Enabling nature-based solutions for climate change on a peri-urban sandspit in Christchurch, New Zealand

Barrier sandspits are biodiverse natural features that regulate the development of lagoon systems and are popular areas for human settlement. Despite many studies on barrier island dynamics, few have investigated the impacts of sea-level rise (SLR) on sandspits. In peri-urban settings, we hypothesised that shoreline environment change would be strongly dependent on contemporary land use decisions, whilst modern engineering capabilities also present new opportunities for working with nature. Our study site in Christchurch, New Zealand, included a unique example of SLR caused by tectonic subsidence and an associated managed retreat initiative. We used a novel scenario modelling approach to evaluate both shorelines simultaneously for 0.25m SLR increments and incorporating open coast sediment supply in 25-year periods. Our key questions addressed the potential impacts of shoreline change on open coast dune and estuarine-coast saltmarsh ecosystems and implications for the role of 'nature-based' climate change solutions. The results identify challenges for dune conservation, with a third of the dune system eliminated in the '1-m SLR in 100-year' scenario. The associated exposure of urban areas to natural hazards such as extreme storms and tsunami will likely fuel demand for seawalls unless natural alternatives can be enabled. In contrast, the managed retreat initiative on the backshore presents an opportunity to restart saltmarsh accretion processes seaward of coastal defences with the potential to reverse decades of degradation. Considering both shorelines simultaneously highlights the existence of pinch-points from opposing forces that result in small land volumes above the tidal range. Societal adaptation is delicately poised between the paradigms of resisting or accommodating nature and challenged by the long perimeter and confined nature of the sandspit feature. The use of innovative policy measures in disaster recovery contexts, as highlighted here, may offer a beneficial framework for enabling nature-based solutions to climate change and natural hazards.YY