Identification of inundated area by debris flow using LAHARZ model - A case study in Ho Bon, Mu Cang Chai, Yen Bai

Debris flow is one of the most common geological disasters in Vietnam, occurring in mountainous areas and causing catastrophic impacts on both the economy and human lives. This article shows the results of a debris flow simulation that took place on August 5, 2023, in Trong La village, Ho Bon commune, Mu Cang Chai district, Yen Bai province, through an empirical model called LAHARZ and digital elevation model (DEM). The debris flow also was assessed for damage to the built-up area. The LAHARZ model is based on empirical equations which were derived from historical debris flood statistics. The equations include A = 0.05 V^(2⁄3) and B = 200 V^(2⁄3), in which A is the cross-sectional area, B is the planimetric area, and V is the volume. This study uses drone images and digital elevation model with 0.5m spatial resolution, which were created on August 12, 2023 by using Phantom 3 Professional drone. The debris flow's source area is roughly 78104 m², corresponding to a volume of 8000–10000 m³. For this reason, the LAHARZ model is simulated with volumes of 5000; 8000; 10000; 15000 and 20000 m³. LAHARZ simulation results were validated by comparing them to field survey evidence. The result shows that the model results are quite similar to the actual inundated area with TPR and TS values being 0.717 and 65.9%, respectively. This study also demonstrates that the false irregular edges in the delineated inundation zones supposedly originated because of a lack of DEM accuracy. The LAHARZ model simulation has many advantages in terms of time and the few parameters used, which enable rapid evaluation of debris flow scenarios.

Keywords: Debris flow, LAHARZ, Ho Bon, Mu Cang Chai