MEASUREMENTS OF PACK ICE STRESSES IN THE BALTIC

摘要

Wind and oceanic forcing generates dynamic stresses in a compact ice cover. To this is related the stress propagation phenomenon where stress is transmitted along chains of contacting floes. To the ice navigation community ice stresses manifest as compression that closes channels and slows down ship's progress. The including of compression to ice forecasts is the aim of SAFEWIN project. This development work and the relating of the model quantities to the forces a ship actually experiences requires understanding of how ice cover stresses are generated and transmitted. Experimentally this can be approached in terms of local ice stress measurements that are then compared with the larger scale values from the ice forecast model. Results of a two-week stress campaign conducted in the Bay of Bothnia in 2011 are described. The measurement site was on a floe that drifted to NE direction close to Tankar lighthouse and the 1 hour average wind speed was mostly above 10 m/s. The instruments included a biaxial stressmeter of the same type as has been used in Arctic campaigns, and two arrays of pressure cells. The principal stresses were calculated by rosette theory and the difference between major and minor principal stress was interpreted as dynamic stress magnitude. The dynamics stress had intermittent character with peaks separated by low stress periods and the maximum stresses were about 80 kPa. The peaks could be related to several types of events: dynamic drift, ridging, stress buildup in static ice cover, and stress buildup when ice converges against coast. The results from the biaxial stressmeter and the arrays had a good correlation for the occurrence of stress peaks while stress values could be different.