Emil writes*

The Netherlands is famed for its water management prowess. This is not for nothing, from the construction of the Afsluitdijk after the Zuiderzeevloed in 1916, to the Delta works after the Watersnoodramp in 1953, water is, and always has been, a genuine threat to livelihood in the Netherlands. Through this decorated history of water management, the Netherlands has built up a complex network of governmental institutions that divide the responsibilities of domestic water management. An important responsibility of this network is flood management. The relevant institutions use a multilayer safety framework, in which flood prevention takes priority over spatial design (tailored to flood damage curtailment) and crisis management (in case of flooding).

In 2017, the norms of flood prevention were updated through the adoption of the new Waterwet (Water Law). Flood management must now be assessed based on risk, which is defined both by the probability of a flood occurring, and the consequences that flooding entails. Based on this, the probability of flooding must be low in places where the consequences would be great in order to minimize risk. Risk is calculated per dike trajectory, of which the Netherlands has 255. (A “dike trajectory” is a set of dikes for organisational and management purposes in the larger institutional framework. A trajectory ranges from 0.2 km to 47 km in length.)

The accepted probability of a flood occurring is calculated based on two different methods: Local Individual Risk (LIR) and Societal Cost Benefit Analysis (SCBA). LIR refers to the probability of death by flooding and must be ≤0.00001 death per year. Mortality is a constant between zero and one, determined on a neighborhood scale. It represents the average mortality by flooding based on historical data. The highest mortality value within an area behind a dike trajectory determines the mortality constant for the whole trajectory. The evacuation fraction is the estimated proportion of the human population behind a single dike trajectory that can be evacuated preventatively. With these equation terms, the flooding probability for a dike trajectory can be calculated as:

LIR = Flooding probability * Mortality * (1 – Evacuation fraction)

Just like LIR, the SCBA [pdf] calculates flooding probability per dike trajectory, but based on economic value. It is a very complex cost benefit analysis, but notable factors include a discount rate of 5.5% and “value of a statistical life” of € 6.7 million.

For each dike trajectory, the lowest probability calculated, by either LIR or SCBA, is used for the entire dike trajectory. Whether a dike trajectory meets the required safety norm is determined through annual inspection but can change over time. Land subsidence or sea level rise could alter the flooding probability. With the determined flooding probability, it is possible to calculate the economically optimal time to initiate flood defense construction. The flooding probability sharply decreases after the construction of new defense works, after which it increases slowly for the aforementioned reasons. The flooding probability follows a downward trend over time because of increasing population and economic value behind the dikes, which therefore increases the risk of flooding.

The Netherlands’ flood defense management seems impeccable, but the costs of defending against sea level rise of 45 cm to 85 cm by 2085 may be steep.

**Bottom Line:** Facing a 1/100000 chance of dying in a flood, every year, may seem terrifying, but it’s subjective and should make you grateful for being born here and not in another low-lying, coastal area.

* Please help my Environmental Economics students by commenting on unclear analysis, alternative perspectives, better data sources, or maybe just saying something nice :).

Very interesting read! The math behind the flood water management is very nicely laid out. I would like to ask, in the end you mention that ‘the costs of defending against sea level rise of 45 to 85 cm may be steep’. According to the KNMI report [Wetenschappelijk rapport KNMI’14-scenario’s (2014)], this number is associated with a 3.5 C temperature rise by 2085. The report calculates the potential sea level rise according to a moderately warming scenario (G, 2050 +1°C, 2085 +1.5°C), and a warm scenario (W, 2050 +2°C, 2085 +3.5°C). The warm scenario they use is that the one that corresponds with the number you mention at the end? Moreover, does this imply that they deem scenarios in which global temperatures would exceed +3.5°C, statistically too unlikely to be included in potential sea-level rise scenarios?