Since 2017 the Dutch flood risk legislation builds upon a risk-based approach (see visual). In this approach, the flood risk is formed by the probability of a flood and the consequences of a flood (risk = probability x consequences). The water level, hydraulic load, strength and height of the dike are factors that have an influence on the probability of a flood. The consequences consist of (direct and indirect) economic damage and (direct and indirect) mortality. The consequences of a flooding are highly affected by the flood progress and pattern, and the evacuation rate. Following this approach, there are three main aspects that come back in the new Dutch flood protection legislation which are important for the technical implementation: basic level of flood protection, limit value and signal value.
Basic level of flood protection
The purpose of the Dutch flood protection program is to make sure flood defences comply with the safety standards, thereby providing a basic level of protection for the population living behind the dike and preventing substantial economic damage. The Local Individual Risk (LIR) on fatalities by flood may not exceed the probability of 1:100.000 a year as follows:
Maximum permissible failure probability for a flood defence according to the new safety standards.
- This basic level of protection is provided by law in the future Omgevingswet or Environment and Planning Act (art. 2.15 lid 2). It is the underlying starting point of the safety standards that are incorporated in ‘Bijlage III Waterwet (Annex to the Water Act) and soon in Bijlage II Besluit kwaliteit leefomgeving or: Bkl (Annex to the Decree on the quality of the living environment).
- A safety standard in line with the basic level of protection has been established for each dike segment. Also group risk and economic damage are part of determining the safety standards. The more serious the consequences of a flood, the smaller the permissible flood probability and the stricter the standard for the dike section. The figure shows the maximum permissible failure probability for a flood defence also called to as limit-value.
- Another important factor is the outcome of a cost-benefit analysis, where the reduction of flood-damage is compared with the investments for a better flood protection.
Figure adapted from STOWA
Signal value and limit value
In the Dutch flood protection legislation, the limit value shows the minimum level of protection that the dike section must offer. The ‘limit value’ for each dike segment can be found in Bijlage III Waterwet (Water Act Annex). In the Omgevingswet (Environment and Planning Act) the ‘limit value’ will be expressed in ‘environmental values’ (art. 2.15 lid 1 sub d, art. 2.0a e.v. and Bijlage II Bkl).
To guarantee that the ‘limit value’ will not be exceeded, the signal value shows at what point the process to reinforce the dike should start. The signal value for each dike segment can be found in Bijlage II Waterwet. When the Omgevingswet is in effect the signal values will be identified as “other parameters” for signalling the safety of primary flood defences (art. 10.8b and Bijlage II Bkl).
When the dike section reaches the signal value, the responsible authority can sign the dike section up for the Dutch Flood Protection Programme (Hoogwaterbescherming programma, HWBP) to be granted a subsidy for 90% of the reinforcement (7.23 lid 2 and 7.24 Waterwet).
From the technical side, the current challenge is to develop and bring into practice design codes that are consistent with the new safety standards: “The main difference with the former codes is that instead of considering a load event (e.g. storm, river flood or a combination with a certain return period), we now need to consider a wide range of possible events with their respective probabilities. Also uncertainties on the dike resistance side need to be considered explicitly” (Schweckendiek and Slomp, 2018,on page 2).
This challenge is the main motivation for more than half of the All-Risk program so we invite you to read the related research projects (A to D).
Last modified: 24/03/2021