New member here. First, I want to say it's refreshing to see such a thoughtful discussion being led by Mick.
I'm not expert on any of this, but just for some background, I have worked as an engineering tech in the realm of foundation soils for close to 25 years. As such, I've been appalled by the ignorant commentary on many sites other than this one.
Second, I have just a few general comments, to which I'd welcome any feedback.
There's been a lot of talk about a "boil" being observed below the emergency spillway last night, but I'm having real doubts that that's what was seen. Clearly there's been some deep, localized erosion, but if a boil had developed, I really think it would still be active, and here's why. Any water which had somehow leached through the soil below the concrete lip of the emergency spillway to the point that there was enough upward/outward pressure on the downstream side to lift that soil and then move with significant flow, this would result in a pathway through the soil where the soil had been displaced and severely weakened. If such a boil occurred at the base of one of those deeper holes, well, I don't know how deep they are - 20 feet? - then the new hydraulic head would be roughly 0.9 of that occurring last night, and once a weakened pathway had developed through the soil below that spillway lip, I can't see that such head reduction would stop the flow. I suspect that some reporter used the word "boil" to describe an area of concern, and things took off from there. As I write this, I'm missing the current official update, and maybe they will say something about it.
On the topic of reporters getting things wrong, note that for the official-update website, the reporter who wrote the introductory remarks said that last night there were concerns that a "30-foot wall of water" would go rushing down the river valley. Well, that wasn't what any of the officials said. The officials were concerned that they might lose as much as the top 30 feet of some portion of the lip of the emergency spillway. That would cause the lake to quickly drop 30 feet, but how fast the water actually exited would depend on the size of the hole. A small hole that's 30 feet deep would release water much more slowly than if the whole emergency spillway dropped by 30 feet. As long as the hole in the spillway lip were significantly smaller in cross section than the cross section of the river valley below needed to carry that flow, you wouldn't get "a 30-foot wall of water" gushing down the valley. This is a perfect example of how reporters tend to get things wrong.
On a similar note, last night, one poster made a comment which I don't think that Mick addressed, suggesting that the designers probably knew that the dam as a whole (referring particularly to the area of the emergency spillway) could not tolerate being full to the brim for more than a very short time, and the poster suggested that this was probably what caused things to fail. Since that post hasn't been addressed, I will. There has been no bursting type of failure so far, that I've seen evidence of or heard about. All the damage has been on the downstream face, caused by erosion. In regard to resistance to bursting, and also resistance to "boils", the designers would have made the structure thick enough to prevent that, then applied a safety factor, probably a very generous one. In some of the older photos taken during construction and later when the lake level was lower than now, you can see how great the horizontal thickness of earth is near the top of the emergency spillway. Of course, that varies with elevation because of the very gentle downward slope toward the lake on the upstream side, but it looks like within the upper 20 feet or so of lake level, there would be hundreds of feet in horizontal thickness, with the concrete lip of the spillway on the downstream face of that broad thickness of earth. That hardly seems like a failure-prone design, from a bursting standpoint, as such a massive amount of earth won't be affected by the minor static force applied by water in that upper zone (say, within the upper 20 to 30 feet) when the lake is full. Again, comments regarding this are welcome.
Again, my compliments on a good site. This is the second "debunking" site I've found in a week, and I'm so encouraged by that, and I'm really pleased with the nature of the discussion here.