High-tide hysteresis effect of low-frequency debris flow after earthquake: a case study of Xifan Gully

To address the issue of debris flow high-tide often lagging behind earthquakes by 1–2 years in a region, this study uses the case of the Xifan Gully debris flow, which occurred on June 25, 2018, in the Jiuzhaigou area. The research was conducted as follows: First, the amount of new material sources in Xifan Gully was determined by comparing drone images taken before and after the earthquake. Second, regional daily rainfall data from meteorological stations were used to calculate the runoff and infiltration in the gully. Third, indoor shear tests were conducted on soil samples collected on-site to determine the relationship between cohesion (C) and internal friction angle (φ) with changes in moisture content. Finally, numerical simulations were used to calculate how the factor of safety (FS) of the soil in Xifan Gully changes with rainfall. Results show that the peak acceleration brought by the Jiuzhaigou County earthquake to Xifan gully was 164.3 Gal. The materials of Xifan gully and newly added landslide and channel materials occupied 78.81 × 10⁴ and 16.07 × 10⁴ m³, respectively. Although the rainfall in September 2017 was the highest in the last decade, the loose material did not reach saturation. The peak rainfall before debris flow eruption in the Xifan Gully (June 21, 2018) was 21.8 mm, and the effective rainfall reached 68.5 mm until the occurrence of debris flow (June 21–25). At this time, the loose source reached saturation and debris flow started. The results demonstrated that High-tide hysteresis of post-earthquake debris flows is due toe the earthquake not only amplifying the amount of loose material but also increasing the amount of rainfall required to saturate the soil, thereby extending the time needed for the soil to reach saturation. Overall, our results are beneficial for monitoring and early warning of debris flow disasters in mountainous areas.