Page - 160 - in Book of Full Papers - Symposium Hydro Engineering

Fig. 10 Ultimate damage at the service spillway 4. THE PHYSICS OF WHAT HAPPENED 4.1. SERVICE SPILLWAY The IFT believes that the service spillway chute failure most likely initiated by the uplift and removal of a section of the chute slab in the vicinity of Sta. 33+50, at about 10:10 am on February 7, 2017. Once the initial section of the chute slab was removed, the underlying moderately to strongly weathered rock and soil-like foundation material beneath the slab at this location (see Fig. 6) was directly exposed to high velocity spillway flow. The high velocity flow rapidly eroded the foundation materials at this location, removed additional chute slab sections in both upstream and downstream directions, and quickly created the large erosion hole that was observed by 12:30 pm on February 7 (see Fig. 9), as flows diminished following spillway gate closure. These findings are based on eyewitness accounts, as well as photographic and videographic records, as explained in detail in the full forensic investigation report [1]. While there are numerous physical factors that contributed to the chute failure, which the IFT studied in detail, the IFT focused on the factors that it believed likely played the most significant roles in the failure of the service spillway chute. The IFT concluded that the initial uplift and removal of a slab section was most likely caused by water uplift pressure beneath a section of the chute slab, producing an uplift force that exceeded the uplift capacity of that particular section 160