Cascading Flood Hazard Following the 2009 Typhoon Morakot, Southern Taiwan

Abstract:

Floods negatively impact more human lives than any other natural hazard on Earth. Prediction of flood magnitude and patterns of inundation is critical in minimizing impact to life and property. Most models of flood hazard assume static channel geometry through time. However, the conveyance capacity of a channel can be quickly and lastingly altered by extreme natural events such as large storms, earthquakes, and wildfires; this can drive a lasting change in flood behavior, a phenomenon referred to as a cascading hazard. Here we quantify cascading flood hazard following the 2009 super-typhoon Morakot in southern Taiwan. This storm triggered >22,000 landslides, causing extreme channel aggradation and elevated flood hazard. We employed a method of regional frequency analysis to predict flood discharge in locations where gauging data is unavailable. We combine flood inundation modeling using HEC-RAS 2D with photogrammetrically derived high-resolution digital elevation data from before and after typhoon Morakot to assess the presence of cascading flood hazard at two field locations in southern Taiwan. We found that flood hazard is substantially impacted by this event, with inundation extents increasing by as much as ~10% after typhoon Morakot. This elevated hazard is likely to persist for decades to centuries as the fluvial system returns to its pre-Morakot condition. This work highlights the need to quantify propagating impacts of natural disasters to better understand the true magnitude and duration of the dangers they pose.