Wave-ice experiments

 

A series of experimental tests were carried out to investigate the interaction between waves and ice floes. Wave generation included both monochromatic and random waves with varying periods and steepnesses. floes were modelled both using plastic plates and freshwater ice. The main purpose of the research was to investigate the reduction in wave transmission and the role of the overwash, the phenomenon where water washed the upper surface of the plate.

 

An experimental model of wave attenuation in pancake ice

In the winter, when the Antarctic sea ice cover is expanding, the far edge of the marginal ice zone is populated by small floes with characteristic diameters much smaller than ocean wavelengths and known as pancake ice. To cast new light on the propagation of waves in ice, an experimental model was setup in the Sea Ice Wind Wave Interaction (SIWWI) flume at the University of Melbourne.

Sea Ice Wind Wave Interaction (SIWWI) flume at the University of Melbourne

 

Energy dissipation in pancake ice for two different ice concentration

Results indicate that wave attenuation depends on ice concentration, with as low as 20 – 40% of energy going through high density covers. Although observations reveal that the ice cover attenuates wave energy over the entire spectrum, energy attenuation is more effective at high frequencies, inducing a significant downshift of the spectral peak.

Alberello, A., Nelli, F., Dolatshah, A., Bennetts, L., Onorato, M. and Toffoli, A., 2018. An experimental model of wave attenuation in pancake ice. Proceedings of the 29th ISOPE, Honolulu, Hawaii, USA. 2019.

 

Hydroelastic interaction between water waves and floating freshwater ice 

Hydroelastic interactions between regular water waves and floating continuous freshwater ice cover are investigated using laboratory experiments in the  for a range of incident wave periods and steepnesses. 

The experiments were conducted at the University of Melbourne in a facility consisting of a wave flume (see Fig. 1), housed inside a refrigerated chamber, where air temperatures can be reduced to −15◦ C. The flume is made out of glass supported by a wooden frame, ensuring optical access and that the structure experiences minimal contraction or expansion during freezing and defrosting. The flume is 14m long, 0.76m wide, and was filled with fresh water 0.45 m deep. It is bounded at one end by a computer-controlled cylindrical wave-maker; and at the opposite end by a linear beach with slope 1:6, which absorbs incoming wave energy (95% energy-effective for waves tested).

Figure 1: Schematic of wave flume, with the wave maker at the left-hand end and the beach at the right-hand end. The light-blue is the water and the dark blue is the initial ice cover. Red rectangles indicate the camera locations.

Results indicate that short-period, small-steepness incident waves travel only a short distance into the ice-covered water without breaking the ice (Fig. 2). As the incident waves get longer and steeper, the waves propagate farther into the ice cover and rapidly break up the ice cover over an increasing distance at least 5 wavelengths for the longest waves tested, which would be on the order of kilometres or more at field scale, and over only a few minutes. Most striking, a sharp transition was noted to rapid breakup of the entire ice cover and wave propagation along the full length of ice-covered water, indicating the existence of a positive feedback loop between increased breakup and increased propagation. Moreover, the increased propagation and breakup led to extensive overwash, submerging some of the broken floes, which would likely affect floe melt rates in the field. The results cast new light on hydroelastic waveice interactions, showing, for the first time, simultaneous observations of wave propagation and wave-induced ice breakup, and paving the way for less idealised investigations into the natural, field-scale phenomenon.

Figure 2: (a–c,e–f,i–k) Mean ice (green squares) and wave (red circles) amplitudes at x = xi (i = 1,. . . ,4) – see supplementary material for definitions of parameters. The continuous blue and jagged purple lines at the bottom of panels (a–c,e–f,i–k) indicate the unbroken and broken status of the ice cover respectively at end of each tests; the lengths of broken ice floes is the measured dimension, while vertical displacements are arbitrary. (d,h,l) Example water surface elevation time series for the highlighted (light blue background) cases at x1 = 1.05 m; if no overwash, water and ice surface elevations coincide.

Dolatshah, A., Nelli, F., Bennetts, L.G., Alberello, A., Meylan, M.H., Monty, J.P. and Toffoli, A., 2018. Hydroelastic interactions between water waves and floating freshwater ice. Physics of Fluids, 30(9), p.091702, doi: 10.1063/1.5050262.

A preprint is available here

Reflection and transmission of regular water waves by a thin, floating plate

Measurements of the wave fields reflected and transmitted by a thin floating plastic plate were produced for regular incident waves over a range of incident periods (with wavelengths comparable to the plate length) and steepnesses (ranging from mild to storm-like). Two different plastics are tested, with different densities and mechanical properties, and three different configurations are tested. The configurations include freely floating plates, loosely moored plates (to restrict drift), and plates with edge barriers (to restrict waves overwashing the plates). The wave fields reflected and transmitted by plates without barriers are shown
to become irregular, as the incident waves become steeper, particularly for the denser plastic and the moored plate. Further, the proportion of energy transmitted by the plates without barriers is shown to decrease as the  incident wave becomes steeper, and this is related to wave energy dissipation.

PVC plate in the moored case (left-hand panel) and moored with barriers case (right).

 

Reflection plus transmission coefficient, as a function of the incident wave steepness,
for the mooring-with-barriers case (blue circles),mooring-without-barriers case (magenta asterisks),
and freely floating case (green squares).

 

Nelli, F., Bennetts, L.G., Skene, D.M., Monty, J.P., Lee, J.H., Meylan, M.H. and Toffoli, A., 2017. Reflection and transmission of regular water waves by a thin, floating plate. Wave Motion, 70, pp.209-221.