Decoupling hydraulic and treatment optima in floating treatment islands: implications for nature-based solutions in stormwater systems

Floating treatment islands (FTIs) are increasingly deployed as nature-based solutions (NbS) in urban stormwater systems. However, conventional design relies on empirical hydraulic proxies, lacking a physics-based framework that integrates multi-scale flow processes with localized reaction kinetics. This study presents a technical increment by quantifying the 3D multi-scale coupling between system-scale hydrodynamics and localized root-zone pollutant uptake using validated computational fluid dynamics (CFD) simulations of a full-scale stormwater pond. The results reveal a clear physical decoupling between hydraulic and treatment optima. Hydraulic efficiency, governed by flow redistribution and short-circuiting mitigation, associated with improved flow distribution and reduced short-circuiting, is maximised at a root-length-to-pond-depth ratio (LR/DP) of approximately 0.50. Conversely, FTI pollutant mass removal – driven by local advective flux and residence time within the porous root matrix – reaches an individual maximum near LR/DP ≈ 0.65, with the upstream island achieving 52% removal. Crucially, system-scale pollutant removal plateaus beyond LR/DP = 0.50 (62% at both 0.50 and 0.65), as flow blockage by deep upstream roots induces hydraulic shielding over downstream structures. Consequently, LR/DP ≈ 0.50 represents a robust, physics-informed design optimum, maximizing water quality benefits while preserving hydraulic performance, structural stability, and cost-efficiency.