On Sunday, February 12th 2017, authorities evacuated nearly 200.000 people in the downstream area of the Oroville Dam. Not without reason, as the dam became a severe threat that almost led to a major disaster resulting from widespread floods, reaching as far as the city of Colusa and Sacramento, CA. The Oroville Dam, with a height of 770 feet (235 meters), is the tallest dam in the United States and plays a major role in the water and (renewable) energy supply of Northern California. The reservoir, Lake Oroville, contains approximately 1.1 trillion gallons of water.
On February 7th, an enormous sinkhole occurred damaging the main spillway of the dam, which is normally used to control the water level in the reservoir. For days, the spillway was said to be inoperable, but due to continuing, heavy, winterly downpours, the engineers were forced to use it. The situation got worse as the ongoing precipitation was too heavy and water levels in the reservoir gained a critical height where the auxiliary "emergency" spillway started to overflow. Large amounts of land just beneath the emergency spillway eroded as the water kept rushing down:
The flow continued for hours and the authorities warned that the spillway was likely to fail leaving them with no choice but to call for a mass evacuation downstream of the dam. Very fortunately, the spillover didn't collapse and nobody was harmed. Currently, the direct threat of failure appears to be over.
What would have happened if the auxiliary spillway had failed? The animation below results from a 3Di-model that simulates the failure of the auxiliary spillway. It shows the arrival time (in hours) of the flood wave over a timespan of 20 hours (lower right corner). The effects would have been catastrophic...