Evaluating urban green infrastructure for runoff reduction: A flow-based approach

Abstract

Urban green infrastructure (UGI) is widely recognized as an effective strategy for mitigating urban flood risk by reducing rainfall runoff. However, the influence of UGI configurations on surface runoff reduction, especially concerning flow paths in two- and three-dimensional (2D/3D) environments, remains underexplored. Taking Kunshan City, China, as a case, this research developed a flow-based approach to estimate the runoff reduction performance of UGI. The airborne Light Detection and Ranging (LiDAR) point cloud data were used to measure the 3D built environment indicators, including terrain characteristics and vertical vegetation structures. The Soil Conservation Service Curve Number (SCS-CN) model and the Arc-Malstrøm tool were employed to simulate surface runoff and spillover volumes along flow paths at a high spatial resolution. Additionally, the XGBoost machine learning model and interpretability methods, including SHapely Additive exPlanations (SHAP) and Partial Dependence Plots (PDP), were applied to elucidate the relationships between various 2D/3D configurations and UGI’s runoff reduction performance during specific rainfall events. The findings indicate that UGI (including forests, open spaces, orchards, and croplands) can reduce total runoff by up to 28.2 % and spillover volume by up to 48.9 %. UGI coverage had the most significant positive impact on reducing runoff (22.74%), while impervious surfaces exerted the strongest negative influence on spillover mitigation (21.63%). 3D vegetation structure, green volume and canopy coverage, also impacted runoff reduction, contributing 16.3 % and 4.4 %, respectively. Optimal runoff reduction performance was observed when UGI coverage ≥ 35 % and green volume ≥ 0.4 m³/m². This study proposes a novel approach to evaluate the influence of 2D/3D environmental factors on runoff reduction and flow paths, offering valuable insights for urban planning and guiding the implementation of UGI to enhance urban stormwater management.