Oroville Dam Crisis: Potential Emergency Spillway Breach (February 2017) - Part I

The Oroville Dam in Northern California is the tallest dam in the United States, rising ~770 feet (235 m) high. It was built on the upper Feather River in 1968, serving mainly for water supply, hydro-electric power generation and flood control. The dam impounds Lake Oroville, the 2^nd largest man-made lake in California, capable of storing more than 3.5 million acre-feet (4.4 km³), almost 1.1 trillion gallons of water for supplying farms and cities across the state. It’s a vital piece of infrastructure located in the Sierra Nevada foothills east of the Sacramento Valley.

Storms in early February, 2017 caused significant damage to the primary spillway when flood was released at 50,000 cubic feet per second (cfs), hindering the safe release of floodwater. The threat of uncontrolled release of water from the dam threatened more than 180,000 people living downstream along the Feather River. To alleviate the flood risk, there are four intended routes that engineers can use to allow water to pass downstream, from most to least preferred:

1.   Through the hydro-electric generators, which allow a maximum flow rate of 16,950 cfs.

2.   Through a river outlet or bypass valve, when operational it has a capacity of 5,400 cfs. The outlet was damaged during an accident in 2009 and has not been used since.

3.   Through the main (primary) spillway, which is used to quickly release large amounts of excess water down a concrete channel and control the height of the reservoir. It is controlled by gates and has a maximum capacity of 150,000 cfs. Based on the dam operator's contracted agreement for flood control, reservoir engineers lower the height of the reservoir before storms arrive. The practice allows the dam to capture storm runoff, reduces spillway flows, and lessens the likelihood of flooding below the dam.

4.   Over the top of an ungated emergency spillway, a 1,700-foot long and 30 ft high weir built several feet below the height of the main dam. Once the lake reaches an elevation of 901 ft, this emergency spillway naturally flows over the weir and down the unprotected natural hillside, which is used as the spillway channel.