Generic adaptation pathways for coastal archetypes under uncertain sea-level rise

Adaptation to coastal flood risk is hampered by high uncertainty in the rate and magnitude of sea-level rise. Subsequently, adaptation decisions carry strong risks of under- or over-investment, and could lead to costly retrofitting or unnecessary high margins. To better allocate resources timely and effectively, and achieve long-term sustainability, planners could utilise adaptation pathways, revealing the path-dependencies of adaptation options. This helps to identify low-regret short-term decisions that preserve options in an uncertain future, while monitoring to detect signals to adapt. A major barrier to the application of adaptation pathways is limited experience. To facilitate this, here we generalize this pathways approach for six common coastal archetypes, resulting in generic pathways suitable to be adjusted to local conditions. This provides a much richer analysis of coastal adaptation than provided by any previous analysis, by assessing the solution space and options over time for a variety of coastal regions. Based on this analysis, we find that the number of adaptation options declines while sea-level rises. For some archetypes, it becomes clear that long-term thinking is needed now, about if, how and when to move to transformative options, such as planned retreat, which may presently not be considered or acceptable. Our analysis further shows that coastal adaptation needs to start earlier than anticipated, especially given time required for local debate and choice and to implement measures.