1. Ph.D. Student, Dept. of Civil Engineering, National Taiwan Univ., Taipei 106, Taiwan.
2. Graduate Student, Dept. of Civil Engineering, National Taiwan Univ., Taipei 106, Taiwan.
3. Research Engineer, Hydrotech Research Institute, National Taiwan Univ., Taipei 106, Taiwan.
4. Exchange Student, Univ. degli Studi di Trento, via Mesiano, 77, 38123 Trento, Italy.
5. Professor, Dept. of Civil Engineering and Hydrotech Research Institute, National Taiwan Univ., Taipei 106, Taiwan (corresponding author).