Against the background of potentially substantial sea-level rise, one important question is to what extent are coastal societies able to adapt? This question is often answered in the negative by referring to sinking islands and submerged megacities. Although these risks are real, the picture is incomplete because it lacks consideration of adaptation. This Perspective explores societies’ abilities to adapt to twenty-first-century sea-level rise by integrating perspectives from coastal engineering, economics, finance and social sciences, and provides a comparative analysis of a set of cases that vary in terms of technological limits, economic and financial barriers to adaptation and social conflicts.

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.

A simple formula to calculate the settling velocity of natural sediment particles for grain sizes between 0.063 and 1 mm is presented. The formula has been derived from the previous work of Dietrich, and it predicts the dimensionless settling velocity W* as a function of a dimensionless fluid-sediment parameter S*, provided the sediment shape factor and roundness are known. In case no information on shape and roundness factors is available, this paper recommends using the formula with a shape factor of 0.7 and a roundness value of 3.5 for naturally worn particles. The formula is tested against several independent data sets, and its performance is compared to other existing simple settling velocity formulas. For fine sediments with nominal diameters dN between 0.063 and 0.25 mm, for which sediment suspension in natural conditions is most likely to occur, the proposed formula is shown to perform the best in terms of standard error of the estimation.