Flash floods are typically triggered by extreme rainfall events, representing a severe natural hazard. With increasing human activity in mountainous areas, the occurrence of rare, large-scale flash floods is on the rise. On August 21, 2023, a mega flash flood hazard occurred in Lugaolin river (LGLR), Sichuan Province, resulting in 52 casualties. This study takes the flash flood hazard in LGLR as a case study, investigating how localized rainfall events can trigger mega flash floods under the influence of mountain engineering. It is found that the return period for precipitation in the LGLR basin was 20 years, mainly in the upstream region, and the return period for flash flood exceeded 100 years. Disaster magnified after bridge jammed by sudden increase in flood debris due to collapse of spoil heap. Spoil heap collapse leads to an increase in the proportion of angular particles. When the bridge is located upstream it can lead to a significant drop in water levels downstream, but there is a need to guard against the risk of bursting. These findings on how climate and human engineering collectively contribute to extreme flash flood events can provide a foundation for adopting nature-based solutions in disaster mitigation for mountainous engineering projects.