The ever-growing climate change, extreme weather events, and continuous expansion of human populations into mountainous regions have contributed to the increase in the frequency, intensity, and duration of catastrophic geophysical mass flows (Fowler et al., 2021;Guerreiro et al., 2018;Hirschberg et al., 2021), such as debris flows, lahars and avalanches. In practical combats of these flows, flexible, slit, and rigid barriers (see Figure S1 in Supporting Information S1) are designed to reduce their peak discharge, flow velocity, erosion, and run-out distance (Gong et al., 2021;Iverson et al., 2016;Marchi et al., 2019). Nonetheless, current practice on the selection of barrier types and associated design has been largely empirical, since there is no unified analytical tool available for systematic analysis and comparison of their performance in arresting impinging flows of variable nature. This is due to the difficulties posed by the modeling of multi-body and multi-phase interactions. For

Geophysical mass flows impacting flexible barriers can create complex flow patterns and multiway solid‐fluid‐structure interactions, wherein estimates of impact loads rely predominantly on analytical or simplified solutions. However, an examination of the fundamental relations, applicability, and underlying mechanisms of these solutions has been so far elusive. Here, using a coupled continuum‐discrete method, we systematically examine the physical laws of multiphase, multiway interactions between geophysical flows of variable natures, and a permeable flexible ring net barrier system. This model well captures the essential physics observed in experiments and field investigations. Our results reveal for the first time that unified bi‐linear laws underpin widely used analytical and simplified solutions, with inflection points caused by the transitions from trapezoid‐shaped to triangle‐shaped dead zones. Specifically, the peak impact load increases bi‐linearly with increasing Froude number, peak cable force, or maximum barrier deformation. Flow materials (wet vs. dry) and impact dynamics (slow vs. fast) jointly drive the patterns of identified bi‐linear correlations. These findings offer a physics‐based, significant improvement over existing solutions to impact problems for geophysical flows.

Flood retention lakes (RLs) are widely employed in rural‐urban catchments with low impacts on the natural environment. However, insights are lacking regarding the control of climate conditions on RLs' performances and how they are affected by different geographic configurations. This study applies a 2D hydrodynamic model to perform a catchment‐scale performance assessment of RLs beyond the scope of analytical and hydrological models. We conduct extensive numerical experiments of rainstorm‐induced flooding in a rural‐urban catchment with a constructed RL and blueprinted ones upstream. Results demonstrate an L‐shaped band of satisfactory performance of the current RL in the frequency‐duration diagram, which coincides with short return periods (<5 years) and long durations (>4 hr), or short durations (<3 hr) and moderate to long return periods (5–50 years); such L‐shaped pattern is also valid for additional RLs and their combinations. With the increase in event size, the first two modes of RL performance (out of four) correspond to effective flood mitigation. When working jointly, RLs with series configurations are more effective in reducing the mainstream flood peaks, while parallel connections provide a greater spatial extent of flood hazard mitigation. For both series and parallel configurations, the upstream‐weighted settings tend to outperform downstream‐weighted ones under more extreme events; the decentralized arrangement in the urban area yields more benefits in spatial flood hazard mitigation compared to the centralized case. The study highlights the critical role of rainstorm severity (with possible spatiotemporal variabilities) in controlling RL performances despite various configurations and hydraulic settings.