Assumptions & limitations for coastal and fluvial floods
The probability of a certain flood depth in the location-specific flood probability maps is a conservative estimate, and therefore relatively high. It is a conservative estimate because the failure probabilities are determined based on the assumptions used for standardising and assessing flood protection measures. Three factors are of importance:
- The failure definition used. Models are used to assess the reliability of flood defences. These sometimes reflect only some of the physical processes that ultimately lead flood defences to fail (i.e. failure definition). In practice, we observe that when failure occurs according to the definition, there is still residual strength present.
- Hydraulic interdependencies are not taken into account. In reality a breach in a river levee leads to a lowering of the water level, which in turn leads to a lower failure probability of the levee on the opposite side of the river and downstream. Multiple breaches are not considered.
- Emergency measures taken by water boards during a flood, such as placing sandbags, are not considered.
The Expertise Network for Flood Protection (ENW) also observed that the failure probability estimates resulting from the Statutory Assessment of primary flood defences were unrealistically high and did several recommendations to calculate more realistic values.
When calculating flood risk, one should be aware of these factors, and not simply multiply the consequences with the estimated failure probabilities. We recommend using the information from the location-specific flood probability maps only for an initial estimate. If the risks are high or significant, further investigation into the failure probabilities is advised.
Alternatively, the website My Water Risk Profile applies a new method, BREACH, which corrects for these limitations based on expert judgement and multiple breach scenarios.
