Sounds like some serious neglect on the part of dam owners. Were told repeatedly to increase the capacity of the spillways and never did it/slash tied it up in courts.
I don't know their exact probable max flood threshold, but they are usually a 1/1,000,000 of occurrence in a given year (very conservative for a reason, since the high risk dams can kill people if they give way). They argued that the chances of that happening in the next 5-10 years was 1/5,000,000-10,000,000 (for some reason adding the yearly occurrence together). The true chance of a PMF happening in the next 5-10 years is actually 1 - (the chance that the PMF doesn't occur in 5 or 10 years in a row). They were also cited for not being able to even contain a half PMF. So the true probability is more like 1/50,000 instead. What's a 100-fold error between friends?