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"...how do you rate the failure of the spillway after it ran for 24 hrs at about one-third capacity its planned capacity?"

There is a bigger engineering picture here that frames the question on the "rating of failure". In this "picture" is the sequence of engineering judgements and events that continued in the face of stark evidence where other dam spillway designs have failed. I haven't put together the full sequence in a single post - only pieces of the whole story & history. Remember this dam was intended to sustain 1861/1862 flood scenarios.

Just a few notable points... the flood capacity of the combined spillway design was intended to handle "standard project flood" conditions of input rates of 440,000 cfs, by designing & rating the total combined spillway capacity of the dam at 646,000 cfs.

+Emergency Spillway stated design rating: 350,000 cfs. Failed. It couldn't sustain a meager 12,000+ cfs.

+Main Spillway stated design rating: 296,000 cfs. Failed. Blowout at 55,000 cfs.

In Engineering, designs have safety factor ratings (Factor of Safety FoF or Safety Factor SF) that are higher than the stated specifications (i.e greater than 1). IN the case of the spillways, this would translate into structural integrity greater than the stresses placed from the design rating - including knowing the erosion performance of the geology in the case of the emergency spillway.

One way of comparing the magnitude of failure: Right now the only operating spillway is damaged & limited to 50kcfs. This is a far sight short of a 646,000 cfs designed rating. In such high impact structures such as dams, there are no excuses. You must know its performance and be darn sure any unknowns are proven out. btw- They had later knowledge of failure modes after dams failed & they had time to evaluate/fix these issues in their spillway. This too should be factored into the failure consideration of "engineering judgement".

Further note: The "Standard Project Flood" of the 440,000 cfs was estimated to occur once in 500 to 1000 years. The Maximum Flood condition, of a 720,000 cfs reservoir inflow was stated as "infrequent" in archives and also estimated at more than 10,000 years. That is where the 646,000 cfs design specification was derived.

They had later knowledge of failure modes after dams failed & they had time to evaluate/fix these issues in their spillway.

Well, another difference between a private enterprise running the show and government running the show - corrective action. In government no one is really accountable. And if someone were accountable in a case like this, they're long gone. I mean, nobody in government actually "owns" the Orville Dam and it is built and maintained entirely with other people's money.

If it were somehow profitable and feasible for private enterprise to own the dam, at least somebody would have ownership and it would be their own money on the line to keep it maintained.