[AzNASCARfan]

It appears to be unreinforced concrete, that just seems insane to me in a state with that much seismic activity.

Concrete cracks, plain and simple, then you put that much water over cracked concrete and this is a fairly obvious result. Especially if fill washed out under the spillway over years and the unreinforced, cracked concrete was spanning over voids.

[dr_lew]

I don't think it's that simple. There are images of workers attending to a repair of the "floor" of the chute in that area, which of course subseqently failed. It seems to be some kind of surface effect, which I am tempted to attribute to the location below the downward bend.

I ask myself, "Why did it fail in the first place?" and "Why did it fail again in the same spot?"

My hypothetical answer is that the location just below the fall-off of the chute causes increased stress due to cavitation, or what have you, and it failed the second time ( after the repair ) for the same reason that it failed the first time ... faulty design!