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Looking at flood risk management developments that have lately taken place in the Netherlands, the emergence of a virtual liking of disaster preparation measures is perceptible. Investments in multilayer safety projects are one of the most popular outlets of this trend. Multi-layer safety has raised several discussions about its cost-efficiency. This paper contributes in these discussions by clarifying the economic implications of multi-layer safety from a rational perspective.

This paper presents an assessment of the multi-layered safety system in Tohoku, Japan based on the tsunami disaster of March 2011. The performed analysis has been based on data provided by local researchers and field observations. First an overview of the tsunami behaviour along the affected coastline of Tohoku is presented, which shows clearly that the disaster has site-specific features. The assessment that follows has a descriptive character and it is divided in two parts. First the performance of each safety layer in Tohoku is separately assessed, and conclusions are drawn for the efficiency of the system. The second part points out some implications of this disaster for the use of multi-layered safety in flood risk management.

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After the flood disaster of 1953, the Netherlands adopted a rational approach to flood risk management with the use of protection standards determined by means of costbenefit analysis. Due to scientific and political developments that have recently taken place, an update of the Dutch protection standards is being undertaken. One of the major priorities considered, is the need to address three issues, namely: (1) expressing the protection standards as failure probabilities of the flood defences, i.e. probabilities of breaching, instead of exceedance frequencies of water levels that is currently the case, (2) taking into account a spatial variability of those failure probabilities, and (3) considering various flooding scenarios. These aspects have been comprehensively addressed within a national flood risk analysis project, and partly considered in a numerical cost-benefit analysis approach, developed for the determination of new protection standards in the Netherlands. This paper presents an analytical economic optimization approach that makes an explicit link with all results of the national flood risk analysis project. In particular, an approach is outlined for the approximation of economically optimal design values of the failure probabilities along dyke-ring segments, which are treated as a series system of flood defences. The approach can assist in the determination of new protection standards in the Netherlands, but also in the design of flood prevention systems elsewhere.

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