Threshold between mechanical and thermal breakup of river ice cover

Extreme ice-jam flood events in rivers occur during a type of breakup that is partly governed by the mechanical properties of the ice cover, and known as “mechanical”. By contrast, thermal breakups are preceded by advanced thermal decay of the ice and can only produce insignificant, if any, jamming. Identification and quantification of threshold conditions that demarcate the two types of breakup events will improve current capabilities in a variety of issues, including river ice–aquatic ecosystem interactions, socioeconomic impacts of river ice, and climate-change impacts via modified river ice regimes. Extending recent advances in the prediction of the onset of mechanical breakups, a simple threshold criterion is postulated and its quantitative consequences are worked out. It is shown that there is a site-specific rise in water level above the freeze-up elevation, which delineates mechanical from thermal events. The threshold value is approximately proportional to the thickness of the ice cover, and also depends on local river morphology and hydraulics. These predictions are tested against three extensive case studies and found to perform satisfactorily. Environmental implications of the present results are discussed.