Oroville’s auxiliary spillway – crisis engineering

As of Saturday, February 11 2017, the crisis with the damaged main spillway at Oroville is continuing to unfold. The reservoir  began to spill over the 1,703 foot auxiliary spillway lip this morning, and as of 2:00 pm, the California Data Exchange Center elevation sensor shows 0.9 feet of flow over the spillway lip.

Engineers for the Department of Water Resources made a heroic, last-ditch effort to armor the auxiliary spillway.   According to a post by Capitol Public Radio:

Cement trucks have been pouring concrete at the mouth of the emergency spillway since Thursday. It had previously been a simple dirt channel.

The problem is that even rapid hardening cement takes at least two days to cure.   So if the emergency spillway has to be used on Saturday, the concrete and rock band-aid could just wash down into the Feather River.   This is desperation.

Emergency work at base of Oroville auxiliary spillway

The Department of Water Resources has long been informed of this worst-case flood scenario. The issues of the unarmored auxiliary spillway were discussed regularly with the Department of Water Resources during relicensing of Oroville by the Federal Energy Regulatory Commission (FERC), but after several years it became apparent that no action was going to be taken.   Environmental and local groups, including Sutter County, intervened in the FERC relicensing, in part to force DWR to do a study of the adequacy and integrity of the auxiliary spillway and remedy any deficiencies.

The 2007 Environmental Impact Statement for the Oroville FERC relicensing (FERC Project 2100) states:

Sutter County also requests that the Commission issue several relicensing orders, including …(2) direct the licensee to investigate the adequacy and structural integrity of Oroville dam’s ungated auxiliary spillway that may currently pose a risk to the project facilities and downstream levees in Sutter County and take all necessary actions to correct identified deficiencies; and (3) direct the licensee to investigate the levees on the Feather River, in the context of its hydroelectric, water supply and flood control operations and to repair, replace, and maintain those levees to provide appropriate levels of flood protection in light of license operations. (C-21)

The Federal Energy Regulatory Commission declined to even require a study of the adequacy of the unarmored auxiliary spillway, although it is part of the dam’s maximum rated discharge of 250,000 cfs in a maximum flood event.

Post updated on February 11, 2017.

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