The Association of State Dam Safety Officials (ASDSO) in the US has highlighted the need for emergency planning during all phases of dam construction and operation as the 50th anniversary of a dam failure approaches.

December 23 marks 50 years since the failure of the Lower Hell Hole Dam in California US. Following five days of heavy rainfall, the partially completed dam failed, causing 30,000 acre-feet of water and 700,000 cubic yards of rock to surge 60 miles down the Rubicon River canyon before terminating into Folsom Lake four hours later.

The dam, located about 70 miles east of Sacramento, was designed as a 410-foot high zoned rockfill structure. During the first year of construction, progress was slower than anticipated, and the dam was only completed to a height of 220 feet before the start of the flood season. In late December 1964, record rainfall of 22 inches over five days filled the reservoir, causing the incomplete dam to fail.

Thankfully, no one was killed as a result of the failure, but rock from the failed dam was carried downstream for miles and five bridges, including two suspension bridges and the California Highway 49 bridge over the American River, were washed out before the water flowed into the Folsom Reservoir. The lack of more severe consequences can be attributed to the remote location of the reservoir as well as the event occurring during a time of the year when downstream recreational use was low, ASDSO said

Dam safety experts attributed the cause of failure to incomplete construction of the dam coupled with record rainfall. The dam design was not judged to be a contributing factor, and it was subsequently reconstructed in the wake of lawsuits for damages. Lower Hell Hole Dam is owned and operated by the Placer County Water Agency and continues to provide flood control, water storage, power generation, and recreational opportunities.

The anniversary of this dam failure is a reminder that, as more people live and work in locations downstream from new and existing dams, it is important to be prepared and to have complete and routinely practiced emergency action plans, ASDSO said.

"Emergency action plans are valuable tools that can help save lives by putting important safety and evacuation procedures in place before an emergency occurs," said Lori Spragens, executive director of the Association of State Dam Safety Officials (ASDSO). "Everyone has a role to play in creating a future where all dams are safe, and the anniversary of the Lower Hell Hole Dam failure reminds us of the importance of understanding the risks associated with potential dam incidents and failures."