1. Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Massachusetts, 223 Marston Hall, Amherst, MA 01003 (corresponding author).
2. Graduate Student, Dept. of Civil and Environmental Engineering, Cornell Univ., Ithaca, NY 14853.
3. Associate Professor, Dept. of Natural Resources Conservation, Univ. of Massachusetts, Amherst, MA 01003.