Page - 73 - in Book of Full Papers - Symposium Hydro Engineering

small discharge of 340 m3/s (12,000 ft3/s) could quickly lead to destabilizing these monoliths/weir sections and result in an uncontrolled release of the reservoir (see Fig. 6). There had also been concern regarding the sliding stability of the larger monoliths during extreme flood events and seismic loadings. Fig. 6 Aerial view of head-cutting erosion threatening the Emergency Spillway monoliths/weirs during the Feb. 12, 2017 spillway releases Four alternatives were considered during the initial alternatives analysis phase for the Emergency Spillway. All four alternatives included: a buttress or a rock-anchoring system for the larger monoliths, replacement of the small 30 cm (1-ft) high weir on the right side of the Emergency Spillway, a concrete apron or splash pad for erosion protection over the rock surface for About 100 meters downstream of the monolith/weir sections, and a secant pile wall located at the downstream edge of the concrete apron to protect the apron from undermining by head-cutting erosion. The four alternatives were evaluated and ranked, using the same criteria that were used for the FCO Spillway chute alternatives, and the following measures were selected for the Emergency Spillway:  An RCC buttress on the downstream side to reinforce the larger monoliths/weir sections (2018)  A 2 m (6-ft) high concrete wall/weir crest to replace the small 30 cm (1-ft) high weir on the right side of the Emergency Spillway (2017)  An RCC apron downstream of monoliths/weir sections (2018)  A secant-pile wall at the downstream edge of the RCC apron (2017) The selected measures for the Emergency Spillway’s monoliths/weir sections are illustrated in Fig. 7. The 2-m-high concrete crest on the right side of the Emergency Spillway and the secant pile wall were to be constructed primarily in the 2017 construction season, but unlike the measures for the FCO Spillway 73