Landslide damming is a typical landslide disaster 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 (about 290×10⁶ m³) ever in Taiwan. The landslide formed a dam on the Matai’an River, approximately 200 m in height and 91×10⁶ m³ 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. The disaster chain caused 19 deaths, 5 people missing, and 156 injured.
We reconstructed the entire hazard chain process and analyzed the risk of the landslide dam breach event with different mitigation measures. We found that the 2024 Hualien Earthquake, Typhoon Kong-rey in November 2024 and Typhoon Wipha in July 2025 jointly resulted in this giant landslide.
During 22-23 September 2025, Typhoon Ragasa brought nearly 800 mm of rainwater to the landslide dam area, and the dam was overtopped at around 12:00 23 September. Numerical simulation suggests that 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 m³/s, and the second peak occurred at 16:24 with an outflow rate of 9309 m³/s.
Risk assessment further suggests that most fatalities occur in low-rise structures, and relocating people from one-story buildings is the most effective way to avoid casualties. The evacuation carried out by local authorities might have reduced fatalities from potentially 136 without human intervention to 19 deaths and 5 others missing.
This disaster chain was a typical landslide hazard chain triggered by earthquakes and a series of typhoons/heavy rainfall in East and Southeast Asia. Although the lake water had been almost released, around 253×10⁶ m³ of unstable deposits remained, posing risks of remobilization, new landslide dams, and elevated riverbed during heavy rainfall. Recommended engineering measures including targeted excavation, dredging of river sediment, elevating bridges and levees, and constructing multi-levee systems.

Landslide dam,Disaster chain,Risk assessment,Dam breaching flood