Accurate flood risk assessment is crucial for community resilience. However, official FEMA flood maps (the National Flood Hazard Layer) often underestimate actual flood risk. This is partly because maps aren't always updated to reflect new conditions or account for pluvial flooding (floods from heavy rainfall).

Another factor contributing to this 'undermapping' is Letters of Map Revision (LOMRs). These are official changes granted to real estate developers who demonstrate they've reduced flood risk through new construction or other measures.

To better understand these changes, we built a new time-series database tracking all LOMRs across the U.S. from 2013 to 2025. This database connects each floodplain reduction to urban growth and nearby disaster declarations. We collected and processed data from various sources, matching each LOMR with its prior map to quantify the actual changes in flood zones over time.

Earlier research showed that between 2006 and 2019, floodplains decreased by 1.4 million properties in predominantly Black and Hispanic neighborhoods, resulting in reduced insurance coverage. Our new database extends this analysis to 2025, providing 20 years of regulatory change data. This will help us gain a deeper understanding of flood hazards and inform urban planning to support flood adaptation.