PROLONGED FLOOD RISKS FOLLOWING TYPHOON MORAKOT LINKED TO CHANNEL CONVEYANCE CHANGES

Extreme flood events reshape rivers through sediment deposition and channel geometry changes, directly impacting channel conveyance and flood inundation. Channel conveyance, the volume of water that a river channel is able to hold within its banks, is vital for precise flood risk assessments, yet geomorphic changes are often overlooked in this assessment. We address this gap by investigating the impact of sediment-induced changes in channel conveyance on flood inundation, particularly following Typhoon Morakot. In 2009 Morakot triggered more than 15,000 landslides across southern Taiwan’s steep mountainous region, rapidly aggrading river channels and halting incision in previously bedrock eroding channels. In the steeper regions, channels aggraded by more than 10 meters, elevating flood risks into the future. This sequence of events triggered by an initial extreme event is termed cascading hazards and is important to understand for hazard mitigation and landscape evolution. Utilizing historical aerial photography and structure from motion photogrammetry, we developed high-resolution Digital Elevation Models that allowed quantification of sediment deposition and channel geometry changes. We performed hydraulic simulations using HEC-RAS 2D to assess flood inundation patterns across a wide range of discharges estimated from a regional frequency analysis. Simulations indicate that post-Morakot geomorphic alterations significantly influenced flood inundation patterns. For instance, the Ailiao River experienced increasing inundation in subsequent years following the initial hazardous event in 2009. By 2015, inundation for a 100-year discharge had increased by ~19% compared to pre-Morakot levels. These results underscore the dynamic nature of channel conveyance, challenging the assumption of its stability over decadal timescales. This research emphasizes the need for integrating geomorphic changes into flood risk models, improving accuracy in predicting flood hazards.