1. a Department of Civil Engineering , National University of Singapore , 10 Kent Ridge Crescent, Singapore , 117576 E-mail:
2. b Department of Civil Engineering , University of Nebraska-Lincoln , 1110 S 67th St, Omaha , NE , 86182 , USA E-mail:
3. c State Key Laboratory of Hydraulics and Mountain River Engineering , Sichuan University , Chengdu , Sichuan , 610065 , China