Risk amplification effect caused by blockages in the main stream road bridges and culverts due to the debris flow

On August 17, 2020, four debris flows successively occurred along four ravines on the main stream in Yazhezaozu Village, Pingwu County, Sichuan Province. This event resulted in multiple collapses and interruptions along the G247 national highway, extensive destruction at the Jiumian expressway project site and laborer residence, among other areas. A significant volume of driftwood carried by the debris flow converged into the main stream, leading to downstream road bridge and culvert blockages, causing backflow and flooding of villages, thereby exacerbating the disaster risk. To prevent similar disasters from recurring, post-disaster investigations using field surveys and remote sensing interpretations explored the characteristics of this debris flow disaster’s risk. A primary focus was placed on analyzing the risk amplification effect caused by blockages in the main stream road bridges and culverts due to the debris flow. The results indicated: (1) The August 17 debris flow in Yazhezaozu Village was a rare, large-scale, low-frequency event occurring approximately every 50 years. The debris flows in Azu Gully and Zapai Gully were extremely large-scale, while the Mashizha No. 3 Gully and Duobu River No. 5 Gully were large-scale, with an affected debris fan area of about 16.66×10⁴ square meters. (2) Improper layout of the main stream road bridges and culverts resulted in the blockage of driftwood carried by the debris flow, forming dammed bodies, increasing the affected area by 16.78×10⁴ square meters, and enlarging the risk range by about 1 time. (3) In well-vegetated, moist mountainous areas, when constructing main stream road bridges and culverts, it is advisable to appropriately increase the axial spacing between bridge piers, allowing for a certain width and operational space in the river channel. This will prevent the amplification of debris flow disaster risks caused by driftwood blockages. This study aims to provide guidance for the reasonable planning of main stream road bridges and culverts in similar mountainous areas and relevant prevention and early warning of debris flow.