On the morphodynamic instability of water-ice interface in a brackish lake

Periodic morphologies, such as ripples, can be found under ice cover in rivers and lakes, and in glacial meltwater streams (e.g. Ashton and Kennedy, 1972; Parker, 1975; Camporeale and Ridolfi, 2012). Stability analysis suggests that a plane ice surface in contact with water at a temperature slightly above freezing can become unstable, with the appearance of ripples (e.g. Thorsness and Hanratty, 1979). Investigations of freshwater lakes (e.g. Stefan et al., 1998) suggest that in lakes that have an ice cover in the winter, temperature, and therefore density is stably stratified. In this condition, without any further driving mechanism, convective mixing due to unfavourable density stratification and, also, wind forcing are absent. In the case of brackish lakes, however, gradual freezing at the interface causes the formation of black ice, from which salt is excluded. As the ice cover thickens, a net downward flux of dissolved salt is generated in the below water at the interface. Both salt exclusion from the ice, as the ice cover thickens, and subsequent freshwater ice melt can modify the seasonal structure of a brackish lake. Pieters and Lawrence (2009) documented, through temperature and conductivity measurements along the water column, the effect of salt exclusion on seasonal circulation in Tailings Lake, a brackish lake located in the Northwest Territories, Canada. In particular, they showed that for Tailings Lake, during winter, the formation of significant black ice, and the resulting flux of salt beneath the ice, was sufficient to break down the stable temperature density stratification to the degree necessary to fully redistribute this salt throughout the water column. In the present work, we study the morphodynamic instability of the water-ice interface. Results provide a mechanistic explanation for the field observations collected by Pieters and Lawrence (2009).