Examining the Mismatch between Regulatory Flood Boundaries and Actual Flood Extent: Evidence from the 2019 Nebraska Floods

Flood maps are fundamental tools for flood risk assessment as well as communication and are instrumental in determining flood insurance rates in various communities across the United States. However, limitations of flood maps have been highlighted in previous literature and through some of the recent flood events, as significant flood inundation and damage were located beyond the Federal Emergency Management Agency (FEMA) floodplain boundaries. Although there is growing consensus regarding the limitations of the floodplain maps, less evidence is available on the magnitude of the ‘mismatch’ in regulatory boundaries and actual flood extent, especially in inland areas. So, the study utilizes countywide FEMA floodplain and 2019 flood inundation layer provided by FEMA, spatially integrated with census population datasets, parcel data, and building footprints data to assess the mismatch in the three most-affected counties in Nebraska during 2019 floods. Our analysis shows that 24% of the flooded areas were outside the FEMA 100-year floodplains, with rural areas experiencing the highest proportion of inundation beyond mapped flood zones. Our findings reveal considerable spatial variations highlighting hotspots of mismatches at census block level in terms of proportion of exposed elements and their socio-economic characteristics influencing vulnerability, characterized by population age structure, poverty levels, building improvement value, and land value. The concentration of mismatches in rural areas underscores the need for more equitable and timely flood map updates, improved integration of revised flood information into local land-use planning and building regulations, and enhanced watershed-based coordination to address transboundary flood dynamics and strengthen regional resilience. • Quantifies mismatches between FEMA floodplains and 2019 Nebraska flood extent at census block scale. • Integrates flood, census, parcel, and building footprint data for spatial analysis. • Identifies rural hotspots where mapped and actual flood risks diverge significantly at census block level. • Reveals socio-economic disparities in populations exposed beyond mapped flood zones. • Recommends equitable flood map updates and watershed-based resilience strategies.