LAUSR

Abstract Deltas and other multi-channel river systems in cold regions are particularly prone to ice jam flooding. The Hay River delta in the Northwest Territories, Canada is a unique full-scale field laboratory for studying such events because of its relatively simple network geometry and easy access throughout. This paper presents detailed analyses of ice formation and consolidation events at the main channel junction of the Hay River delta. These analyses describe the dynamic, two-phase, open channel flow (DTPOCF) of water and ice in a multi-channel environment. Water waves and ice movements were tracked continuously at 22 locations over three breakup seasons. In the six analyzed events, ice jam movements were initiated at the upstream end of the ice jams while the stopping of these movements was initiated at the downstream end. This resulted in ice movements that alternated between the delta channels, due to the unsteady flow conditions at the channel junctions and the momentum of the ice upstream of the junction. This alternating pattern of ice movement had been observed in the Hay River delta for many decades; but until this study, it had remained unexplained. These findings suggest that the prediction of the extent of ice jam consolidation, and thus ice jam flooding, may require consideration of unsteady ice jam dynamics. Finally, a unique cause for ice jam consolidation was observed: a wave generated by the melting and “creeping consolidation” at the upstream end of an ice jam caused the jam to consolidate and release.