The process of delineating areas adjacent to rivers and streams that are subject to inundation during flood events of specified return periods. Floodplain maps are essential for land use planning, insurance, and emergency management.
Floodplain mapping involves the delineation of areas subject to flooding along rivers and streams, typically for flood events with specific recurrence intervals such as the 100-year (1% annual chance) and 500-year (0.2% annual chance) floods. In the United States, FEMA's National Flood Insurance Program (NFIP) uses Flood Insurance Rate Maps (FIRMs) to designate Special Flood Hazard Areas (SFHAs) that determine insurance requirements for federally backed mortgages. Modern floodplain mapping integrates high-resolution LiDAR-derived DEMs, hydrological modeling (to estimate flood discharges), and hydraulic modeling (HEC-RAS, MIKE, TUFLOW) to simulate water surface elevations and inundation extents. Two-dimensional hydraulic models are increasingly replacing one-dimensional models to better represent complex floodplain flow patterns, especially in urban areas. Remote sensing contributes through LiDAR for terrain data, SAR for real-time flood extent mapping during events, and historical satellite imagery for validating model predictions. Climate change, urbanization, and aging infrastructure are driving the need for more frequent map updates, as many existing floodplain maps are based on outdated data and methods. The USGS StreamStats application and other web-based tools provide flood frequency statistics that serve as inputs to floodplain mapping studies.