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Engineers err in Oroville Dam drama

Oroville Dam auxillary spillway

The decision to close off a damaged spillway may have increased the risk of flooding at Oroville Dam in California over the weekend, according to a leading dam expert.

British Dam Society chairman Andy Hughes has criticised the decision to block off the primary spillway at the Oroville Dam in California after a hole appeared in the middle of the concrete chute last Tuesday (7 February).

Soon after the discovery, officials closed the structure for inspections before gradually reopening it to controlled water flows.

The dam has two spillways, which handle water overflowing from the dam. It first flows into the primary, or main, spillway but the auxilliary spillway provides extra capacity in an emergency. It came into action for the first in the dam’s 48-year history on Saturday morning (11 February).

The normally drought-ridden state experienced winter storms that resulted in heavy rainfall throughout January and into February, adding pressure on the damaged primary spillway.

The auxiliary spillway is a channel cut into the hillside. Like the primary spillway, it developed signs of erosion on Sunday – this time at its head.

This led to fears that downstream areas could be flooded and California governor Jerry Brown issued an evacuation order to 180,000 residents.

“I think they [officials] actually made a little bit of a mistake because it appears that they blocked off the [primary] spillway – they closed the spillway off – and then the rain continued,” Hughes told New Civil Engineer.

“So then the water level rose to a point where the auxiliary spillway started to operate, and then that auxiliary spillway started to [experience] damage, and then you’ve got nothing, you’ve got absolutely nothing.”

Hughes described how the amount of water running down the auxilliary spillway could cause significant damage, even without failure of the dam.

“It’s easy to criticise when you’re not in that situation but I think because it was well away from the dam, actually they would’ve been better just letting the [primary] spillway continue to erode – and fly some concrete in with helicopters or something [similar] – and not put the auxiliary spillway at risk,” he said.

“It’s been a very, very serious incident.”

The California Department of Water Resources, which operates the reservoir, yesterday said it believed it was better to continue “damaging” the spillway “than have the water get too high again”.

Hughes said the incident highlights the need for surveillance and maintenance to ensure reservoir safety.

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