STUDIES OF ICE RIDGE SHAPE AND GEOMETRY FROM UPWARD LOOKING SONAR DATA
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Ice ridges may pose a serios threat to structures and vessels in ice covered waters. To appropriatly account for the actions that could be imposed by the ridge keels their properties should be described statistically. A useful tool to observe ridge keels are upward looking sonars. Upward looking sonars, which can be mounted on submarines or moored to the seabed, observe the ice draft directly above. Such observations are suitible for the derivation of ridge keel geometry statistics. The current work focused on the derivation of ridge keel geometry and shape from upward looking sonar data. From 2006 to present the Norwegian Polar insitute has monitered the ice export out of Fram Strait using upward looking sonars with a time resolution which enables individual ridges to be identified. Data from this instumentation during the period 2006 to 2011 is the primary source of data used in the present study. Ice ridge keels were identified using the Rayleigh criterion with a threshold value of 2.5m and a minimum draft of 5m. Key statistics of the geometry parameters draft, width and area are provided. The deepest keel observed in the Fram Strait in the period was 35m deep and an extreme value analysis suggest that a 100 year return value is in the range 37-41m. The shape of the ridge keel is studied through two symmetry parameters and the keel area coefficient. The symmetry parameters confirmed the common assumption abut symmetric ridge keels, but also quantify the variation of the location of the deepest point of the ridge keel and the centroid keel. The keel area cofficient is the ratio between the area of an idealized triangular ridge keel, spanned by the keel width and keel draft, and the observed keel area. A triangular keel would underestimate the keel area for 78% of the ridges. A better generalization of the shape of first year ridges is a trapezoidal keel shape rather than triangular. Based on the observatons the mean trapezoidal keel, representing both first year ridges and old ridges, has a keel bottom-width which is 10%-15% of the keel width. There was a loss of deformed ice in the Transpolar Drift in the Fram Strait during 2006-2011. In this period the relative contribution from defromed ice to the mean ice thickness decreased from 76% to 52%. This loss was coused by a reduction in ridge draft and frequency which declined with approximately 2% annually. An increasing fraction of first-year ridges – a result of the recent basin wide decrease in ice age – could be an important contributing factor, through its greater sensitivity to melting/disintegraton than old ridges.