The immediate outflow area from that dam is not heavily populated, per Google maps. It feeds into a canyon surrounded by public forests, but near the coast, two population centers, Quebradillas/Cacao, and Isabella, pinch in on it, so the key to this event will be depth of the outflow versus capacity of the canyon.
I haven’t found a heigth on the dam yet, looking.
I’ve looked around quite a bit trying to learn just what authorities mean by “dam failure”. All I have learned is the failure is somewhere at the northern part of the dam. As yet they’ve not specified the nature of the problem. So is it a crack? Erosion underneath? Overtopping? Orrrrr???
The lake/reservoir holds some 11plus billion gallons and is over a square mile, closer to two..
The NWS twitter is keeping up but they have very little info themselves.