Unfortunately, the rules for defining "FLOOD STAGE" are not identical for all locations and rivers. But for the Mississippi River sites, "flood stage" is the height -- measured relative to sea-level -- at which flooding would occur ** IF ** there were no levees.
"Flood Stage" for Baton Rouge is 35 feet (above sea-level), but the levees protect to roughly 48-51 feet (above sea-level). So why not set flood stage to the tops of the levees? Yes, in many ways that might make better sense, but there are reasons. In part, due to history . . . and the flooding that occurred before the Corps of Engineers levees were built. Also flood stage serves as some reference for the threat of "sand boils" and seepage. Also, flood stage as a water level DOES have meaning for operations "inside" the levees . . . boat & barge traffic, river docks, etc.
I believe, but am not 100% sure, that all stages along the Atchafalaya are also referenced to sea-level.
Most . . . but not all . . . flood stages on our local area and regional rivers (like the Amite, Comite, Tickfaw, Tangipahoa, etc.) have also been re-established with reference to sea-level. However, there are a few locations where the flood stage value is set to a local reference point rather than sea-level. The local reference point is something established by the local community - a level that has local "meaning" for those impacted.
Does this help?
WAFB Storm Team