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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.

USGS website for Gaujataca Dam:

https://pr.water.usgs.gov/public/rt/pr_lakes/lake_50010800.html

It holds back zero acrefeet at water elevation 557.4 ft. It holds back 34276 acre feet at elevation 646.0 feet above mean sea level. That indicates a head height of an instantly failing Dam of 88.6 ft.

A water level graph shows the level topping out at 646 ft so it’s full, it topped, and erosion from water spilling over the dam is most likely cutting a V-Trench, into the crest of the embankment.

That means it probably won’t fail all at once and you won’t see an 88 foot high wall of water roaring down that Canyon. A very rough estimate in cases like these historically, is around a half the head height, so up to 40 45 feet.

On the minus side, the outflow from that Waterway drops 650 ft in the 10 miles between the the dam itself and the coast. That will add a lot of energy to what will become a wall of water with a whole bunch of dead national forest trees, both recently standing, and down by the hurricane.