I may be off on this...but why couldn't you ask those workers come back and dig a 6-8 inch trench 8 inches deep (guesses) around the new bed..and fill with pretty gravel/ rock? This rain is unusual for you..and for the few times it floods the water will run off it will hit the rock first and sink. Plus, it would give the present bed edging a finished look. Eventually, your plantings can overhang it a bit. And, if you hired a business with any suggestion in their ads of landscaping savvy...then I would ask them to come back..trench that..fill that at no cost. It won't cost them hardly anything but labor time. They should have known better. Advised you differently. If that is not possible...then you should raise that up as others have suggested, ...maybe with the cement blocks the big box stores sell..maybe you would just need one row...could you do it yourself?
Last..stock tanks are relatively inexpensive maybe two sitting in front of the shrubbery for now....fill with things you love -- things with height and things that grow over sides and no bending!
I would not do bog situation if the rain is a rare occurrence.
