1. Public Works Research Institute, 1-6 Minamihara, Tsukuba, Ibaraki 305-8516, Japan (corresponding author).
2. Hydrologic Research Laboratory, Dept. of Civil and Environmental Engineering, Univ. of California, Davis, CA 95616.
3. Public Works Research Institute, 1-6 Minamihara, Tsukuba, Ibaraki 305-8516, Japan.