Page - 29 - in Book of Full Papers - Symposium Hydro Engineering

Even though the Butte County Sheriff’s office and others received an electronic message from DWR to inform them that they were investigating the damage at the flood control outlet of Oroville Dam, it was through social media that the Sheriff learned of the hole in the FCO Spillway chute. After the initial communication issues were cleared up, the Sheriff remained in continuous contact with the EOC to ensure that public safety was the top priority. At this point, DWR officials presumed that the remaining storage area within the reservoir was sufficient to capture the expected flood flows from the rainfall during the rest of that week without causing any threat to the dam or the public. It was the goal of the dam operators to avoid using the emergency spillway while the flood flows were contained, but to be prepared in case using the Emergency Spillway became necessary. DWR and the Butte County Sheriff’s Office kept the public informed through their respective social media platforms. On February 9, rainfall in excess of 32.5 cm (12.8 in) over the Feather River Basin caused the inflows into Lake Oroville to exceed 5,380 m3/s (190,000 ft3/s). The peak inflow to the reservoir was significantly higher than originally forecasted. The following morning, dam operators raised flows through the main service spillway to 1,840 m3/s (65,000 ft3/s) and the hole within the spillway chute continued to expand and erode portions of the adjacent mountainside. DWR personnel decided to use the emergency spillway in an effort to minimize the increasing erosion at the main spillway. They reduced flow through the main service spillway to 1,550 m3/s (55,000 ft3/s) and water level within the reservoir continued to rise. It is important to note that debris deposited in the river downstream of the dam from concrete slab failure and foundation erosion of the FCO Spillway chute raised the tailwater on the Edward Hyatt Power Plant and threatened to flood it out. DWR implemented emergency measures to flood proof the plant with the goal of preserving power and water supply capabilities post event. Without that effort, water supply for the region would have been in jeopardy. Although not an evacuation issue, it was foremost on the minds of the officials during that critical time. The Emergency Spillway had not been used or tested since the dam was constructed. A large amount of erosion was expected in the Emergency Spillway and DWR employees prepared the face of the mountain by removing brush and trees in the areas where the water was likely to go, trying to limit further impact to the power plant. Due to the erosion concerns on the FCO Spillway chute and higher than anticipated inflows, the water level in Lake Oroville reached the Emergency Spillway crest early in the morning of February 11. 29