The incident in question regarding the drowning of slaves happened at Ebeneezar Creek, which is a branch off the Savannah River. The bridge was not burned but removed. It was a military pontoon bridge more than likely put in place by the 58th IN. It was recovered before the blacks in the trail of the formations could cross over safely. The Union reasoning - security of a military system that was threatened with Wheeler's Cav approaching.
It caused quite a fuss for Sherman, once word got North to the Anit-Slavery groups. Didn't help matters that the Corps Commander responsible for action at Ebeneezar was Jefferson Davis, no friend of the Abolitionists. A modern day Congressional inquiry followed which occupied quite a bit of Sherman's time when he was trying to prepare for the next phase of his operations.
I will add that Union Soldier accounts exist that describe in detail the event with disgust.
I would suggest reading Dr. Mark Grimsley's book on Sherman's March and how he practice what he calls the "the Hard Hand of War." It might help address the issue of Sherman's March and how it impacted on the civilian populace.