Antarctic tabular icebergs in ocean waves
The thesis presents a detailed account of our existing knowledge of Antarctic tabular icebergs and their development. Data from in situ field experiments and automatic stations placed on the surface of icebergs are presented and analysed. The data comprise information on the sea state, iceberg geometry, structure and other features, rigid body motions and surface strains. We show that tabular icebergs respond to ocean waves of medium (15-30 s) and long (>30 s) periods, but that they act as filters to very short (<10 s) period waves. A two-dimensional, linear model of rigid body motions, including hydrodynamical effects of added mass and damping and the forcing from the ocean waves, is presented. Good correspondence with the field data is generally obtained, but the causes of long period rigld body motion response of tabular icebergs, especially in roll, cannot be fully explained by a linear, two-dimensional model. We present evidence for substantial bending of the bergs in response to resonance periods in the rigid body motion spectra. This evidence is found both in results from the analysis of field data, and from modelling of the flexure of icebergs. The largest strains, of the order of microstrain and larger, are experienced when icebergs respond to storm swell of periods between 15 and 40 s. Finally, we discuss evidence for the break-up of tabular icebergs and conclude that the shape and size, as well as inherent flaws are important in determining the conditions necessary for their fracture. Thus, heavily crevassed and thin icebergs deteriorate rapidly, while square, thick icebergs relatively free of crevasses can last for several years at sea.
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