R.B. Jongejan

The use of individual and societal risk criteria within the Dutch flood safety policy-nationwide estimates of societal risk and policy applications

The Dutch government is in the process of revising its flood safety policy. The current safety standards for flood defenses in the Netherlands are largely based on the outcomes of cost-benefit analyses. Loss of life has not been considered separately in the choice for current standards. This article presents the results of a research project that evaluated the potential roles of two risk metrics, individual and societal risk, to support decision making about new flood safety standards. These risk metrics are already used in the Dutch major hazards policy for the evaluation of risks to the public. Individual risk concerns the annual probability of death of a person. Societal risk concerns the probability of an event with many fatalities. Technical aspects of the use of individual and societal risk metrics in flood risk assessments as well as policy implications are discussed. Preliminary estimates of nationwide levels of societal risk are presented. Societal risk levels appear relatively high in the southwestern part of the country where densely populated dike rings are threatened by a combination of river and coastal floods. It was found that cumulation, the simultaneous flooding of multiple dike rings during a single flood event, has significant impact on the national level of societal risk. Options for the application of the individual and societal risk in the new flood safety policy are presented and discussed.

Quantifying Flood Risks in the Netherlands

The Flood Risk in the Netherlands project (Dutch acronym: VNK2) is a large-scale probabilistic risk assessment for all major levee systems in the Netherlands. This article provides an overview of the methods and techniques used in the VNK2 project. It also discusses two examples that illustrate the potential of quantitative flood risk assessments such as VNK2 to improve flood risk management processes: (i) informing political debates about the risks of flooding and the effectiveness of risk management actions, and (ii) (re)directing research efforts towards important sources of uncertainty.

A Risk-informed Approach to Coastal Zone Management

Abstract Economic and population growth have led to an unprecedented increase in the value at risk in coastal zones over the last century. To avoid excessive future losses, particularly in the light of projected climate change impacts, coastal zone managers have various instruments at their disposal. These primarily concern land-use planning (establishing buffer zones) and engineering solutions (beach nourishment and coastal protection). In this paper, we focus on risk mitigation through the implementation of buffer zones (setback lines). Foregoing land-use opportunities in coastal regions and protecting coasts is costly, but so is damage caused by inundation and storm erosion. Defining appropriate setback lines for land-use planning purposes is a balancing act. It is, however, unclear what level of protection is facilitated by current approaches for defining setback lines, and whether this is, at least from an economic perspective, sufficient. In this paper, we present an economic model to determine which setback lines would be optimal from an economic perspective. The outcomes of the model provide a useful reference point in the political debate about the acceptability of risk in coastal zones. The main conclusions are: (i) that it is useful to define setback lines on the basis of their exceedance probabilities; (ii) that the exceedance probability of an economically efficient setback line will typically be in the order of magnitude of 1/100 per year; (iii) that it is important to distinguish between situations in which morphological conditions are stationary and non-stationary; and (iv) that long-term uncertainties (eg. due to climate change) influence the exceedance probability of efficient setback lines but only to a limited extent. The economic model stresses the need for a probabilistic approach to beach erosion modelling. The recently-developed Probabilistic Coastal Setback Line was applied at Narrabeen beach, Sydney, Australia, to illustrate how economically optimal setback lines can be derived for specific sites.

Probabilistic modeling of wave climate and predicting dune erosion

ABSTRACT Li, F., van Gelder, P.H.A.J.M., Callaghan, D.P., Jongejan, R.B., den Hijer, C. and Ranasinghe, R., 2013. Probabilistic modelling of wave climate and predicting dune erosion considering sea level rise Knowledge about future oceanographic events will assist governments to better manage risk in coastal zones, a crucial task in the light of projected sea level rise, population growth and economic development. In this study, a 31-year data set of deep water wave climate parameters and bathymetry measurements (yearly cross-shore transect surveys) at Noordwijk, the Netherlands, were analyzed (1) to jointly estimate storm events variates of deep water wave conditions, and (2) to probabilistically compute dune erosion volume and the resulting coastal retreat distance with the simulated wave climate and plausible local sea level rise scenarios by 2100. The probabilistic coastline retreat models were applied and adjusted to the study site. Based on the outcomes of this application, a modeling technique can be established to propose a framework for probabilistically describing the coastal risk along the Dutch coast.

A comment on “Changing estuaries, changing views”

The provision of flood safety is of paramount importance in densely populated deltaic regions. The Dutch rely on flood defences to protect their lives and livelihoods from large-scale floods. The paper “Changing estuaries, changing views” (Smits etxa0al., Hydrobiologica 565:339–355, 2006) criticizes this strategy and presents an alternative that could be summarized as a proposal to leave deltas untouched and to rely on natural sedimentation to reduce the impact of floods. It seems questionable, however, whether such a strategy will often be compatible with population pressures and efforts to stimulate economic growth. Moreover, it presupposes morphological conditions that seem highly unrealistic, not just in the Netherlands but also in many other sediment-starved coastal systems. Other than recommending countries not to implement the Dutch flood protection strategy and to leave deltas untouched, it should be recommended that solutions be tailored to local circumstances. The choice of a flood protection strategy should be based on a balanced evaluation of alternatives, including a realistic assessment of physical conditions.