Overflow over the top of a dam this size and type will almost certainly result in complete structural failure. The overflow we've seen so far is a secondary canal diverting excess water away from the main dam.
Is there an article discussing the dangers of continuing to use the main or emergency spillways? I'm not clear on why the downstream destruction is so dangerous. (And I apologize if it's completely obvious, maybe I'm just dense on this.)
California's damming of the Sierras and construction of the aquifers were primarily about flood control. Supporting the growing population came secondary (that's the easy part). An enormous effort was put into diverting melting snowpack into mountain valleys. This essentially created the northern and middle parts of the Central Valley, on top of creating sustainable year-round sources of fresh water for the developing coastal cities.
Downstream destruction is of concern because that entire region depends on flood control. The habitability and agricultural production of the corridor between Oroville and Sacramento relies upon, and was created by, the flood control of Plumas, Yolo, Butte, etc counties. There are a number of canals, diversions, and reservoirs both above and below the Oroville to further control flow the dam as it reaches our rivers. However, most of the re-routing is also man-made and thus untested for such an event.
Dirt hillsides are not designed to have 100,000 cubic feet of water rolling down them. They tend to erode extremely quickly and can quickly turn into uncontrolable discharges if the lip erodes.
For the main service spillway, the main concern is that the flow of water will cause erosion damage upstream of the existing damaged site[1]. If the damage to the concrete continues up the spillway to the top, it could render the spillway inoperable.
For the emergency spillway, the main concern is that a continued flow of water would erode the soil off the hill to the point that the hill would no longer support the spillway (the concrete lip at the top of the hill). If this happened, the spillway would fail. The effect wouldn't be as severe as if the dam failed (because there would still be a large hill in between the water and where it wants to flow), but the erosion along the path of the water would get out of control pretty quickly.
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.
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.
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.
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.
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.
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!
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.
