Reframing Nature-based Solution Valuation through Complexity and Life-Cycle Planning: Evidence from Raingarden Performance in Auckland, New Zealand

Urban Nature-based Solutions (NbSs) offer multiple hydrological, ecological, cultural, and wellbeing benefits, yet their uptake remains constrained by economic appraisal frameworks that prioritise monetised, short-term outcomes and overlook wider co-benefits. Building on a recently developed economic valuation Decision Support Framework (DSF), this doctoral research advances a holistic approach that integrates measurable and non-measurable benefits across the life cycle of stormwater infrastructure. The framework is applied to a detailed raingarden typology case study in Auckland, New Zealand. Four raingarden typologies were identified across greenfield developments through field surveys, distinguished by size, configuration, and location. The DSF’s complexity-aware decision pathway (adapted from Cynefin) was used to identify the appropriate valuation methods. This led to a combined qualitative and semi-quantitative assessment of benefits using the More Than Water (MTW) framework, alongside a life-cycle cost (LCC) evaluation. The DSF enabled explicit alignment of benefit types with decision-making contexts, revealing where monetised assessment is suitable (e.g., avoided material costs) and where semi-quantitative or qualitative valuation is required (e.g., biodiversity enhancement, urban cooling, shading, safety, placemaking). MTW results showed that raingarden typology strongly influences both the type and magnitude of multi-benefit outcomes, with media depth, vegetation complexity, and surface area emerging as dominant drivers of non-water benefits. While hydraulic and water-quality performance was broadly comparable across typologies, more complex designs generated markedly greater ecological, resilience, and wellbeing outcomes — benefits typically overlooked in conventional cost-efficiency analysis. LCC results further indicated that larger typologies achieve lower life-cycle costs due to economies of scale. The case study demonstrates that the DSF offers a practical, transferable method for embedding plural valuation and complexity-aware decision-making into stormwater planning. Its application reveals patterns of benefit realisation not captured by traditional cost–benefit analysis, supporting a shift toward more resilient, equitable, and system-aligned NbS investment in New Zealand and internationally.