Low probability, high consequences
The probability of urban flooding from the sea, major rivers, or major lakes is very low in the Netherlands, yet if such a flood occurs, it will have a major impact. The probability of urban flooding from regional waters is higher, but the impact will be less devastating (with the exception of the deep polders that are protected by regional flood defence systems: here, the risks will be quite high, especially if protection is provided by peaty dykes). In the past, floods used to be taken into account in spatial planning, for example, by choosing or constructing elevated sites. In flood risk management, attention has gradually shifted to raising and improving dykes, and creating more room for rivers. However, despite the strong dykes and wider rivers, factoring in the impact of a flood continues to be important, by limiting the damage, fatalities, and social disruption if a flood occurs nonetheless, and improving the water resilience of urban and rural areas. In the long run, this may even reduce the need for dyke improvements and river widening.