The landslide dam breach disaster in Hualien, Taiwan on 23 September 2025

Abstract Landslide damming is a typical landslide hazard chain in regions with strong tectonic activities or frequent rainstorms. On 19 July 2025, Typhoon Wipha caused severe rainfall in Hualien, Taiwan, triggering the largest landslide (23.715°E, 121.299°N) (about 290 × 10 6 m 3 ) ever in Taiwan. The landslide formed a dam on the Matai’an River, approximately 200 m in height and 91 × 10 6 m 3 in lake capacity. Two months later, on 23 September 2025, the lake was filled and breached during Typhoon Ragasa, eventually leading to a catastrophic dam breach flood. As of 11 October 2025, the disaster caused 19 deaths, 5 people missing, and 156 injured. The objective of this study is to simulate the entire hazard chain process and analyze the risk of the landslide dam breach event. Results show that the giant landslide was likely attributed to the 2024 Hualien Earthquake, Typhoon Kong-rey in November 2024 and Typhoon Wipha in July 2025. Typhoon Ragasa brought nearly 800 mm of rainwater to the landslide dam area during 22–23 September, and the dam was overtopped at around 12:00 23 September. The dam breach hydrograph has two peaks due to the development of breach geometry. The first peak occurs at 15:38 with an outflow rate of 6612 m 3 /s, and the second peak occurred at 16:24 with an outflow rate of 9309 m 3 /s. Results also suggest that the evacuation carried out by local authorities might have reduced fatalities from potentially 136 to 19 confirmed deaths, with 5 others missing. This disaster marked a typical landslide hazard chain triggered by earthquakes and a series of typhoons/heavy rainfall in East and Southeast Asia. Notably, there were only two months of mitigation time from the formation to the breach of the landslide dam, allowing for very limited engineering mitigation measures. In such a case, an effective contingency plan assisted with timely evacuation and public education becomes critical.