We've commented recently about Oroville Dam in Northern California, the full one with the disintegrating spillway. Let me pass along what honesty demands I characterize as "gossip" thereabouts.
A shirttail relative/friend writes that the wife of his good friend worked for the designers of said defective spillway. She reports they left recommended steel rebar out of the concrete structure's design because it was "too expensive."
Anybody who has done concrete knows it is brittle without rebar, much tougher with. The informant assumes that the vibratory pounding of thousands of tons of falling water pulverized the concrete like a jackhammer, turning it into pebbles and boulders which were swept downstream. An embedded rebar cage likely would have prevented the damage.
Photos I've seen of the huge hole in the spillway suggest she is correct. You could park several SUVs in that hole.