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News » International » Fears of spillway collapse trigger evacuation near USA’s tallest dam » published 13 Feb 2017

Fears of spillway collapse trigger evacuation near USA’s tallest dam

Some 130,000 people living around Lake Oroville in the US state of California have been evacuated because of fears that that the dam’s auxiliary spillway is about to collapse.

There are concerns 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, said California Department of Water Resources (DWR). At one stage, DWR said that the auxiliary spillway was predicted to fail within the hour.

Oroville Dam – the tallest in the USA - itself is sound and is a separate structure from the auxiliary spillway.

DWR said that it plans to use helicopters to drop rocks to fill in the gouge in the Oroville auxilliary spillway to stabilise it.

DWR has been monitoring conditions at Lake Oroville’s main and auxiliary spillways around the clock for signs of erosion that could threaten the integrity of the emergency spillway. To lower the lake level and thus reduce flows and the potential for erosion, DWR increased flows down the main spillway’s damaged, concrete chute to 100,000 cubic feet per second from 55,000 cfs. DWR tweeted late on Sunday evening local time that flows over the auxiliary spillway had ceased but that water was continuing to flow at 100,000 cubic feet a second down the main spillway.

Total releases from the reservoir do not threaten the dam or downstream communities and fall well within the capacity of the Feather River and other downstream channels, said DWR. Flows into the auxiliary spillway are far less than the volume of water washing into the lake from the Feather River watershed. The 16,000-acre surface of the lake acts as a buffer, spreading and attenuating inflow.



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This article was published on 13 Feb 2017 (last updated on 13 Feb 2017).

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