Sitting here in the comfort of my kitchen, it is easy to second guess the situ...but, why would an engineer recommend the construction of an oil refinery on a flood prone river plain....?
Several reasons. First, a large part of the oil comes from offshore locations (Gulf of Mexico, Venezuela, Saudi Arabia). So the refineries are located close to where the oil can be offloaded and piped for refining. These refineries aren't knew. Most are well over 60 yrs old and were built to pre WWII.
Second. The refinery process, like most large chemical processing plants, paper mills, steel mills, etc. require large amounts of clean water for use in their boilers and cooling towers. So they have to be located near a large supply of fresh water, a river or a very large lake. The Mississippi River is the largest river in the US. So would expect that it would have a large amount of industry located next to it.