Spatiotemporal aspects in coastal multi-risk climate change decision-making: Wait, protect, or retreat?

Climate change-induced sea-level rise will have major impacts on coastal infrastructure. Gravity-based drainage systems are particularly susceptible due to the potentially compounding effects of sea-level rise, more frequent severe rainfall events, and rising groundwater. In the area of Petone in New Zealand, an accelerating rate of drainage system failures is certain over the coming decades, and over time managed retreat could be a viable adaptation response due to technical, environmental, economic, and social constraints. Adapting coastal cities such as Petone to climate change means resolving the individual, compound, spatially, and temporally varying risk contributions from multiple interacting hazards and comes with high complexity and risk of maladaptation. Here, we introduce a novel spatially explicit application of the Dynamic Adaptive Pathways Planning (DAPP) approach to address these challenges. Through stakeholder workshops, descriptive conditions for adaptation are formed alongside projections of service decline including economic, socio-technical, and environmental dimensions. Based on an identification of spatially heterogeneous vulnerabilities, the Petone area is disaggregated into smaller units based on patterns of vulnerability defined by unique combinations of thresholds and decision points in each subarea. Based on a numerical analysis we show that a spatially explicit DAPP can be developed with area-specific thresholds under non-stationary, multi-hazard risk. We find that accounting for local vulnerabilities in a multi-hazard environment better informs the timing of decision triggers, informed by a spatially-explicit understanding of thresholds where otherwise it would be difficult to aggregate the risk across the study area. Our analysis identifies opportunities to influence the timing of adaptation responses through area-specific interventions aligned with synergies and conflicts between adaptation responses, and with conditions and dependencies that affect the spatial sequencing of adaptation pathways in each subarea. For example, incorporating wider threshold considerations from the co-production into the quantitative DAPP may affect the adaptation timing. The spatiotemporal staging of adaptation responses using the spatially explicit DAPP could also help decision makers keep their options open while transitioning towards different adaptation options in response to declining service levels.