M.J.P. Mens

Enhancing flood risk system robustness in practice: insights from two river valleys

ABSTRACT Decisions about flood risk management are usually based on the reduction in flood risk compared to the cost of the strategy. It is common practice to express this flood risk (the combination of flood probabilities and potential flood damages) in a single number. The downside of this approach is that explicit information about how the system responds to the whole range of possible water levels or river 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 possible river discharges. This paper analyses system robustness for different system configurations of two embanked river valleys in the Netherlands: the IJssel River valley and the Meuse River valley. Comparing the results of these cases provides us with clues about how to enhance a flood risk systems robustness. The IJssel case shows that a system with embankments that will not breach when overflown scores best on overall system robustness. The Meuse case shows that systems with differentiation in protection levels along the river score best on overall robustness. Furthermore, we found that in systems with high protection standards, the most effective way to increase system robustness is by increasing the systems response proportionality. This means that the consequences of flooding increase proportionally to an increase in river discharge. These findings confirm that a system robustness perspective may help to develop strategies that reduce the flood risks without increasing the vulnerability to beyond-design floods.

Strategies for urban drought risk management: a comparison of 10 large cities

Abstract Sustainable development of cities requires robust water supply systems, yet many cities need to resort to ad hoc measures when faced with a drought. This article aims to explore how cities can do better in reducing the risk of water shortage due to drought. To that end, a classification of drought measures in urban water supply systems is proposed, and then applied to 10 cities that recently faced a drought. We find that these cities used a relatively limited number and variety of measures. The classification can help cities evaluate different types of measures for reducing long-term water stress and limit the impact of extreme droughts.

Flood risk management for an uncertain future: economic efficiency and system robustness perspectives compared for the Meuse River (Netherlands)

Flood risk management planning involves making decisions on which measures to implement and when to do so. This is particularly difficult in view of global changes, which are inherently uncertain. 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 or cost effectiveness analysis. However, these metrics treat low-probability/large-consequence risk and high-probability/small-consequence risk as equal, which is often considered unsatisfactory. Robustness analysis can 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. A robust system can cope with a variety of extreme floods, including those that exceed the design flood. This paper aims to examine how different key decision criteria may advise diverse decisions. To this end, it examines how a variety of strategic alternatives for flood risk management along the Meuse River in the Netherlands score on two different economic criteria and how they would rank 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.