Three immediate thoughts on the Oroville spillway damage:
1) The main spillway is too narrow with insufficient discharge capacity. That would explain the damage to the concrete channel halfway down - the force of the discharge was too great to be supported by the channel walls.
2) The emergency spillway does not appear to have any channeling structure whatsoever to lead discharged water back to the river. So aside from the erosion of the concrete overflow lip itself, that would seem to be a cardinal design error.
3) If, as reported, there is a risk of flooding to the urban areas downstream, then the river leaving the reservoir has insufficient carrying capacity and should have been widened or supplemented with diversion channels.
(Further thoughts added later)...
4) Where is the stilling basin at the end of the main spillway channel?
5) Did the engineers err in calculating the maximum possible inflow of water into the reservoir from its' feeder river? If so, that is a catastrophic error that would not only have meant an inadequately sized main spillway, but may also mean that the dam itself will eventually prove inadequate to the sheer stresses it is currently being put under.
6) The apparent lack of any sort of channel for the emergency open overflow spillway really is astonishing, and this is made more so by the fact that the mountainside below it actually supports a number of pylons to carry power lines. Erosion of the mountainside from that emergency spillway will almost certainly mean that the pylons will fall and the power lines collapse. More importantly, if that emergency spillway collapses, along with the mountainside beneath it, then there is going to be catastrophic flooding downstream.
The design flaws here are of the order of criminal negligence, and it is astonishing that remedial measures were not taken a long, long time ago. But I suppose that's probably down to the stupid public spending priorities of successive Californian State governments and the people who elected them and failed to hold them accountable.
This guy has aerial footage from his own private plane, which is superb, but his nonchalant manner is simply astounding. This is a fairly major disaster already, and could become much worse...
More aerial video available here, start from 2.29...