The Netherlands builds and calibrates its dams and dykes on extreme sea levels, including those estimated by modelling the tail with extreme value theory, not on average sea level.

True: official Dutch flood-protection policy (for example, the Delta Programme, the Water Act or Rijkswaterstaat safety standards) sets design standards for dikes and dams by extreme water levels or exceedance probabilities (such as 1-in-10,000-year levels or flood-probability standards), not by average sea level, and extreme-value or tail statistics are used to estimate those levels. Mostly true: design is based on extreme levels or exceedance probabilities, but the use of extreme value theory specifically is minor, indirect or not documented. Mixed: extremes are used for some flood defences, while a substantial share are designed on other bases. Mostly false: extremes play only a secondary role and the main design basis is something else. False: Dutch policy designs dikes on average sea level, or does not use extreme levels at all.

Set before the research began.

1. E1 An ensemble study of extreme storm surge related water levels in the North Sea in a changing climate Ocean Science (Copernicus Publications), by A. Sterl et al., KNMI · 18 September 2009

   “To determine the 10 000-year return surge height, simulated and observed annual maxima, y, are fitted to a Generalized Extreme Value (GEV) distribution, the theoretical distribution for block maxima (Coles, 2001),”

2. E2 An ensemble study of extreme storm surge related water levels in the North Sea in a changing climate Ocean Science (Copernicus Publications), by A. Sterl et al., KNMI · 18 September 2009

   “By law, part of the coastal defence system has to withstand a water level that on average occurs only once every 10 000 years.”

3. E3 Risk and coastal zone policy: example from the Netherlands Coastal Wiki · 22 February 2025

   “The design water level of flood defences along the dutch coast are designed using the probabilistic approach. These levels are related to extreme storm surge water levels with a certain probability of occurrence.”

4. E4 WBI2017 Code calibration : reliability-based code calibration and semi-probabilistic assessment rules for the WBI2017 Rijkswaterstaat (Ministerie van Infrastructuur en Milieu) / Deltares, by R. Jongejan · 24 June 2017

   “In the past, the standards were defined in terms of exceedance probabilities of loads that primary flood defenses should be able to safely withstand.”

E2 says Dutch law requires part of the coastal defence system to withstand a water level that occurs on average once every 10,000 years. E4 (Rijkswaterstaat/Deltares) says flood-defence standards were defined as exceedance probabilities of loads, and E3 says design water levels along the coast are set with a probabilistic approach tied to extreme storm surges. E1 (KNMI) shows the 10,000-year return surge height is estimated by fitting annual maxima to a Generalized Extreme Value distribution, which is extreme-value theory. Together these show that design rests on extreme levels and exceedance probabilities, not on average sea level, and that tail statistics are used to estimate those levels.