1. Graduate Student, Dept. of Hydraulic Engineering, Tsinghua Univ., Beijing 100084, China.
2. Associate Professor, Dept. of Hydraulic Engineering, Tsinghua Univ., Beijing 100084, China (corresponding author).
3. Professor, Dept. of Civil and Environmental Engineering, Hong Kong Univ. of Science and Technology, Hong Kong SAR, China.
4. Professor, Dept. of Hydraulic Engineering, Tsinghua Univ., Beijing 100084, China.