Influence of mechanical layering and natural fractures on undercutting and rapid headward erosion (recession) at Canyon Lake spillway, Texas, U.S.A

This study investigates the role of mechanical layering and fractures on flood-related erosional undercutting resulting rapid spillway recession. In summer 2002, 86 cm rain fell in an 8-day period across Guadalupe River drainage basin central Texas, causing Canyon Lake reservoir to completely fill overtop emergency for first time. The flood incised a gorge into mechanically layered Glen Rose Formation caused headward erosion (recession) at downstream edge spillway. Comparison pre- post-flood imagery assessment records indicates that maximum recession localized northern end where 28 m occurred. occurred estimated rate up 10 m/day during ~3 days flood, which is among highest rates recorded bedrock Analysis historical photographs, field observations measurement undercutting, along with measurements fracture orientation, spacing, rebound are used understand rock mass characteristics influenced Evidence significant was observed incompetent argillaceous wackestone (marl) underlies competent limestone. These results reveal greatest amount associated toppling collapse fracture-bounded limestone blocks. Block size may be factor continuation process, large blocks accumulate base scarp inhibit continued whereas other areas smaller eroded can carried away by floodwaters continue, facilitating combination contrast between layers natural together contributed exceptionally high erosion. Observed erodibility behavior range medium widely spaced would normally expected have very low erodibility. Bulk this situation similar most erodibile layer, specifically, marl pour-off cliff.