As many of you know, the big news on Tuesday was that the spillway on Oroville Dam had developed a large crater from water eroding away the Oroville Spillway structure. Based on the large hole, DWR stopped all flows down the Spillway to evaluate the damage. Water users downstream started to get nervous as Oroville was rising quickly, and folks feared that water could come over the emergency spillway, which would be uncontrolled.
So this might be time to back-up and offer some context. Oroville is on the Feather River and holds about 3.5 million acre-feet. It was about 80% full when this happened, and the operators were releasing about 70,000 cfs to make room for the storm that was on its way in. Inflow into Oroville has exceeded 200,00 cfs in the past, and the levees downstream have historically started to be stressed at flows approaching the 100-year event, or about 150,000 cfs. Thus, the strategy was to release enough water to make room to hold back the peak of the latest storm. In other words, the reservoir was to function as intended.
As further context, we have also learned that both the Spillway and the emergency spillway are supposed to have been constructed on native bedrock, to ensure that erosion in a case like this could not travel back to the dam itself. As you may also know, the dam is the tallest dam in North America and is an earthen dam. Click here to view an image of the dam and the Spillway. In this image you can see the Spillway running from the top to the bottom of the picture with the gates at the top. If you look to the left of the gates you can see a concrete weir that runs for approximately 1700 feet. That weir is the top of the emergency spillway. It is a passive structure that kicks in only when water rises in the reservoir above the top of the weir, and then water runs down the hillside and connects back up with the river.
By Wednesday afternoon DWR had elected to run two tests of releasing 20,000 cfs through the Spillway to see what happened to the Spillway. As you might expect, doing so resulted in additional erosion. But the pictures after that test show significant additional erosion with huge sections of the Spillway gone. Click here for the photo. DWR then elected to increase the flows down the Spillway to 35,000 cfs, we believe to reduce the likelihood that water would have to flow over the emergency spillway during the incoming storm event. As of late Wednesday DWR was also releasing approximately 13,000 cfs through the powerplant.
Unfortunately, on Thursday we learned that water could no longer be released through the powerplant. Apparently the power lines served by the powerplant run across the Spillway and there was a risk that the erosion from the failing Spillway could erode the foundation of the towers supporting the power lines. (If you look back at that last picture you can see the powerline towers just uphill of the erosion.) As a result, by late Thursday DWR was releasing approximately 35,000 from the reservoir, with all water running through the Spillway. We have also learned that inflow into Oroville is higher than anticipated, and as of Thursday afternoon was running at 185,000 cfs. At that hour the reservoir was at 884.6 feet and rising at about a foot an hour. The emergency spillway is set close to 901 feet. (Go here for the latest data on inflow and outflow.)
As of Thursday afternoon DWR had shared that it believes that no water will spill over the emergency spillway, although it is not clear to us that DWR has yet updated its modeling. DWR had earlier shared that the peak of the storm would be Saturday and that it looked like no water would spill. More recent data (higher inflow and reduced releases as a result of the powerhouse being shut down) suggests that water could start spilling on Friday evening. However, DWR currently concludes that even if water were to spill from the emergency spillway, the flow would be lower than the design flow for the urban levees downstream for the storms forecasted for the next five days.
While there are lots of sources of information on this event, we hope to be able to share a collection of data and insights that bring it all together and make it all make sense. Please check back for updates over the next few days.