From the top right where the ships are in the picture to all away around the bend to the convention center is dry (at least a mile or two thick). From the bank of the river to almost the foot of the Dome is completely dry.
There is a two swathe of dry land with thousands upon thousands of buildings. Go in the other direction for miles and miles (not shown), and you would still be walking across dry land with buildings and supplies.
Now, I am not defending the cops on the bridge, all I am saying is that there were plenty of supplies within the several hundred square miles of both the Dome and Convention Center and it was completely dry land!!!
Look closely at the concentration of buildings, warehouses, skyscrappers, shipping containers, ect... Are you honestly going to say that there were more supplies across the bridge? 
What I am saying is, take a look at the photo link helpfully provided by mosquitobite, and then reevaluate whether you can state unequivocally (as you did in your post I originally replied to) that "The whole downtown area was completely clear of flooding."
You keep posting the same graphic with the information that the bridge is the dry route to safety, and with Algiers, on the other side of the bridge, as dry area with food, water, and shelter. The bridge was closed by armed cops, and Algiers was, therefore, inaccessible.
Look closely at the concentration of buildings, warehouses, skyscrappers, shipping containers, ect... Are you honestly going to say that there were more supplies across the bridge????
Those people weren't looking for "supplies" - - there were no "supplies" in Gretna. They wanted to cross the bridge in order to see what kinds of jewelry, stereo equipment, and silverware were left behind by the people who had evacuated Gretna.