HYDRAULICS BRANCH OFFICIAL FILE COPY SPILLWAY AND DAM FOUNDATION EROSION: PREDICTING PROGRESSIIVE EROSION EXTENTS R. J. Wittles B. W. Mefford S. W. Abt J. F. Ruff G. W. Annandale Presented at The First Intetnatbnai Conference On Water Resources Engineering in Sari Antonioo, Texas