[tlrobinson]

The spillway is damaged and eroding as water flows through it. As it erodes it opens up such that more water can flow, accelerating the erosion to the point where the entire hillside erodes away and the dam essentially fails.

[ChefDenominator]

I can understand those concepts just fine, but when I look at the dam in Google Maps, it is not clear to me how this is specifically going to occur. The main spillway is on the hillside next to the dam, and the emergency spillway is on the same hill, yet further from the dam.

[maxerickson]

The area around either spillway may continue to erode away. The main dam structure is currently not in danger. They don't want to run the main spillway at all but they also don't think they can rely on the emergency spillway.

It would still be a catastrophe, the outflow will go from inches per hour to feet per hour.

[ChefDenominator]

Okay, this makes more sense. The problem is that the spillways will fail in the sense that the downstream capacity cannot handle the volume of flow, so will flood those regions, as well as possibly flood areas not typically even near water as it carves its own paths.

[Naritai]

You have to remember that the hill _is_ the dam (of the type[1]). so when water flows down that spillway, it's going to pick up dirt with it - making the hill smaller and smaller, ie. making the dam thinner and thinner.

[1]https://en.wikipedia.org/wiki/Embankment_dam

[ChefDenominator]

The dam is the earth type of dam, but the portion the spillway is on is not part of the dam, at least as I am looking at it in Google Maps:

https://www.google.com/maps/@39.5400137,-121.4931958,3617m/d...

[anotheryou]

But if it eats up the spillway upwards it engangers the little part of tho damm where the vales are. If you do not have a concrete lip like on the auxillary spillway and it just flushes over the top of the hill it will quickly carve a lot deeper, so you have more water than just what was held back by the smaller wall.

Also: the top 70 meters or something is still a lot of water!

[Naritai]

You're drawing distinction between the 'man made' part of the dam and the 'natural' dam, which exists on paper but not in the static force analysis. If that natural portion of the dam becomes unable to support the weight of the water, there will be a collapse.