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

On August 17, 2020, debris flows successively occurred in four ravines along the main stream of Yazhezaozu Village, Pingwu County, Sichuan Province. This event resulted in multiple collapses and interruptions along the G247 national highway, and extensive damage to the Jiumian expressway project site and laborer residences, among other areas. A significant volume of driftwood carried by the debris flow converged into the main stream, leading to the blockage of downstream road bridges and culverts, causing backflow and village flooding, thereby exacerbating the disaster risk. To prevent similar disasters in the future, 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 main stream road bridges and culverts due to debris flow. The results indicated: (1) The ‘8.17’ debris flow in Yazhezaozu Village was a low-frequency, large-scale, rare, and extensive group occurrence, with an eruption frequency of approximately once every 50 years. The debris flows in Azu Gully and Zapai Gully were extremely large-scale, while 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 main stream road bridges and culverts resulted in the blockages 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-fold. (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 flows.