On Saturday we said, “What a difference a day makes!” And that is true again! As of Sunday night at 11:00 pm when we wrote this we had been through several very scary hours and over a hundred thousand people had been evacuated from their homes. Indeed, the facts are so fluid and the information so incomplete that we debated not posting at all. But we ultimately decided to post what we know, again in a simple format to hopefully make it easily understandable for those trying to follow along. But remember, there is always a risk that by the time you read this, it may be out of date.
Here is our summary of what we knew as of late Sunday night:
- DWR had intended to release 65,000 cfs from the service spillway through the weekend, believing that this level of release would prevent water from eventually spilling over the emergency spillway. That facility had never been used, and better to use a facility with a history, rather than one that was untested. However, after some new erosion near the service spillway, DWR cut back the releases to 55,000 cfs. While we don’t know for sure, our understanding is that the erosion may have been related to the power lines that run over the service spillway and serve power to the gates and dam operation.
- Once the flow was reduced from 65,000 cfs to 55,000 cfs for the service spillway, it was all but certain that water would flow over the emergency (or auxiliary) spillway, and indeed that water started to flow over the second spillway around 8:00 a.m. on Saturday. Various media organizations had spectacular pictures of the water cascading over the 1700 foot long emergency spillway. Here is a picture from DWR.
- Unfortunately, late Sunday afternoon word came out that DWR was very concerned about erosion which might undermine the emergency spillway. At this point the lake was a full foot above the lip of the emergency spillway, which is a lot of water for a reservoir that is 15,810 acres in area. Now remember that the emergency spillway is a passive structure; you can’t turn it on or off. The only way to prevent water from flowing across it is to lower the lake level. In response to the new risk, DWR began dumping water through the service spillway at a rate of 100,000 cfs!! And this increased flow dropped the lake level, such that by a few minutes before 9:00 pm water had dropped below the lip and the erosion of the emergency spillway had been arrested.
- It appears that before water stopped flowing over the emergency spillway, DWR shared with the Butte County Sheriff that there was a very real risk of continued erosion until water stopped flowing, and that the erosion could cause a failure of the emergency spillway (but not the dam). As a result the Butte County Sheriff called for an evacuation of Oroville, eventually extending that evacuation down as far as Gridley. Shortly thereafter, evacuations were called for large parts of Sutter and Yuba Counties as well. The media ultimately reported that over 150,000 people were evacuated.
- So why might evacuations be called for? Well, the consequences of a failure of the emergency spillway would be significant. At the section of the emergency spillway immediately adjacent to the service spillway, the emergency spillway is in excess of 30 feet tall. We have seen estimates that show that in the event that this section of the emergency spillway failed, downstream communities would be flooded with a 30 foot high wall of water and nearly 750,000 acre-feet of water would force its way to the S.F. Bay Delta. This would be a flood of significant proportions.
- With the emergency spillway no longer flowing, DWR continued to release 100,000 cfs from the service spillway and intended to continue to do so as long as it is safe to. At 10:00 pm Sunday DWR shared that it wanted to drop the lake level by 50 feet, creating the space in the reservoir required to handle future storms.
- On Monday morning once the sun comes up, DWR intends to inspect both spillways and to take necessary actions. The emergency spillway needs to be evaluated, and actions need to be taken to stabilize the section of the spillway that is being eroded. There is a rumor that DWR was able to dump several loads of rock on the area before water stopped spilling, but without a doubt more work will be required. DWR has seen an erosion scar, and needs to see what is causing it. And as to the service spillway, with releases of 100,000 cfs DWR will also need to make sure that the service spillway is still safe to operate. Don’t forget, DWR ratcheted back the releases from 65,000 cfs to 55,000 cfs because of erosion. But clearly releases of 100,000 cfs are the lesser of two evils right now.
- As to the evacuation, it is the hope of the emergency leadership to allow people to return to their homes once the inspections are completed, but of course people will not be allowed to return unless it is safe.
- As of 10:00 pm Sunday night, inflow into the reservoir was only 40,000 while outflow was 100,000 cfs. According to DWR Director Bill Croyle, if DWR can operate the spillway for long enough, it will allow the reservoir to drop by over 1 million acre-feet.
- While 100,000 cfs does put a load on the downstream levees, this flow is well within the design levels, and is much lower than flows in both 1986 and 1997, and also significantly lower than the 100-year event. We are grateful that over $500 million has been invested to improve levees on the Feather River system.
- While future updates will focus on the technical fixes, we must all remember that a series of storms are forecasted to be on their way, with the latest on scheduled for Wednesday or Thursday.
For more detailed information about current storage, inflow, and outflow from Oroville Dam, we still recommend the California Data Exchange Center. And for some very responsible reporting, check out the Chico Enterprise Record. We also found KPAY 1290 News-Talk out of Chico to be a great source for information and a reliable source for listening to DWR’s press briefings. You can access it here, and then click the “listen live” link.
Additional sources of information on this incident can be found on the DWR website and DWR has shared this phone number for public updates: 530-872-5951.
Look for further updates as we consider the consequences of future storm events until the primary and emergency spillways can be repaired.