On Dec. 14, 1864, Fort McAllister (CS) having fallen the day before, opening the Great Ogeechee River to Union shipping and rendering Savannah untenable, Lt. Gen. W. J. Hardee, CSA, decided to evacuate the city to save it from a destructive bombardment and to extricate his besieged army. River craft being unequal to the task and no pontoon bridging being available, an engineering expedient was adopted. Directed by Lt. Col. B. W. Frobel, CSA, pontoon type bridges were laid by sailors of the CS Navy and details from the Georgia Militia. Using large "cotton flats" for boats, car wheels for anchors and planks from the city wharves for flooring, a bridge was laid from the foot of West Broad Street to Hutchinson Island, another across Back River to Pennyworth Island, and a third across Little Back River to the South Carolina shore.
On the 19th, orders were issued giving priorities and times of withdrawal. The heavy guns were spiked and carriages and ammunition destroyed. At dark, the garrisons of Whitemarsh Island were withdrawn into the city and evacuated via the bridges.
At dark on the 20th, the garrisons of Causton's Bluff, Thunderbolt and the Savannah River batteries gathered at Fort Jackson and were transferred by steamer to Screven's Ferry; but the main garrison — infantry, cavalry, light artillery and wagons — crossed on the bridges. Well before dawn, the rear-guard had cleared Hutchinson Island, the bridge from West Broad Street had been cut adrift, and the troops were marching via the Union Causeway to Hardeeville.