Give him back his deposit. He got flooded out. Assuming it was a residence--you don't say--it was made uninhabitable by the flood. The warranty of habitability is implied in a residential rental agreement. An act of God is a defense to breach, which means a suit against him for leaving before the lease was up would almost certainly be unsuccessful. I as a juror or judge would be very annoyed at you if you didn't give him his deposit back.
Suppose the place was struck by lightning and burned to the ground. Can you imagine the landlord prevailing in a lawsuit for breach of the lease?
Not legal advice as I don't practice in Tennessee. It's just my two cents. If you need legal advice, please consult a lawyer who holds Tennessee licensure.
I agree with the above answer, especially in light of the facts you provided in the comments.
In landlord/tenant cases, the notion of 'reasonableness' usually wins the day. And while it may be hard to find anything particularly 'reasonable' in the circumstance of a major flooding, the same is true for both parties here.
While you could retain the month's rent, you could also easily be sued and made to go to court by the former tenant, and potentially face damages and/or attorney fees if you lost.
Given that the tenant vacated the next day, it seems perfectly reasonable to return the rent. Or, perhaps you could withhold an amount prorated for the single day of occupancy, if you were really insistent, but even that is splitting heirs, so to speak. (Consider, of course, that you wouldn't have had any rent money to return if the tenant hadn't promptly paid you on the 1st of the month in the first place.)
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If your (residential) tenant moved out because of the flooding (health and safety reason) then they had a valid right to terminate the lease agreement, you should return the deposits as per the normal move out terms of the agreement. You cannot collect any flood damage repair from the tenant.