Heavy rain/flood could be as disastrous as anything you can think of. It's dreadful and has wrecked many lives and properties.
It's never something anyone should pray to experience but if it happens, say these lines of prayer:
Heavenly Father, you've promised to keep me safe from any danger. That includes rain and flood. Please don't allow my life and properties to be destroyed by this, come to my rescue and save me, Lord, Amen.