Updates at the bottom of this post.
The Oroville Dam, just north of Yuba City, has had an interesting week. Last Tuesday, the spillway, the concrete ramp that flows water out of the dam, had an erosion problem. Some of the land beneath the ramp gave way, and part of the ramp collapsed. The flow of water over the collapse further eroded the soil under the ramp, and there’s a pretty large mass of earth missing from the ramp.
Fast forward to the weekend, and after crews were working on clearing the emergency spillway as preparation for the lake to overflow it’s levels, it happened. The lake filled up to past 100% capacity, and began to spill into the emergency spillway for the first time in 49 years. Now, yesterday, after about 36 hours of spilling, crews noticed some erosion at the base of the emergency spillway. This raised concerns of a failure of the emergency spillway (due to potential weakening beneath the foundation of the concrete structure), and prompted evacuation of the area around the dam, almost 200,000 people were told to evacuate last night around 4:30.
As of 10:00 last night, the flow over the top of the emergency spillway finally stopped, but the damage has been done, and now crews are working to reinforce the base of the emergency spillway to prevent further erosion, and prepare for the next round of rain coming this week. The other problem is that the lake is still draining into the primary spillway, which is the one that was damaged from the initial sinkhole that appeared last Tuesday, so more erosion on that ramp will happen. Crews are allowing about 100,000 cubic feet per second of water to flow down the ramp for now, and they’re watching the erosion on both ramps carefully. The lake is filling at a rate of about 45,000cfs, so there’s more going out than in. No word on how much they’re expecting it to fill once again after rain in the forecast.
Traffic was almost gridlock during the evacuations last night, but it appears that everyone has made it out, but there are various lane closures in the area preventing travel in certain directions.
For some more info on the situation, check this post throughout the day. The latest news from the crews is that they’re waiting for sunrise to check on everything so they can get a better picture for what the erosion looks like.
A list of evacuation centers is available here, so if you’re looking for a loved one or helping someone find where to go, check it out.
Governor Jerry Brown has issued a state of emergency, and there are crews on standby (National guard, etc) to help should it be needed.
UPDATE 9:30am: It looks like, following an inspection this morning after sunrise, that the 100,000cfs of water flowing over the sinkhole that opened in the main concrete spillway has not further eroded the ground. Despite the massive hole, they say it’s pretty much done eroding for now, so they’ll continue to use it but keep a close eye on its status. Crews are still working on reinforcing the emergency spillway to prevent possible collapse. Source.