I live in the countryside, where some of the roads wind through grassy hills, populated by wealthy horse people (I assure you I am not one of them) who breed and raise horses for the race tracks.
Today, at the request of my older son, I made a trip into our local village to go to the bank to cash a check (that'd be cheque to some of you) for him. It took me along some of those winding roads, and as we've been having quite a rainy period of several days, the grass is green, and the hills are beautiful. But nearing the entrance to a golf course, I came upon a temporary road sign erected, I imagine, by the highway administration near a low spot in the entrance road. It was a yellow, diamond-shaped sign, affixed to a small portable barrier, and in bold black letters said:
FLOODED
Sure enough, there was water in the roadway going into the course.
Now, as I am always curious about language in its real-world use, I got to wondering just what is the grammar of this word in this particular usage context. I'd be grateful if some of you could help me out here, and 'splain to me how this word works.
Thank you so much for your kind help.

Larry Latham