The influence of ice rubble on sea ice friction

Sea ice floes in the Arctic collide with each other, and this leads to the production of smaller pieces of broken ice, which we call rubble. Rubble is also produced when ice collides with offshore structures, and when ships pass through sea ice. Previous analyses of ice friction have considered the contact between two sliding ice surfaces. Here, we consider the effective friction between two ice surfaces separated by ice rubble. In particular, we present experimental results across a range of scales and environments. We show results from metre-scale experiments in the Barents Sea; from metre-scale experiments in the Hamburg Ship Model Basin (HSVA); and from centimetre-scale experiments in the Ice Physics laboratory at UCL. We show that the effective kinetic friction is consistent across these scales, and comparable to friction coefficients measured without rubble. Looking at static friction, we find that when floes are in static contact for a short time, the presence of rubble acts to reduce static friction. However, if floes and rubble remain in static contact for around 10^4s (a few hours) then the presence of rubble promotes strengthening, and the floe-floe effective friction can be raised by the presence of rubble. This has implications for modelling Arctic Ocean dynamics and for assessing friction loads on ships making repeated passages through a channel.