1. a Dept. of Hydr. Engrg., North China Inst. of Water Conservancy and Hydroelectric Power , 450045, Zhengzhou, P.R. China E-mail:
2. b College of Geography , Zhongshan Univ. , 510275, Guangzhou, P.R. China E-mail:
3. c Hydraulics Lab., Dept. of Civil Engrg. , Aristotle Univ. of Thessaloniki , Thessaloniki, 540 06, Greece E-mail: