With the amount of rain that fell on such a short period of time, flooding is inevitable regardless of the drainage.
The other thing about "normal" Dominican rain is that it's heavy. Many Dominicans compare SD with places like NYC, Boston, etc and often claim a little bit of rain causes flooding in SD. Well, it doesn't truly rains in NYC, Boston, etc as it does in SD except in a handful of days during summer. Guess what happens when is rains like in SD? Even highways floods, that should give an idea.