The Evolution of Flood Zones in southeastern Louisiana

Policy-making, land development, and the management of natural resources by federal government, state, regional, local agencies, and NGOs are generally driven by flood zone maps, namely the 1% annual chance floodplain. Such floodplain surfaces are typically derived from either rainfall-runoff or hurricane storm surge but not their collective interaction. Inundation caused by both, termed "compound flood" events, can have a cascading and profound impact on low-lying coastal communities. This has been observed most recently by the 2016 Louisiana flood and Hurricanes Harvey, Irma, and Maria.

Recent research has introduced and improved the capability to delineate flood zones based on the driving mechanisms, such as rainfall-runoff, coastal surge, and combination (Bilskie & Hagen, 2018). As a result, it is now possible to delineate flood transition zones, where rainfall-runoff and coastal surge interactions are large. The flood transition zone demonstrates regions that are vulnerable to the combined effects of rainfall-runoff and coastal surge. Alternatively, the zones that are dominated by either hydrologic or surge flooding can be defined. This delineation, integrated with socio-economic data, can prove useful in better defining and quantifying flood risk.

This presentation will consider how such a flood transition zone has changed in the past half century based on historic changes to the coastal land margin, with focus in south Louisiana (subsidence, marsh fragmentation, etc.). These advanced scientific capabilities provide a better understanding of the current and historic flood response to the combined impacts of rainfall-runoff and coastal surges. Such advancements lead to new awareness of the reciprocal feedbacks between the coupled natural and human system concerning flood events in low-lying coastal and deltaic regions.

M.V. Bilskie & S.C. Hagen (2018). "Defining Flood Zone Transitions in Low-Gradient Coastal Regions." Geophysical Research Letters. doi: 10.1002/2018GL077524.