New images reveal the scale of damage to the Oroville spillway

In the last few days the California DWR has shut off flow down the Oroville Spillway whilst they start to clear debris from the lower channels, which in turn should allow them to restart flow through the dam itself.  This allows the level of damage to the spillway to be assessed properly – and there is no doubt that it is extremely serious.  This image, from Reuters and Buzzfeed, provides an overview of the state of the Oroville Spillway now:

An overview of the Oroville Spillway, via Reuters and Buzzfeed.

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The damage is better illustrated in the Buzzfeed image, which shows the enormous erosion that has developed on the margin of the spillway, and the very large hole that has been created in the midsection of the spillway itself:

The scale of the erosion of the Oroville Spillway, via Buzzfeed.

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The challenges of rebuilding the spillway are illustrated by this image of the upper section of the spillway.  The new structure will need to be properly founded onto competent rock.  It appears from this image that sections of the spillway that initially appear to be undamaged are in fact weakened:

Damage to the main section of the Oroville Spillway, via Rich Pedroncelli / AP.

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Meanwhile there is a very nice article in the Sacramento Bee that provides an overview of the crisis, and starts to probe what went wrong at the dam:

Some outside experts already have weighed in with theories as to why the chute, rated to handle 250,000 cfs, broke open Feb. 7 with a relatively modest 55,000 cfs pouring down.

Some, including Tullis, point to a phenomenon known as “cavitation,” in which the blast of tiny water bubbles gushing down the chute at 50 mph effectively jack-hammered holes in weakened sections of concrete. Others, such as Robb Moss, a professor of geotechnical engineering at Cal Poly San Luis Obispo, speculate that roots from trees growing along the chute expanded weaknesses in the concrete.

Rogers, the dam failure expert from Missouri, said the spillway may have had cracks that weren’t properly patched. He also theorized the failure may have been tied to California’s five-year drought: The aging spillway could have weakened as it underwent contractions due to the sudden heavy soaking following years of dry weather.

An investigation is underway.  There is now enough spare storage capacity in the reservoir that the emergency spillway should not be needed again this year.  The challenge will be to rebuild the main spillway in time for winter.