A few things re: Oroville.
In the third hand information department... 2 unofficial reports (truck driver and helicopter pilot both dumping rocks in the spillway) say there appears to be a sand boil under the failed section.https://en.wikipedia.org/wiki/Sand_boil
Also, the lifting of the evacuation may have been at least partially driven by looting and the inability to adequately patrol the vast evacuated areas.
Coincidentally, yesterday, I finished reading Colossus by Michael Hiltzik, about the building of Hoover Dam. In 1941 they raised the level of the lake to test the spillways for 2 months. When they were through, the spillway tunnel concrete was reduced to rubble, and there was a 115 foot long x 45 foot deep hole.
Years after repairs, similar damage occurred in 1982. Also in 1983, Glen Canyon Dam spillways were damaged. The root cause was finally determined to be cavitation.
It seems like the Oroville damage should have been anticipated.