That is way too unpredictable to assume that the enemy will charge and then you blow the dam. Past history has shown that blowing dams is to prevent enemy from getting to the other side.if you worry about that , you let enemy attack occupy those low lands then destroy the dam and wash them and inflict more damage and casualties to them than all the period of war.
its as simple as that , you don't destroy it to prevent enemy coming there , you destroy it to destroy your enemy who are stationed there