I would not jump to conclusions about the original designers and builders of Oroville dam. 50 years of drought and wet years it did its job. But I agree that the sooner researchers capture historical data before it's deleted is essential to laying blame where I suspect it ought to be.
I did not mean to impugn the original designers / builders. MOST engineers in the US are responsible types. I think it is VERY likely the soils info. is available.
THIS situation is almost certainly more a problem of poor maintenance, planning, and “CYA’ism”, than defects in the original design. Grass overflow spillways can work fine, at least for a while, IF properly maintained.