To the contents of Vol. 49, issue 1

Risk based water defence

Risk-based maintenance and design is nowadays, at least in principle, an accepted approach for civil engineering structures. This special issue of Heron shows the development of those methods for application to water defence systems.

In recent years Europe was shocked with many floods. These floods, however, are not completely new phenomena. Throughout the ages many inundations occurred as results of insufficient strong or high water protection structures. Following the event the dikes were rebuilt to meet the most severe conditions known at the time, often the conditions causing the latest disaster. After strengthening the dikes people believed to be fully safe, but this idea proved to be wrong equally many times.

Nowadays complete safety is believed not to be feasible. Additionally, not only raising dikes is believed to be a solution. Widening rivers, creating wetlands or water retaining areas might help as well to mitigate the risk against flooding. The idea is that risk reduction can be achieved not only by a reduction in flood probability but also by a reduction of potential damage and numbers of casualties.

Given all options for water defence, or better, risk reduction there is need for a proper and transparent decision model where costs and benefits are balanced. The papers in this special issue of Heron focus on the quantification and visualisation and of the flood probability, the consequences of flooding and the risk management i.e. the balanced decision on optimal flood defence.

This risk-based decision support has been developed in the Netherlands during the last 60 years. The framework was set after a large flooding in the southwest of the Netherlands in 1953, killing about 2000 people. The basis of the framework was a minimisation of risk, where risk is a function of probability and consequence. Firstly, in the 60’s, only the extreme water levels were based on a statistical approach. The system was extended with the randomness and uncertainties in the water defence system themselves in the 90’s. At present also the consequences are taken into account. Consequences are not only direct economic damage and casualties, but also environmental damage, indirect economic damage, loss of life lines, and long term agricultural damage.

Finally, the papers in the special issue show that that the methods are not only used in the Netherlands. The risk-based decision models are spread in other European countries as well.

Dr. P.H. Waarts

Prof. A.C.W.M. Vrouwenvelder