Four stages of pressure ridging

The pressure ridging process is simulated using a two-dimensional particle model. Blocks are broken from an intact sheet of relatively thin lead ice pushed against a thick, multiyear flee at a constant speed. The blocks of ice rubble accumulate to form the ridge sail and keel. During the simulations the energy consumed in ridge growth, including dissipation, is explicitly calculated. On the basis of the results of simulations performed with the model, the ridging process can be divided into four distinct stages. The first stage begins with an intact sheet of lead ice impacting a flee and ends when the sail reaches its maximum height. In the second stage the ridge keel deepens and widens. The stage ends when the maximum keel draft is reached. In the third stage the direction of growth is leadward creating a rubble field of more or less uniform thickness. The third stage ends when the supply of thin ice is exhausted. In the fourth stage the rubble field is compressed between converging flees. The results of simulations establish the dependence of ridging energetics in the first and second stages on the thickness of the ice sheet and the amount of ice pushed into the ridge. The average profiles of the simulated ridges delineate the growth process in the first, second, and third stages. The energetics and profiles of the fourth stage were described by Hopkins et al. [1991]. Lead ice extents of up to 1300 m are pushed into ridges to determine maximum sail heights, keel drafts, and ridging forces.