Page - 159 - in Book of Full Papers - Symposium Hydro Engineering

emergency spillway at the peak reservoir level was estimated to be about 354 m3/s (12,500 ft3/s), less than 4 percent of ultimate capacity. The emergency spillway discharge channelized as it flowed across the natural terrain downstream of the crest structure and caused extensive erosion, with some of the erosion areas headcutting aggressively back toward the emergency spillway crest structure. According to Incident Command notes [7], at 3:44 pm on February 12, an evacuation order was issued for about 188,000 downstream residents, because of the rapidly advancing erosion areas in the emergency spillway’s discharge channel. DWR increased the discharge through the service spillway, beginning at 3:35 pm on February 12, about nine minutes before the evacuation order was issued, according to the Incident Command notes [7]. Service spillway discharge was increased to about 2,830 m3/s (100,000 ft3/s) by about 7:00 pm on February 12 and this discharge level was maintained through 8:00 am on February 16. Discharge over the emergency spillway crest ceased at about 8:00 pm on February 12, about 36 hours after it began and about 5 hours after the flow had peaked. At about 3:30 pm on February 14, the evacuation order was changed to an evacuation warning, under which residents were allowed to return but were advised to monitor the media and be prepared to evacuate again, if necessary. No further evacuation orders were judged necessary, and the evacuation warning was lifted five weeks after the evacuation order was first issued. DWR established a target reservoir level at Elevation 259.1 m (850 ft), which is 15.2 m (50 ft) below the normal full-pool level. This target elevation for Lake Oroville was reached on February 20. The service spillway gates were closed on February 27 to allow for on-site investigations to support remedial actions. After that time, investigations and remedial actions were interrupted occasionally for service spillway releases to manage the reservoir. The service spillway gates were closed for the season on May 19, 2017, so that construction of spillway repairs could begin. During service spillway operations between February 8 and May 19, 2017, additional spillway chute slab sections were lost and the slab failure and foundation erosion at the service spillway enlarged significantly, as shown in Fig. 10. 159