General Discussion

The City had sirens installed with radio activation from the Police Dept. At first as soon as a tornado warning was sent by the NWS they set off the sirens.
They set it off when a tornado warning was issued for the city but it did not hit anywhere in the city. The Powers That Be were so annoyed that the city council hearing determined that in the future that police cars had to be dispatched to the projected area. Only when an officer radioed in that indeed a tornado was on the ground in the city would the sirens be activated.

So in the flood area I would imagine it would be the same.. Dispatch a deputy to indeed verify that flood water was heading to the area. The deputy would then radio in about the water and the sirens would be activated. Probably people would hear them turn on as they were being swept away.

Such is rural and small town emergency preparedness in the south.