Shokotsu River Ice Jam Formation

Hung Tao Shen
Visiting Professor
Hokkaido River Disaster Prevention Research Center
Chuoku, Minami-1, Nishi-1, Sapporo, 060-0061

要旨

The Shokotsu River is one of the major rivers in Hokkaido. The River is 84 km long. It flows through mountainous areas to a narrow coastal plain in northeastern Hokkaido, before discharges into the Sea of Okhotsk. A spring ice jam occurred in March 1995. The ice jam flood threatened the Town of Upper-Shokotsu and National Highway No. 273. This jam was the only jam occurred in this river in the last ten years. Since the flow and ice conditions of the river are similar to most of the rivers in Hokkaido, an analysis of this jam may serve as an example for possible ice jams in Hokkaido rivers. This study investigated the reasons for the occurrence of this ice jam by analyzing the available hydro-meteorological data as well as hydraulic and geometric characteristics of the River. This analysis showed that the breakup of the ice cover was triggered by a sudden increase in basin runoff produced by the rainfall during a warm spell. The jam was initiated at a section where there are rapid reductions in channel slope, flow velocity, as well as the top width as the ice run approached from the upstream. Numerical simulation was carried out to analyze the dynamics of the ice jam formation to supplement the limited information obtained from the field observation.