The scale of the erosion problem at the Oroville Dam site

As the fairly desperate attempt continue to shore up the spillways at the Oroville Dam site, and to lower the water level ahead of the next rainstorm, better images are emerging of the scale of the problems on both spillways. If we start with the main spillway, which suffered the original erosion events last week, it is hard to appreciate the serious nature of the problems. However, this image (via the Sacramento Bee) from the weekend shows workers in the hole eroded in the main channel:-

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Of course the high flows down the spillway since then are likely to have exacerbated the problem significantly. Meanwhile, the headward erosion problem in the emergency spillway is now garnering most of the attention, primarily because of the potential for a major collapse. This image, from Hamodia, shows the nature of the erosion that has developed at multiple locations immediately down slope of the emergency spillway at the Oroville dam site:

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The most serious problem appears to be a gully towards the bottom of the image, but there is also substantial amounts of erosion occurring on the other side too. The danger is of course that the these gullies will suffer headward erosion until they undermine the spillway lip. whereupon collapse may occur. One challenge is that the quality of the rock does not appear to be high, which accounts for the rapid erosion in both cases. This image, via Twitter, shows the nature of the bedrock with which the crews are dealing:-

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The challenges are substantial. Fortunately, as before, there is no threat to the dam itself.