Quantitative assessment of physical fragility of buildings to the debris flow on 20 August 2019 in the Cutou gully, Wenchuan, southwestern China

Abstract

Catastrophic debris flows are frequently triggered by rainstorms after the 2008 Wenchuan earthquake and pose great threats to residential buildings and human lives in the mountainous areas. However, little effort has been devoted to studying the quantitative assessment of the physical vulnerability of buildings to debris flows in the Wenchuan earthquake-affected areas. To cope with this deficiency, this paper presented a quantitative method to assess the physical fragility of buildings related to debris flows by analyzing the debris flow activity and building damage features. First, we constructed a building database in the area affected by the Cutou debris flow and determined the degree of damage to each building. Then, the momentum flux was selected as a debris flow intensity parameter, considering the combined effect of flow velocity and flow depth. We used the FLO-2D software to reproduce the process of the debris flow that occurred on 20 August 2019 in the Cutou gully and obtained the intensity parameters of the debris flow. Finally, we proposed physical fragility curves for different types of building structures to the debris flow based on the model describing the probability of exceeding different damage states corresponding to different intensities of debris flows. In the case of brick-concrete buildings, when the momentum flux was 8 m3/s2, buildings were moderately damaged at the probability P = 1 with a 95% confidence interval, and complete damage was sustained from a momentum flux of 36 m3/s2. Masonry-wood buildings sustained complete damage from a momentum flux of 5 m3/s2. The structural resistance of buildings to debris flows in the Wenchuan earthquake-affected region can be assessed by using the proposed physical fragility functions, which provide a scientific basis for risk management and mitigation strategies.