Oroville Spillways Incident Background

February 7: As water releases from the gated flood control spillway ramped up to 52,250 cubic feet per second (cfs) in anticipation of significant inflow to Lake Oroville expected from the largest forecast storm of the winter, DWR employees noticed an unusual flow pattern. Spillway outflow was stopped for inspection. Engineers found a large area of concrete chute and foundation erosion - much too significant to repair. DWR began ongoing consultation with the Federal Energy Regulatory Commission Dam Safety (FERC) and California Division of Safety of Dams (DSOD) experts to carefully consider the steps forward.

February 8: After a collaborative decision with FERC and DSOD, DWR ran short duration flows of 20,000 cfs down the damaged gated spillway to monitor further erosion and then re-inspected to find the initial damaged area had essentially doubled in size. As a contingency, crews from CAL FIRE and DWR began preparing for possible use of the emergency spillway by clearing the hillside of trees, rocks, and debris to the extent feasible as well as grouting and shotcreting some areas. DWR activated four 24/7 emergency interagency operations centers to study and implement responses to flood control spillway and related structures, carefully monitor weather forecasts, and coordinate operations going forward. As gated flood control spillway releases were resumed and initially ramped above 20,000 cfs, significant headward (uphill) erosion occurred as well erosion downslope towards the diversion pool. DWR, FERC, and DSOD engineers and geologists were certain this would happen, but uncertain how far upslope the gated spillway erosion and damage would go. This was the primary concern at the time because if the erosion got too close to Oroville Dam and the gate control structure, the spillway would have to be shut down. Fortunately, DWR had its full 750,000 acre-feet of flood storage available at the onset of this storm. To aid in this decision making, engineers and geologists monitored the gated flood control spillway around the clock with personnel on site, as well as video from ground and drone-based cameras.

February 9: Upslope erosion of the gated spillway chute had been initially severe upon re-operation on February 8, but appeared to be stabilizing some 400 feet upslope from the initial damage discovered early on February 7. DWR, FERC, and DSOD were cautiously optimistic that releases down the gated spillway chute could be increased. DWR ramped up gated spillway releases to 35,000 cubic feet per second (cfs) and then 45,000 cfs, high enough to avoid spilling over the emergency spillway while utilizing all of the available flood reservation space in Lake Oroville. By 8 pm the forecasted peak of 2017's largest storm had increased more than 25 percent from the previous forecast just 6 hours earlier. In response, DWR, FERC, and DSOD agreed to increase gated spillway releases in an attempt to avoid an emergency spillway release. Concurrently, outflow from the Hyatt Powerplant was halted because debris below the damaged spillway caused water to back up into the diversion pool thus raising water levels near Hyatt Powerplant. These elevated water levels near the generating turbine outlets, known as the tailrace, precluded DWR’s ability to safely operate the power plant. DWR continued, as a contingency, to prepare for use of the emergency spillway. An additional concern emerged as the upslope erosion along the gated spillway was within approximately 100 feet of Tower #4 of PG&Es Table Mountain 230kv transmission line with serves Hyatt Powerplant. It is estimated that a loss of this tower would likely have caused a cascading failure to a multitude of adjacent towers, thereby impairing outflow capabilities at Hyatt Power plant for months.

February 10: Releases down the damaged gated flood control spillway were increased to 55,000 cfs hours after midnight and then to 65,000 cfs. This operation, based on a peak inflow of over 190,000 cfs having already occurred late on February 9, , was designed to reduce the use the emergency spillway ,while also utilizing all of the available Lake Oroville flood storage space below the emergency spillway crest. The precise nature of this operation (leaving no buffer for forecasting error) represented a balancing of two risks: 1) potential spill over the emergency spillway; and 2) the risk of severely damaging all controlled outlet capabilities of the Dam with higher gated spillway releases (the gated spillway chute and the Hyatt Powerplant). The risk associated with both was perceived to be roughly equal at the time. As a safety contingency, emergency spillway preparations continued. In addition, as tailrace water elevations reached historically high levels, DWR crews shifted work at the Hyatt Powerplant to emergency flood fighting preparations inside the plant. This effort was critically important to prevent flooding of Hyatt Powerplant, which would have rendered it inoperable for up to a year or more and thus significantly limiting DWR's ability to manage lake elevations as well as to meet all beneficial water use releases from Lake Oroville including those to support Delta water quality, senior water rights, and endangered fish in the Feather River for an extended period of time – the consequences of which would be devastating to the aforementioned resources. With DWR assistance, the California Department of Fish and Wildlife evacuated millions of baby salmon from Feather River Fish Hatchery in Oroville, California to nearby fish ponds at the Thermalito Afterbay Hatchery Annex. A stall of the trailing edge of storm over the Feather River watershed increased the total storm volume just enough so that by late on February 10, 2017 it was apparent water would flow over the emergency spillway for the first time in the operational history of Oroville Dam.

