1. Professor, Dept. of Water Resources Engineering, Lund Univ., Box 118, 22100 Lund, Sweden.
2. Graduate Student, Dept. of Water Resources Engineering, Lund Univ., Box 118, 22100 Lund, Sweden; and, Graduate Student, Institute of Mechanics, Vietnamese Academy of Science and Technology, 264 Doi Can, Hanoi, Vietnam (corresponding author).