Assessing nature-based coastal defense.

Nature-based Coastal Defense is increasingly used to reduce climate risk, because considered effective, inexpensive and cost-effective, easy to implement, and no-regrets. This article discusses this positive framing through the analysis of 23 projects implemented in French overseas territories, using an ex-post expert judgment method considering enabling conditions (context, governance, funding, social acceptability), risk reduction (technical effectiveness; studies, monitoring and evaluation) and externalities (co-benefits and disbenefits; contribution to adaptation). 80% of projects aimed at reducing coastal erosion; 47.8% were implemented in natural or rural areas; 87.1% included restoration; 82.6% targeted one single ecosystem and 51.7% beach/dune systems; 47.8% were led by public actors; all relied on multiple funding sources. Performance indices range from 39.4 to 77.2%. The highest scores concern governance and social acceptability, and the lowest scores risk reduction. No project included an evaluation of risk reduction and was calibrated for future risk. Internal (i.e. project related) and external (more general) levers and barriers to effectiveness were identified. Internal barriers include the lack of political support to nature-based options, the difficulty to secure the required long-term funding and to upscale action in the face of strong land tenure constraints.