Threshold condition between mechanical and thermal breakup

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. Quantitative conditions that demarcate these two types of breakup events can find application in a variety of issues, such as interaction between river ice processes and the aquatic ecosystem, socio-economic impacts of river ice, and effects of climatic change on river ice regimes. Using recent advances on the onset of mechanical breakup, a simple threshold criterion is postulated and its quantitative consequences are worked out. At a given river site, there is a site-specific rise in water level above the freeze-up elevation, which delineates mechanical from thermal events. This rise is approximately proportional to ice thickness but also depends on local river morphology and hydraulics. Data from case studies on three very different rivers are used to test this prediction and show encouraging results. Respective site-site specific coefficients are determined and environmental implications of the present findings are discussed