February 11: As the runoff from the largest storm in twenty years (which would total more than one million acre-feet since February 6) slowly receded, Lake Oroville's level rose above 901 feet. At approximately 8:00 a.m., water flowed over the lip of the 1700-foot-long concrete weir, engaging the emergency spillway for the first time in the facility's 49-year history. Observation and monitoring of the emergency spillway and flows down the hillside to the diversion pool were carefully performed by engineers and geologists on the top of Oroville Dam, and other locations deemed safe inspection vantage points as well as through the use of drones.

February 12: In mid-afternoon, anticipated erosion began progressing faster than previous observed rates near the top of the slope beneath the emergency spillway crest. After conferring with DWR, CAL FIRE, FERC, and DSOD on risk this erosion posed to a portion of the emergency spillway crest structure, the Butte County Sheriff's Office issued mandatory evacuation orders for the Oroville area, as did cities in Yuba and Sutter Counties. To more quickly cease flow over the emergency spillway, the flood control spillway outflow was increased to 100,000 cfs. At approximately 8:00 p.m., the lake level dropped below 901 feet and flows over the emergency spillway stopped. Erosion to the emergency spillway hillside was assessed via aerial and direct inspection.

February 13: DWR crews began working around the clock to fortify the emergency spillway hillside slope. Evacuation orders remained in effect. A Temporary Flight Restriction, including recreational drones, was enacted to allow crews to survey and conduct their work including the use of helicopters to bring in rock for revetment. Acting DWR Director William Croyle announced his goal to reduce the lake level to 850 feet or lower

February 14: As the lake level continued to drop, the mandatory evacuation order was reduced to an evacuation advisory warning. Crews continued working around the clock to repair the emergency spillway, placing 1,200 tons of material per hour in the erosion gullies below the spillway crest using helicopters and heavy construction equipment. Sustained 100,000 cfs releases from the gated flood control spillway added significant debris mass in the diversion pool and raised the tailrace elevation to even further record high levels. Flood fighting inside the plant increased with extraordinary measures.

February 15: Lake level continued to drop as DWR worked day and night to reinforce the emergency spillway. State and federal engineers performed continuous monitoring of the construction area, dam, spillways, and other structures from the ground and with the use of drones. Barges and cranes were stationed to remove debris and sediment from the diversion pool.

February 16: Flood control spillway outflow was reduced to 80,000 cfs to facilitate the clearing of debris from below the spillway, but the effectiveness of excavating from accessible diversion pool shore areas proved to be limited. Lake level dropped further and construction to armor the emergency spillway continued. Extraordinary flood fighting measures continued inside of Hyatt Powerplant. The tailrace elevation reached a record 256 feet, or 4 feet above the level at which Hyatt Powerplant would otherwise be flooded.

February 17: Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Divers entered the water near the Hyatt Powerplant to assess and relieve pressure on the river valve chamber flood relief wall. As directed by FERC, DWR formed an independent Board of Consultants to review and assess operations, conditions and risk reduction measures associated with the dam and appurtenant structures, and a separate team to investigate the cause of the main spillway failure. Outflow from the flood control spillway remained at 70,000 cfs.

February 18: Outflow from the flood control spillway decreased from 70,000 cfs to 55,000 cfs. Lake level came down to 854 feet. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Barge construction began in order to remove debris from the diversion pool beneath the spillway.

February 19: Lake level was at 852 feet. Outflow from the flood control spillway increased outflow 55,000 cfs to 60,000 cfs in anticipation of the expected increase in inflows due to forecasted weather conditions. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Assessment teams viewed debris build-up and dredged material piled below the flood control spillway.

