Tens of thousands of Californians fled their homes on short notice Sunday evening about 60 miles north of Sacramento when the emergency (auxiliary) spillway at Oroville Dam--the nation’s tallest dam--showed signs of failure, prompting mandatory evacuations along the Feather River downstream, including the entire city of Oroville (population 16,000.) It was a dramatic turn of events in a situation that has gotten progressively worse over the last few days. Record amounts of precipitation to date this winter across the northern Sierra (see Figure 4)—much of it falling as rain or melting prematurely—have filled Lake Oroville to capacity for the first time in years.
On Tuesday, February 7, as massive amounts of water poured through the main spillway (see figures below), a patch of erosion developed on the spillway’s concrete base. This erosion has since spread across a wide stretch of the spillway, spanning roughly 200 by 150 feet and extending 40 to 50 feet deep. To reduce further damage and to help keep debris from flowing downstream, water was shunted from the main spillway to the emergency spillway starting on Saturday morning, February 11. This was the first time the emergency spillway had been used since the dam was put into service in 1968.
A hole developed in the broad concrete lip at the top of the emergency spillway on Sunday afternoon, as several inches of water cascaded over the top of the spillway. By late afternoon, the California Department of Water Resources had issued this dire statement: “Officials are anticipating a failure of the Auxiliary Spillway at Oroville Dam within the next 60 minutes.” Residents of Oroville and neighboring areas were ordered to leave the area as soon as possible, sparking major traffic jams. By around 8:45 pm PST, water was no longer flowing over the lip of the emergency spillway, reducing the immediate threat that the spillway would fail. The lake’s water level had dropped an additional three-plus feet by 7:00 am PST Monday. However, mandatory evacuations remained in effect as officials examined the state of the dam’s infrastructure.
Along with these photos, see the embedded video at bottom for added perspective.
Figure 1. Undated photo of Oroville Dam and Lake Oroville, with the Feather River at bottom. The main spillway (left side of image) is used to relieve pressure on the dam. The emergency spillway, a broader earthen structure just to the left of the main spillway, had never been activated before this past weekend. Image credit: Wikipedia/California Department of Water Resources.
Figure 2. This aerial photo from Saturday, February 11, 2017, shows the main spillway, bottom, and a broader auxiliary spillway, upper, of the Oroville Dam at Lake Oroville. The dam is located on the other side of the main spillway, off the right side of the photo. Image credit: Albert Madrid/California Department of Water Resources via AP.
Figure 3. Another aerial photo from Saturday, February 11, 2017, shows the damaged main spillway. Water pouring through the damaged area had already caused significant erosion to the hillside (orange areas). Image credit: William Croyle/California Department of Water Resources via AP.
Figure 4. The cumulative precipitation across eight key stations in the northern Sierra was at 226% above the 1922-1998 average for the season to date as of Saturday, February 12, 2017. This is more than 50% more precipitation than had fallen by this point during the record-wet year of 1982-83 and the very wet year of 1997-98 (both of which were "super" El Niño events, along with 2015-16). The year-to-date total of 68.05" through February 12 is already well ahead of the record for any year up through the end of February of 65.86" (1955-56), according to Jan Null, @ggweather (these records extend back to 1921-22). Frequently mild conditions this winter have pushed a higher fraction than usual of this precipitation into the reservoir system, either as rainfall or as melted snowfall. Image credit: California Department of Water Resources.
“1. Calm down. 2. Evacuate if ordered. 3. The dam won’t fail, but 4. Severe flooding still possible if emergency spillway fails.”
Point 3 is based on the fact that the spillways are independent structures from the dam itself. It’s extremely good news that the Oroville Dam itself is sound, because the amount of water behind it would produce a truly cataclysmic flood, inundating much of California’s Central Valley. However, residents downstream are by no means out of the woods. Now that the only two spillways from Oroville Dam are both damaged, officials face a very challenging task as they attempt to carry out repairs on the emergency spillway while keeping the main spillway functional. If either spillway were to fail at this point, catastrophe could still result.
When the crisis arose with the emergency spillway on Sunday evening, engineers roughly doubled the flow through the damaged main spillway--from around 50,000 to around 100,000 cubic feet per second (cfs)--in order to quickly relieve pressure on the emergency spillway. The Sacramento Bee reported late Sunday that officials will examine the main spillway on Monday to see how much additional damage was incurred by Sunday’s sudden release and to ensure that the main spillway can still be used without risking more serious damage or failure.
“It’s a dynamic situation,” said Bill Croyle, acting director of California’s Department of Water Resources, in a press briefing Sunday night. “The key here is we need some drier weather or some cooler weather to keep those inflows down….We want to maintain the system at a rate that doesn’t further degrade the infrastructure that we have.” Croyle said that the hope is to keep the main spillway flow at the newly accelerated rate of around 100,000 cfs, if the structure permits. Given the huge amount of runoff still flowing into the lake, water levels will remain quite high. The net goal is to cut the total amount of water in the lake by about 3% by midweek.
Figure 5. Rivers throughout the northern part of California’s Central Valley were swollen with runoff on Saturday, February 11, as seen in this NASA satellite image annotated by the National Weather Service office in Sacramento. Oroville is located along the Feather River north of Yuba City. Image credit: NWS/Sacramento.
The forecast Northern California will get a much-needed break from rain and snow during the first part of the upcoming week. Then the forecast turns more ominous. Light rain and mountain snow will begin Wednesday and intensify by Thursday, and there will be additional pulses of rain and mountain snow throughout the following week. On Sunday night, the National Weather Service office in San Francisco identified five separate frontal passages in GFS model output for the week starting this Wednesday, February 15. At present, these do not look like record-smashing storms, but they could add up to 2” or more of valley rain falling on saturated soils, with 4” - 6” possible in the Sierra foothills and even more at higher elevations (much of the latter as snow). The rainfall will keep water flowing into Lake Oroville and keep the pressure on engineers and officials scrambling to protect downstream areas from a hydrologic threat whose end is not yet in sight.
There is sure to be controversy over the emergency spillway itself and why it nearly failed in its very first weekend of use. The flow on Sunday through the emergency spillway was only around 6,000-12,000 cfs, far less than the engineered maximum flow of 350,000 cfs, as reported by the San Jose Mercury News. In 2005, three environmental groups filed a motion with the Federal Energy Regulatory Commission to require that the earthen spillway be reinforced with concrete. State and local water agencies concluded that these upgrades were not necessary, and the federal government agreed.
In a harrowing interview with the Sacramento Bee, a member of the Central Valley Flood Protection Board describes how a breach of the emergency spillway could have devastating consequences. The Bee’s website also features a livestream from the Governor’s Office of Emergency Services.
We’ll be back with a new post on Tuesday.
Figure 6. Total precipitation (rainfall and the amount of water in snowfall) projected by the 00Z Monday run of the GFS model for the ten-day period ending at 4:00 pm PST Wednesday, February 22, 2017. Image credit: tropicaltidbits.com.
Figure 7. Reservoir conditions in California as of February 11, 2017. The state’s second largest reservoir, Lake Oroville, was at 101% of capacity, forcing use of its emergency spillway for the first time. California’s largest reservoir, Lake Shasta, was at 96% of capacity. Image credit: California Department of Water Resources.
Video 1. Multiple vantage points of Oroville Dam and its main and emergency (auxiliary) spillways, as captured by drone by the California Department of Water Resources. Image credit: California DWR.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.