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UPDATE 2/14/17 - Evacuation order lifted.
UPDATE 4:50PM 2/12/17
Officials Warn of possible collapse of the emergency spillway. Evacuations ordered
(The above explains why the dam will not collapse. The emergency spillway might collapse, but is separated from the dam itself)
On Feb 8th 2017, the spillway of the Oroville dam in Northern California failed after a 150ft wide hole formed and significant amount of the underlying hillside were washed away. The large hole forced operators to shut off the flow at a time when the level of water is rising at unprecedented rates. Currently (Feb 9th) the inflow is 130,000 cubic feet per second, and at the time of the failure the spillway was releasing 50,000 cubic feet per second.
A significant function of dams in California is flood control. If the dam were not there, then the full 130,000 cubic feet per second would be heading down the Feather river through Oroville, Yuba City, and Sacramento. Many areas along the way are behind levees.
If, as seems quite likely, they cannot quickly repair the spillway then they are faced with a difficult choice. They can either continue to let more water flow down it in a controlled manner, which will cause far more damage to the spillway, or they can let the level of water rise until it goes over the emergency spillway (just visible on the left of the above image).
Neither option is good. More damage to the spillway will mean very expensive and time consuming repairs, but using the emergency spillway will be uncontrolled, with the possibility of levees being breached downstream. There will also be very significant local erosion, with the greatest risk being that this leads to the failure of the emergency spillway itself.
The spillway is essentially a lower dam, to the Northwest of the main spillway. It is designed to be overtopped before the dam is.
The worst case would be if the emergency spillway (which was built in the 1960s and has never been used) starts to erode - potentially leading to the formation of a deeper channel allowing for a rapid uncontrolled outflow from the dam.
The dam itself would not fail, however a failure of the emergency spillway would lead to some flooding, and significant rebuilding costs. Even the use of the emergency spillway intact would likely mean some downstream flooding if there's a sustained storm. This is referred to in the FERC filings as "loss of crest control"
Update 2/9/2017: Significant changes overnight with a limited test release. The damage is going in the wrong direction.
Update Feb 11 2017:
At around 8:00 AM on Feb 11, the water started to flow over the emergency spillway:
- CDEC Oroville levels and flow data.
- DWR HD Public Domain Photos
- 10 day weather forecast in the watershed (Berry Creek)
- RAP 7-day Precipitation forecast.
- Sierra Club motion expressing concerns about the emergency spillway
- CA DWR (Department of Water Resources) Twitter
- YubaCity filings with Oroville Dam operating instructions and blueprints
- 1975 Earthquake Investigation
- California State Water project - Volume III - Storage Facilities
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