February 20: Outflow from the flood control spillway remained at 60,000 cfs. Despite wet and windy weather, erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities.

February 21: Outflow from the flood control spillway remained at 60,000 cfs anticipation of the expected increase in inflows due to forecasted weather conditions. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities.

February 22: Outflow from the flood control spillway remained at 60,000 cfs. Governor Edmund G. Brown Jr. visited the Incident Command Team for a briefing on emergency activities and to greet and thank workers. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities.

February 23: Outflow from the flood control spillway reduced from 60,000 cfs to 55,000 cfs at 1200 and to 50,000 cfs at approximately 1400 to manage reservoir elevations based on forecasted weather and resulting inflows. U.S. Senator Kamala Harris, members of U.S. Senator Dianne Feinstein's staff, and members of the California State Legislature attended a briefing on the activities and toured the site. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Additional safety planning and repairs to the emergency spillway, including check dams and armoring, are approved.

February 24: Lake level was 849 feet. Outflow from the flood control spillway remained at 50,000 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Dredging operations between flood control spillway terminal structure and Hyatt Powerplant are approved and planned, withsafety plans, in concurrence with FERC, DSOD, and USACE.

February 25: Lake level reached 846 feet. Outflow from the flood control spillway remained at 50,000 cfs. DWR partnered with Caltrans to address roads in Oroville and surrounding areas impacted by trucks and heavy machinery responding to the Oroville Spillway Incident; Canyon Drive identified as a first priority.
Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. FCO Radial Gates Structural Inspection for all 8 gates is approved and planned including safety plans in concurrence with FERC, DSOD, and USACE.

February 26: Lake level decreased to 843 feet -- 58 feet below the maximum level of the reservoir, with outflow exceeding inflow. Outflow from the flood control spillway remained at 50,000 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Incident Command held dry-run on “zero-flow plan” with expected inspections and repair plans starting on February 27 in concurrence with FERC, DSOD, and USACE.

February 27: Lake level reached 838 feet. DWR started to incrementally decrease outflow from the flood control spillway from 50,000 to zero cfs. Outflow was reduced gradually following the USACE flow reduction criteria down to 35,000 cfs, and then to zero. Thermalito Afterbay releases were increased to maintain the USACE flow reduction criteria downstream in the Feather River to minimize risk to levees. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Dredging Operations between FCO Spillway Terminal Structure and Hyatt PP is approved and planned including safety plans and equipment mobilized in concurrence with FERC, DSOD, and USACE.

February 28: Outflow from the flood control spillway remains at 0 cfs. Lake level rose 2 feet to 840 feet since the gates were closed on 2/27. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Drone and physical inspections were performed in concurrence with FERC, DSOD, and USACE. Flood control radial gate structural inspections started on gates 1 through 8 and were completed on March 6th. With flood control spillway outflow curtailed, the enormity of the debris pile below it was revealed. Large excavators and several barges were used to begin removing sediment and debris. Additional heavy equipment was immediately mobilized to assist in the removal process.

March 1: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue. 60,000 cubic feet of material was removed as of the morning, with lake level at 843 feet. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Drone and physical inspections continue. Flows to meet fishery requirements continued to be released at 2,500 cfs from Thermalito

March 2: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue. 110,000 cubic feet of material was removed as of the morning, with lake level at 845.6 feet.
Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Drone and physical inspections continue.

March 3: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue. 168,000 cubic feet of material was removed as of the morning, with lake level at 848.3 feet. The Hyatt Powerplant resumed operations at 7:00 a.m., with flows ramping up to 2,650 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Drone and physical inspections continue.

March 4: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue. 216,000 cubic feet of material was removed as of the morning, with lake level at 850.3 feet. After a successful restart of the Hyatt Power Plant on Friday, a temporary shutdown began around 10 a.m. to focus on deepening the water channel so that the plant could reach its full capacity. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Preparation work and safety controls are being put in place to begin work on flood control spillway.

