What Conventional Appraisal Misses: Rain Garden Typologies, Multi-Benefit Performance and Complexity-Aware Valuation

Urban Nature-based Solutions (NbSs) provide ecological, hydrological, cultural and wellbeing benefits; however, conventional economic appraisal often privileges monetised, short-term outcomes and overlooks a broader range of benefits. This study demonstrates the application of an economic valuation Decision Support Framework (DSF) using four rain garden typologies identified in Auckland, New Zealand. The DSF’s complexity-aware decision pathway, adapted from Cynefin, guided the selection of context-appropriate valuation methods. Benefits were assessed qualitatively and semi-quantitatively using the More Than Water (MTW) Tool, alongside a quantitative life cycle costing (LCC) evaluation. MTW results showed that rain garden typologies exhibited clear differences in assessed benefit potential. Although hydraulic and water-quality outcomes were broadly comparable, greater surface area, soil and vegetation volume, vegetation complexity and landscape integration were associated with higher assessed levels of ecological, resilience and wellbeing benefits. LCC results indicated that larger typologies achieved lower costs per unit area through economies of scale. The case study demonstrates that the DSF provides a practical method for combining monetised and non-monetised values within stormwater appraisal. Its application reveals differences in potential benefit realisation that conventional cost-efficiency analysis may overlook, supporting more comprehensive NbS investment decisions in New Zealand, and offering a transferable model for other jurisdictions, subject to local calibration of costs and benefit data.