1. Deputy Team Leader, River Engineering Research Team, Civil Engineering Research Institute for Cold Region, Toyohira-ku Hiragishi 1-3-1, Sapporo, Hokkaido 062-8602, Japan (corresponding author).
2. Professor, Laboratory of Hydraulic Research, Graduate School of Engineering, Hokkaido Univ., Kita-ku Kita 8-5, Sapporo, Hokkaido 060-0808, Japan.