The meaning of system robustness for flood risk management

► System robustness is the ability to remain functioning under a range of disturbances. ► Robustness indicators are derived from system response curve. ► System response curve is compared with a recovery threshold. ► Response curve of robust flood risk systems stays far from recovery threshold. ► Robustness indicators may be useful in comparing flood risk reduction measures. This paper explores how the concept of robustness can be defined and made operational for flood risk management. Decision making about taking actions to reduce the flood risk is complicated by uncertainties about the current and future flood probabilities and impacts. Because decision makers are increasingly aware of uncertainties, new decision-making approaches are being explored. Among other concepts, robustness is put forward by decision makers in the Netherlands as a way to deal with uncertainties. However, they cannot specify what makes a system more robust. This paper defines robustness as the ability of a system to remain functioning under disturbances, where the magnitude of the disturbance is variable and uncertain. We present a conceptual framework for analysing system robustness, and we suggest how the robustness of flood risk systems can be quantified. The ideas are tested on the Westerschelde flood risk system in the Netherlands. This case shows that analysing system robustness provides additional insight into the effect of flood risk reduction measures on system behaviour.