Robustness analysis for flood risk management planning: on risk-based decision making beyond simple economic reasoning, exemplified for the Meuse River (Netherlands).

Flood risk management planning involves making decisions on which measures to implement, and when to do so. Rational decision making on which comprehensive strategy to implement, or on which measures to take first, requires ex-ante assessments that question whether flood risk is effectively reduced, and against which societal costs. Such decision making is usually supported by cost benefit analysis (CBA) or cost effectiveness analysis (CEA). The key economic assessment criterion applied may be the ratio between the benefits and costs of a measure or strategy (B/C), or, alternatively, the minimum of the sum of costs and (residual) flood risk. However, these metrics treat lowprobability/large consequence risk and high-probability/small consequence risk as equal, which is often considered unsatisfactory in a decision making context. Robustness analysis can be used to account for this ‘flaw’, as it gives insight into the relationship between flood magnitude and flood consequences at the scale of an entire flood risk system, thus revealing how sensitive such a system is and whether it can still recover. A more robust system is able to deal with a variety of extreme floods, including those that exceed the ‘design flood’. This paper examines how a variety of strategic alternatives for flood risk management along the Meuse River in the Netherlands score on various economic criteria and how they would be assessed from a robustness perspective. The strategies include making room for the river, strengthening embankments, and various combinations of these. The results show that the three criteria indeed lead to a different ranking of which strategy to prefer. This supports our claim that a robustness perspective may help to select a strategy that is not only economically efficient, but may also be more sustainable in view of uncertainties into the future.