There are two sets of gates at ANY lock. If both are open then the up level quickly runs to the low level (kind of a fluid thing). The River is much higher than any thing around the NOLA area except the Rivers own levees. If both sets of gates of the Missippi/Industrial Canal lock were open all that River water would flood into the Lake, over the canal and lake levees. and the city would flood fast. ALL of it - even the bits such as the French Quarter, which are above sea level.
See my last post to Knitting_a_Conundrum.