Assessing Fluvial Flood Risk Changes Using an Updated Digital Elevation Model Post-Earthquake: A Case Study

Earthquakes can cause rapid changes in elevation and topographic relief, which, in turn, affect hydrologic regimes and modify flood risk in affected regions. The regulatory floodplain, an area of elevated flood hazard adjacent to water bodies, is critical for managing exposure and mitigating flood risk in nations. Shifts in the distribution of flood risk in regions impacted by seismic activity constitute a compound hazard. Tools are needed to enable reevaluation of regulatory flood maps after seismic events, minimizing exposure of affected populations to additional flood risk. In the United States, floodplain mapping is primarily implemented by the Federal Emergency Management Agency (FEMA), known as regulatory flood mapping. They are based on Hydraulic modeling and delineate the floodplain for areas representing a 1% annual chance of flooding. The floodplain map is not updated regularly by FEMA; it relies on manual, costly revision processes and does not consistently use current, high-resolution, and up-to-date elevation data. Therefore, these maps will struggle to detect recent flood behavior, thereby increasing flood risks and limiting the effectiveness of regulatory flood mapping management. This study presents a rapid, satelliteintegrated framework for updating regulatory flood maps in regions exposed to topographic shifts from earthquakes. Using the 2019 Ridgecrest earthquake sequence as a case study in the North and South Fork Kern River basin, California. Specifically, we used the U.S Geological Survey 3DEP/NED with 10-m resolution DEM, which represented the pre-earthquake topography, integrated with a vertical displacement data derived from InSAR time series analysis to generate a corrected post-earthquake DEM. Both DEMs were then used in the HEC-RAS model to quantify changes in floodplain extent and inundation patterns under multiple return-period scenarios. To assess model performance and quantify the accuracy improvements in regulatory flood mapping, observed flood inundation maps derived from high-resolution PlanetScope satellite imagery were used in the validation. Our integrated approach demonstrates how InSAR-updated topography improves floodplain mapping accuracy and enables rapid updates to regulatory flood maps. HEC-RAS modeling results across three reaches along the North and South Fork Kern River consistently showed larger flood extents in post-earthquake simulations relative to pre-earthquake conditions. Validation using PlanetScope-derived flood inundation maps demonstrates improved model performance for the post-earthquake DEM, with an F-score 84.52% compared to pre-earthquake simulations, using an optimal NDWI threshold of 0.35.