Extreme Events Reshape Flood Hazards Through Sediment‐Induced Channel Change in Southern Taiwan

Abstract Extreme precipitation events can cause long‐lasting changes to river systems, altering flood hazards beyond the initial disaster. We investigate how Typhoon Morakot (2009) impacted flood inundation patterns across four river catchments in southern Taiwan. Morakot triggered over 17,000 landslides, delivering large volumes of sediment to steep mountain rivers, reducing channel conveyance. Using aerial imagery and Structure from Motion photogrammetry, we constructed pre‐ and post‐event digital elevation models to quantify morphological changes. We simulated flood inundation using HEC‐RAS 2D across a range of discharges. Results show that sediment‐driven channel infilling increased inundation area by up to 44%. Areas with minimal sediment supply exhibited thalweg erosion, vegetation regrowth, and reduced inundation extent. These findings underscore the importance of including geomorphic change in flood hazard assessments, as landscape responses to extreme events can amplify, prolong, or reduce flooding hazards for years after the event.