1. Associate Member, ASCE
2. California Inst. of Technol., 1200 E. California Blvd., Envir. Engrg. Sci. (138-78), Pasadena, CA 91125. E-mail: jhering@caltech.edu (corresponding author).
3. Univ. of California, Los Angeles, Dept. of Civ. and Envir. Engrg., 5732 Boelter Hall, Los Angeles, CA 90095-1593.