New paper on climate change: the costs of raising coastal dykes outweighs the damage caused by floods

A group of European scientists, among whom Philip Ward, published the findings of their research in Nature Communications last week. They conclude that when elevating dykes along one third of Europe’s coastline by on average 90 to 104 cm, at least 83% of flood damage caused by the expected raise in sea level between now and 2100 could be avoided. The study was commissioned by the European Commission.

05/19/2020 | 1:14 PM

Many countries consider elevating their dykes. A lot of people live in a 50 km radius from the coast and the area is economically important. The researchers looked at two scenarios of future sea level rise: (1) 43 to 76 cm (at a global warming of 2.4°C) and (2) 58 to 172 cm (at 4.3°C). These numbers are both higher than the target of 1.5 degrees of the Paris Climate Agreement. The last scenario is the case if we do not switch over to sustainable energy.

They compare the costs (coastal defence) and benefits (avoided damage) against each other. If European countries were to do nothing to defend themselves against a higher sea level, the annual damage would be €209 billion (moderate scenario) to €1,268 billion (high scenario). France, UK, Italy and Denmark show the highest increases in coastal flood risks towards the end of the century.

Even when a country does invest in dykes (cost €1.8 to €2.8 billion per year), there will still be substantial economic damage: €8.9 to €24 billion per year, but this will be much lower than when nothing is done to coastal sea protection.

This study covers the coastline between Norway and Greece. According to the researchers, for two thirds of the coastline dykes may not be economically effective. This is especially the case when the coastline is rugged or mountainous or when it is relatively sparsely populated. Elevating dykes is cost-effective in areas where the population density is larger than 500 people per km2. This applies e.g. for Belgium, France and Italy.

The Netherlands is mentioned specially in this study, because it already has an extensive network of dykes and surge barriers. More protection decreases the chance of a heavy flood further, but because it has extensive low-lying areas and a high population density, the impact of a flood is enormous.

Is raising dykes the best solution? When sea levels increase even further after 2100, you can keep on elevating. During this study the focus was on dykes, but there are also other solutions mentioned like effective forecasting/warning systems and emergency response, or flood proofing of structures. Not to bet on one horse, a combined approach would be best.