Since 1996, the Netherlands has adopted a flood risk management policy based on making more room for the rivers. Currently, the focus in flood risk management is being adapted again, in view of increasing societal vulnerability and foreseeable effects of climate change. In this context, the choice between making more room for the river and/or strengthening embankments is again of major concern. This calls for further quantification of the effectiveness, costs and benefits of all possible measures. Making room for rivers was originally advocated by referring to a reduction of the probability of failure of the embankments and a reduction of the consequences in case of breaching. These arguments still apply, but the effect of making more room for rivers on flood risk has never been properly quantified yet. In this paper we identify three potential risk reducing effects, and quantify their effect for along the Rhine and Meuse Rivers in the Netherlands. We show that lowering the flood levels means smaller flood probabilities, that larger floodplain surface area significantly influences the relationship between discharge and flood level, and that lower flood levels in the river translate into smaller flooding depths and/or flood extent, and thus reduce the consequences of flooding.

The Netherlands is updating its flood protection, whilst fully taking into account climate change and socioeconomic development. This translates in 'anticipatory standards' which need to be met in 2050, and which apply for the then foreseen climate and economy. Whilst the government maintains to have adopted a policy of adaptive planning and management, the new standards are thus based on one future situation, which qualifies as a 'high end scenario' from a flood risk management perspective. The consequences of adopting these new standards are now becoming clear. It is expected that many hundreds of kilometres of primary flood defences need to be reinforced and/or raised, at an estimated investment of about 9-14 billion euros. The many uncertainties about actual future development, however, complicate the decision making about the implementation of individual reinforcement projects: should one aim at immediately meeting the new standard or gradually improve and grow towards it? In this paper we discuss the uncertain decision making context, show that lawfulness (working according to procedures, rules and regulations) and expediency (towards a purpose) may jeopardize the good intentions of adaptive management, and argue that optimization may not provide the most useful answer to this decision making problem.