laboratory experiments or fieldwork for these models to be
validated and improved. Unfortunately, performing detailed measurements of wave
propagation in grease ice, either on the field or in the laboratory, is
challenging. As a consequence, laboratory data are relatively scarce, and often
consist of only a couple of wave elevation measurements along the length of the
wave tank. We present combined measurements of wave elevation using an array of
ultrasonic probes, and water kinematics using Particle Image Velocimetry (PIV),
in a small-scale wave tank experiment. Experiments are performed over a wider
frequency range than what has been previously investigated. The wave elevation
measurements are used to compute the wave number and exponential damping
coefficient. By contrast with a previous study in grease ice, we find that the
wave number is consistent with the mass loading model, i.e. it increases
compared with the open water case. Wave attenuation is compared with a series
of one-layer models, and we show that they satisfactorily describe the viscous
damping that is taking place. PIV data are also consistent with exponential
wave amplitude attenuation, and a POD analysis reveals the existence of mean
flows under the ice that are a consequence of the displacement and packing of
the ice induced by the gradient in the wave-induced stress. Finally, we show
that the dynamics of grease ice can generate eddy structures that inject eddy
viscosity in the water under the grease ice, which would lead to enhanced
mixing and participating in energy dissipation.

Fluid Dynamics