In your twitter link to a whirlpool picture, I will hazard a guess as someone who has built various detention structures.
If this is like the other structure the principle outflow is a pipe of a limited diameter. That has a shut off but after the first day of the storm, it opens and eventually drains the basin after 40 to 50 days. The piped outlet creates a whirlpool but does not necessarily indicate a a problem.
Whirlpool could be the main gate, under water, all the way to a sandboil signifying a major blowout of the embankment.
No way to tell until we get a wider view or more info.