Experiment on the benefits of natural-based solutions for debris flow mitigation using synergistic eco-geotechnical measures

With the growing development of nature-based solutions (NBS) for debris flow hazard mitigation, the eco-geotechnical coupled technical system has drawn public concern. While the scheme of debris flow prevention and control are constantly improved, the current collaborative patterns, mechanism, and effects on debris flow interception are not clear. In this study, a new synergistic mitigation methodology of reducing debris flow impact risk coupling tree-shrub mixed vegetation filter stripes (T-SMVFS) in S-shape flow paths and dams was proposed. Four efforts were achieved stepwise: 1) the optimal row and stem spacing of T-SMVFS were determined by the overspread T-SMVFS type; 2) set “S-shape” flow path parameters: width ratio (30%, 45%, 60%, 75%); 3) Comprehensively compared the effects of synergistic measures and single measures (ecological or geotechnical measure) on debris flow reduction; 4) Calculation equation of flow reduction considering the influence of topographic features (channel width, roughness), vegetation planting pattern (stem spacing and row spacing), physical properties of debris flow (capacity, mass) was constructed. The results showed that the overspread T-SMVFS with the row spacing of 10cm and stem spacing of 6cm, respectively presented the best reduction effects with energy regulation reaching 43%, flow regulation reaching 46%, and flow rate being close to 40%, respectively. As the flow path widths of the S-shape vegetation filter strips increased (0%-75%), the flow reduction rate (≈45-8%), flow reduction rate (≈58-13%), and sediment interception rate (≈78-5%) decreased sequentially, but the transport capacity increased. Synergistic measures achieved 60% energy reduction, which was better than pure geotechnical (8.9-23.6%) and pure biological (11.56-52.72%) measures, and 70% sediment interception, and were also much higher than single measures. In the comparison of multiple synergistic approaches, the coupled s-shape vegetated filter strip with a 45% proportion of flow path and beam dam is more effective in synergistic hazard reduction. The synergistic eco-geotechnical mitigation measures proposed in this study are a pattern and an attempt to mitigate disasters based on the concept of NBS and provide a reference for subsequent more optimal mitigation solutions.