Safety against flooding in the Netherlands

In addition, sea level rise has been approximately one metre over the same period. Currently, the subsidence in the peat polders is still about one centimetre per year. In the clay polders, the total subsidence has often been no more than one metre.

Due to these processes and the increasing standards of safety against flooding, flood protection measures - dikes, storm surge barriers, dunes, etc. - have become increasingly taller, stronger, and more important over the centuries. A distinction is made between primary and regional provisions. In addition, the landscape also includes dormant and drift dikes, which no longer serve a flood defence function. The primary flood protection provisions are located along the sea, major rivers and the IJsselmeer. Regional provisions often include dikes along the collecting and transport systems for superfluous polder water and dikes along inland watercourses.

Initially, after flooding, the relevant dikes were generally raised to a certain level above the water level during the flood. However, especially since the 1953 North Sea Flood, safety against flooding has been determined based on a risk of flooding based on economic and statistical considerations. Regarding the level of protection, a distinction is made between densely populated and less densely populated areas that can be flooded from the sea, areas that can be flooded from the major rivers, and transitional areas that can be flooded from the sea, or from the major rivers. This has resulted in safety levels for flood protection for the primary dikes per dike ring area, as published by the Technical Advisory Committee on Flood Protection Provisions, among others. These range from a probability of flooding of 1/1,250 per year for the river dikes to 1/10,000 per year for the probability of flooding from the sea in the western part of the country.

The high river discharges in 1993 and 1995 led to the Room for the River program, which has since been implemented. This program brought safety up to existing safety standards, but based on a design discharge of 16,000 cubic metres per second of the Rhine River at Lobith, where the river enters the Netherlands, instead of the previous 15,000 cubic metres per second.

After the flooding in the Mississippi Delta caused by Hurricane Katrina in 2005, safety levels in the Netherlands were reassessed. In response, a Delta Commission was established to conduct research and formulate future directions. In 2008, this Commission recommended raising the design standards for flood protection by at least a factor ten. The government adopted this recommendation, and in 2014, new design standards for protection against flooding at primary level were presented. These standards not only reduced the risk of flooding but also shifted from focusing solely on overtopping of the provisions to addressing the failure mechanisms of the flood protection provisions. As a result, the proposed standards, particularly for river dikes, are much stricter than the existing standards.

For the first time, safety standards are no longer based solely on economic considerations, but also on the risk of death from flooding. These standards prescribe the situation as it should be by 2050. Damage calculations are based on an annual probability of damage of € 100 per hectare. The probability of death is assumed to be between one in a hundred thousand and one in a million. This is currently being worked on as part of flood protection programs implemented by the Delta Commissioner, Rijkswaterstaat, and the Water Authorities.

The regional flood protection provisions are subject to the risks of flooding as determined by the Interprovincial Consultation (IPO). Implementation of these flood protection provisions is the responsibility of the Water Authorities.

Thanks to the entire package of measures, the Netherlands will continue to be significantly better protected against flooding in the future than residents in other coastal and deltaic areas in similar situations.