Page - 68 - in Book of Full Papers - Symposium Hydro Engineering

event. Following the incident, the FCO Spillway lower chute was left badly damaged and the Emergency Spillway was found to be incapable of safely passing even 340 m3/s (12,000 ft3/s). The mission of OER was to develop and implement measures to: “Restore the Spillway Capacity to Ensure Public Safety in Advance of the 2017-18 Winter Flood Season.” It was recognized early on that the full restoration or replacement of the damaged areas would require multiple phases and years, in part because the first design and construction season in 2017 was extremely short – only about 8 months between March 1, 2017 and November 1, 2017, which was the assumed onset of the 2017-2018 Winter Flood Season. Accordingly, the first phase would include structural replacements, modifications, and other remedial measures to rebuild the existing spillway structures to safely pass water flows for the upcoming 2017-2018 Winter Flood Season. Later phases will increase the level of protection incrementally each year until the original design capacities are restored. Later phases are estimated to require as much as 5 to 10 years to complete. The specific design objectives for the 2017 construction season were set as follows:  The reliable capacity of the relatively undamaged the FCO Spillway upper chute, approximately 488 m (1600 ft) in length between the upstream gated headworks and the main scour hole, would be restored to safely pass a peak discharge flow of 7600 m3/s (270,000 ft3/s).  The capacity of the damaged FCO Spillway lower chute, approximately 427 m (1400 ft) in length, would be restored to safely pass a peak discharge flow of approximately 2800 m3/s (100,000 ft3/s), and up to 7600 m3/s (270,000 ft3/s) with potential damage.  The capacity of the Emergency Spillway would be restored to safely pass a limited flow of up to 850 m3/s (30,000 ft3/s) without significant damage to the spillway monoliths and weir, but erosion further downslope would be expected. These 2017 spillway design objectives are illustrated in Fig. 2 (flows shown in cubic feet per second – cfs). 68