Sir,

It was interesting to note the comments by Reference AshtonAshton(1974) concerning the Froude number concept for the stability of ice packs as originally developed by Reference KivisildKivisild (1959). The paper by Ashton refers to the case of the stability of a unique ice-floe in front of an ice cover. It should be noted that this is not necessarily the same as the criterion to define the moment at which the whole ice pack stops growing up-stream by accumulating ice-floes at the upper end under which further ice-floes could be washed. It is this criterion for the progression limit of an ice cover which was studied by Kivisild. It results in a different concept of Froude number and it is derived from the behaviour of the whole ice cover. The instability of a single ice-floe docs not prevent the up-stream progression of the ice cover. Individual floes may become unstable and move under the cover. Rafting increases the thickness and the cover is again stable as a whole.

In that case there is still an up-stream progression of the ice cover and the whole mass becomes unstable only when the Froude number based on the water depth reaches a critical value. This will be achieved by a change of water depth up-stream of the cover that may be caused by an accumulation of rafted floes under cover.

In nature, the total ice thickness of the cover does not stay constant, it depends on the quantity of rafted ice in the cover. We have recently carried out further investigations on this problem on rivers in Yukon, confirming that the Froude criterion rather than temperature or ice thickness is the most important parameter in defining the development and break-up of dynamic ice covers, hanging ice dams, and jams.

10 October 1974