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 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 heather 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.

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