1. School of Civil, Urban and Geosystems Eng., (38-312), College of Engineering, Seoul National University, Seoul, Korea (E-mail: rainwater_waterfirst@yahoo.com)
2. School of Civil, Urban and Geosystems Engineering, #39-217, Seoul National University, Shinrimdong, Kwanak Gu, Seoul, 151-742, Korea (E-mail: myhan@snu.ac.kr)