Experimental study on the benefits of nature-based solutions for debris-flow mitigation via synergistic eco-geotechnical measures

Research on nature-based solutions (NbSs) for mitigating debris-flow hazards has increased interest in eco-geotechnical systems. Most studies focused on the efficiency of isolated mitigation measures, while the benefits and mechanisms of coordinated approaches remain unclear. Consequently, this study proposed a novel approach to mitigating debris-flow velocity, sediment transport, and energy by utilizing tree-shrub mixed-vegetation filter strips (T-SMVFS) along S-shaped flow paths combined with dams. The optimal design determination involved four steps: 1) optimal T-SMVFS row and stem spacings were determined; 2) S-shaped flow path parameters were set based on width ratios; 3) effects of synergistic and individual measures on debris-flow reduction were compared; and 4) a flow velocity reduction equation was constructed, considering the influence of topographic features, vegetation planting patterns, and debris flow properties. Results revealed that a completely covered T-SMVFS with row and stem spacings of 10 and 6 cm in 1/50th scale, respectively, exhibited the best reduction effects with 50% energy reduction, 55% sediment interception, and 53% flow discharge regulation. As the S-shaped flow path width increased, the flow reduction and sediment interception rates decreased sequentially, while the transportation capacity increased. Synergistic measures achieved 60% and 70% in energy and sediment interception reductions, respectively, outperforming pure geotechnical and biological measures. Comparisons between different synergistic approaches indicated that a coupled S-shaped vegetation filter strip with a 45% flow path proportion and a beam damweres more effective in reducing debris flow. These findings provide a reference for subsequent optimal mitigation solutions involving NbSs that integrate synergistic eco-geotechnical measures.

Methods
Simulated flume experiments were performed with different stem and row spacings with mixed shrub-tree vegetation to investigate the effects of coupling ecological and geotechnical measures on debris-flow sediment interception, flow discharge, velocity, and grain size. The experiment primarily consisted of three parts: In Part 1, this study explores the interception effect of full-cover vegetation arrangement on debris flow movement and obtains the optimal row and stem spacing. Part 2, it examines the influence mechanism of S-shaped flow path width variation on debris flow movement under optimal row and stem spacing. Part 3, it analyzes the mechanism and benefits of the integrated S-shape vegetation filter strip and geotechnical measures. To monitor sediment deposition, a grid was positioned on the glass retaining wall of the flume along the direction of water flow to track the sediment's height and morphology. Three laser mud level meters (Leuze, ODSL 30/V-30M-S12, 10 Hz) and two cameras (GoPro 9, 3840 × 2160 pixels, 30 fps) were installed directly above the flume. The three laser mud level meters were installed at distances of 0.5, 1.25, and 3.5 m from the material pool to measure variations in the flow-body depth in front of the debris dam, between the debris and vegetation filter strips, and behind the vegetation filter strips, respectively. Two cameras were installed at distances of 1.25 and 3.5 m from the material pool to observe and record the processes, wherein debris flows passed through the debris and vegetation filter strips, respectively. Two additional cameras (Sony FDR-AX60, 3840 × 2160 pixels, 25 fps) were installed on the side and directly behind the flume to observe and record the entire test process.