A Fully Integrated Hydrodynamic-NBS Model for City-Scale Flood Risk Assessment under Extreme Rainfall

Urban Nature-based Solutions (NbS) are critical for flood mitigation, yet existing modelling approaches have limitations in explicitly capturing their performance during extreme events. Conventional approaches typically couple NbS modules with two-dimensional (2D) surface flow models as separate executables. This "loose coupling" fails to capture the two-way dynamic interactions between surface runoff and NbS features, especially when systems approach full saturation or exceed their design capacity.This study introduces HiPIMS-NbS, a high-performance, GPU-accelerated modelling framework that embeds physical behaviours of multi-layer NbS directly into high-resolution 2D shallow water computations. Unlike traditional models with predetermined drainage areas, HiPIMS-NbS calculates flow directions and surface-NbS exchange rates dynamically across the 2D domain. Surface ponding depths influence NbS infiltration rates while NbS storage regulates surface water availability within the unified GPU-accelerated computational framework, achieving dynamic two-way coupling.The model was validated against SWMM benchmarks and field-scale bioretention experiments. To demonstrate city-scale applications, HiPIMS-NbS was applied to the 2012 "Toon Monsoon" flood event in Newcastle upon Tyne, UK, to evaluate various NbS implementation scenarios. Results demonstrate that the model achieves the computational efficiency required for city-scale simulations while capturing key NbS behaviours under realistic overflow conditions. This integrated approach provides a robust modelling basis for urban planners to optimise NbS placement and design for the extreme rainfall events projected under changing climate.