Near-failure of Oroville Dam in the US causes mass evacuation

Posted 17 February 2017

Oroville Dam

Engineers are rushing to repair faults in the tallest dam in the US – Oroville Dam in California – ahead of more rain.

Almost 190,000 downstream residents were evacuated on Sunday amid concerns the emergency spillway could collapse and send a 10m wall of water towards them.

California Governor Jerry Brown said: “We're doing everything we can to get this dam in shape [so] that they can return and live safely without fear. It's very difficult."

The situation arose after a 100m-wide hole in the dam's main concrete spillway was discovered on Saturday following months of winter rains.

In a bid to avoid further erosion, the California Department of Water Resources (DWR) slowed releases down the gated spillway from 1840 cubic metres per second (cms) to 1557 cms.

“The slight shift in operations is intended to balance risks caused by erosion in the dam’s main spillway, but the dam itself is sound and there is no imminent threat to the public or the dam,” the department said at the time.

But with water levels continuing to rise over the weekend, the decision was made to put the emergency spillway to use for the first time in the dam's 50 year history.

At first, this solution appeared to work. 

“The flows we’re seeing are extremely low compared to the design of the structure. Based on our current situation, there is no threat,” said DWR Acting Director William Croyle.

But then a second, more serious threat emerged: water flowing over the emergency spillway was causing extensive erosion.

The DWR said: “The concern is that erosion at the head of the auxiliary spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville. Those potential flows could exceed the capacity of downstream channels.”

After evaluating the erosion on the emergency spillway, a plan was put in place to prevent further erosion.

“Utilising trucks and helicopters, crews moved large rocks and gravel to fill erosion on the emergency spillway. DWR staff continues to inspect and evaluate the emergency and primary spillways for further erosion,” DRW said.

With more heavy rain forecast this week, full repair works ”might not happen until the winter rains end in May,” The Mercury reported, based on comments from DWR senior engineer Kevin Dossey.

“[Repairs then] will involve pouring large amounts of concrete slurry into the hole and capping the top with reinforced concrete.”

The US Federal Government has approved an emergency aid request to support the rebuilding of the shore of the damaged Oroville Dam and to help the affected communities.

Under different circumstances, the heavy rains would have been welcome news, as this incident comes after years of drought conditions in the region.