Answer
Use letters to represent each simple statement in the argument.
p: There is a dam.
q:There is flooding.
Express the premises and conclusion symbolically as
\[\frac{\begin{align}
& p\vee q \\
& q \\
\end{align}}{\therefore \sim p}\ \ \ \ \ \ \frac{\begin{align}
& \text{There must be a dam or there is flooding}\text{.} \\
& \text{This year there is flooding}\text{.} \\
\end{align}}{\therefore \text{This year there is no dam}\text{.}}\]
The argument is in the form of “misuse of disjunctive reasoning.”
So, the argument is invalid.