March 5: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue. 329,000 cubic feet of material was removed as of the morning, with lake level at 852.9 feet. The Hyatt Powerplant was restarted at 6:00 p.m. with one turbine generator moving water at 1,720 cfs. Operations were monitored to determine the efficiency and safety of excavation with Hyatt outflow. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities Work begins to shotcrete under the lip of the broken spillway.

March 6: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue. 427,000 cubic feet of material was removed as of the morning, with lake level at 855.3 feet. Work continued below the emergency spillway and reconstruction of Canyon Road. A second Hyatt Powerplant unit was started at 6:00 p.m. allowing for a total flow of 3,550 cfs, with ongoing operations monitoring. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Shotcreting continues and work begins to install rock anchors to further secure the concrete slab against uplift forces generated during spillway operation.

March 7: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue. 517,000 cubic feet of material was removed as of the morning, with lake level at 857.4 feet. Flows for fishery requirements continued to be met with 2,500 cfs releases from Thermalito. A third Hyatt Powerplant unit was started at 6:00 p.m., allowing for a total outflow of 5,330 cfs. Spillway anchoring continues and spillway (slab and wall) joint sealing begins. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities.

March 8: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue. Approximately 620,000 cubic yards of material have been removed from the debris pile to date. Because the Feather River Watershed experienced cool and dry weather over the past week, DWR was able to make much progress removing debris. Flows to meet environmental requirements were met by releases from Thermalito to the Feather River at 4,400 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Station Shotcrete work completed. Excavation work begins on the bank slope adjacent to the right side of the spillway. Spillway invert spall repairs begin.

March 9: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue; approximately 715,000 cubic yards of material have been removed from the debris pile. Five operational units at the Hyatt Powerplant were running, allowing for a total outflow of 8,800 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Anchoring, excavation, spall repairs, and joint sealing work continue.

March 10: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue; approximately 803,000 cubic yards of material have been removed from the debris pile. Five operational units at the Hyatt Powerplant are currently running, allowing for a total outflow of 12,900 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Anchoring, excavation, spall repairs, and joint sealing work continue.

March 11: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue; approximately 897,000 cubic yards of material have been removed from the debris pile. DWR increased flows to the Feather River at noon, from 11,000 to 13,000 cfs. Five operational units at the Hyatt Powerplant were running, allowing for a total outflow of 12,900 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Anchoring, excavation, spall repairs, and joint sealing work continue.

March 12: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue; approximately 990,000 cubic yards of material have been removed from the debris pile. Five operational units at the Hyatt Powerplant were running, allowing for a total outflow of 12,900 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Anchoring, excavation, spall repairs, and joint sealing work continue.

March 13: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue; approximately 1,075,000 cubic yards of material have been removed from the debris pile. Five operational units at the Hyatt Powerplant were running, allowing for a total outflow of 12,900. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Anchoring, excavation, spall repairs, and joint sealing work continue. During this week, DWR continued to increase flows to the Feather River, with inflows ranging from approximately 14,000 to 20,000 cfs.

March 14: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue; approximately 1,148,000 cubic yards of material have been removed. Five operational units at the Hyatt Powerplant running, allowing for a total outflow of 12,900. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Anchoring, dredging excavation, spall repairs, and joint sealing work continue.

March 15: Outflow from the flood control spillway remains at 0 cfs and dredging operations continue; approximately 1,200,000 cubic yards of material have been removed from the debris pile. Hyatt Powerplant discharging 12,900 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Anchoring, dredging and excavation work are completed. Spall repairs and joint sealing work continue.

March 16: Outflow from the flood control spillway remains at 0 cfs and approximately 1.24 million cubic yards of material have been removed from the debris pile. Hyatt Powerplant discharging 12,900 cfs. Erosion repair efforts were ongoing, as was DWR monitoring of the status of the dam, spillways, Hyatt Powerplant, related structures, and the progress of repair activities. Spall repairs, and joint sealing are completed. Dredging is completed to operate Hyatt when spillway is started on March 17. All work on or around Spillway are demobilized in anticipation of startup.

March 17: Releases down the damaged flood control spillway resume, gradually increasing from 0 cfs to 40,000 cfs. Dredging operations below the spillway are halted while water is released; approximately 1.25 million cubic yards of material has been removed from debris pile. Hyatt Powerplant temporarily shut down, but resumed operation in the evening.