Shokotsu River ice jam formation

A spring ice jam occurred in the Shokotsu River in March 1995. This has been the only known ice jam in this river since 1955. This paper discusses the ice jam processes and investigates the causes of the Shokotsu River ice jam by analyzing the available hydrometeorological data as well as hydraulic and geometric characteristics of the river. This analysis showed that this breakup event was triggered by a sudden increase in basin runoff produced by the rainfall during a warm spell. Snowmelt resulting from warm air temperature alone would not be sufficient to produce the discharge increase for the breakup. The jam was initiated at a section with rapid reductions in channel slope, flow velocity, as well as the top width as the ice run approached from upstream. Numerical simulation carried out to analyze the ice jam dynamics showed that the model is capable of reproducing the jam event and supplemented the limited information obtained from the field observation.