Enhancing flood risk system robustness in practice

Decisions about flood risk management are usually based on the reduction in flood risk compared to the implementation costs of the strategy. It is common practice to express flood risk (the combination of flood probabilities and potential flood damages) into a single number. The downside of this approach is that explicit information about how the system responds to the whole range of possible discharges is lacking. This type of information is relevant when a robust system is desired. We consider robust (fluvial) flood risk systems to have the ability to remain functioning under a range of river discharges. To understand system robustness, it is thus useful to analyze how the flood impact varies over a range of possible events. We applied a method to quantify the robustness of flood risk systems on a fluvial flood risk system in the Netherlands: the IJssel River, a branch of the Rhine River, and its flood-prone area. We compared four alternative system configurations of the IJssel River Valley, and analyzed their robustness, applying the following criteria: resistance threshold, response severity, proportionality and recovery threshold. The robustness analysis provides us with clues about how to enhance a flood risk system’s robustness. We found that a system with unbreachable embankments scores best on overall robustness, because it has the least uncertain resistance threshold, the most proportional damage increase with increasing discharge, and the least uncertainty about where and when floods will occur. This shows that a robustness perspective helps to develop strategies that reduce the flood risks without increasing the consequences of beyond-design floods