Comparative analysis of debris flow mitigation by geotechnical and biological measures: a case study of Shaer Gully, Sichuan, China

Geotechnical measures are widely used for debris flow hazard mitigation, while the complementary role of biological measures, which is regarded as a method of Natural-based Solutions, has received increasing attention. The integration of these strategies combines the immediate mitigation capacity of engineering structures with the long-term stabilizing effects of vegetation. Although the individual effectiveness of each measure has been widely investigated, quantitative evaluations of their combined effectiveness remain limited. Focusing on the debris flow system in Shaer Gully, Sichuan Province, China, a method to estimate forest-belt roughness under debris flow conditions is developed, and debris flow processes are simulated under natural conditions, geotechnical measures, and geotechnical–biological measures across multiple rainfall frequencies. The results indicate that compared with single geotechnical measures, integrated strategies of geotechnical–biological measures consistently achieve greater mitigation effectiveness, with the most pronounced benefits with return periods between 10 and 50 years. At a forest-belt density of 9.07 × 10−3, compared with those under natural conditions, the areas of low-, medium-, and high-hazard zones are reduced by 41%, 65%, and 24%, respectively. In addition, the total hazard area is reduced by 28–35% compared with that under natural conditions and by 13–24% compared with that under geotechnical measures.