Millions of Californians live behind levees constructed to redirect water and prevent floods. Accurate assessments of these levees are crucial in flood hazard maps, which underpin federal floodplain regulations and insurance rates provided by the National Flood Insurance Program. However, when the federal government digitized California’s flood maps in the 2000s, it used a simplistic binary procedure that assessed levees as either meeting the minimum standards for accreditation or not. This study investigates the consequences of the ‘without levee’ procedure, which mapped flood risk as if non-accredited levees did not exist and shaped the 100 year flood zone in California. We combine publicly available datasets on levee systems, flood insurance enrollment, and demographics behind levees. We find that non-accredited levees outnumber accredited levees by a factor of 10-to-1, but accredited levees are concentrated in urban areas and protect more people and assets. Our results demonstrate that households behind non-accredited levees effectively cross-subsidize flood insurance costs in leveed areas—between 2009 and 2020, households behind non-accredited levees paid twice as much for flood insurance premiums than if their levee were accredited and they were removed from the flood zone. While Risk Rating 2.0 insurance pricing is narrowing the premium gap, accreditation continues to shape the 100 year flood zone, with enduring economic implications.

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