The Use of Individual and Societal Risk Criteria Within ... faculteit/Afdelingen... · the Dutch...

Risk Analysis, Vol. 31, No. 2, 2011 DOI: 10.1111/j.1539-6924.2010.01502.x

The Use of Individual and Societal Risk Criteria Withinthe Dutch Flood Safety Policy—Nationwide Estimatesof Societal Risk and Policy Applications

Sebastiaan N. Jonkman,1,2,∗ Ruben Jongejan,1,3 and Bob Maaskant4

The Dutch government is in the process of revising its flood safety policy. The current safetystandards 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 currentstandards. This article presents the results of a research project that evaluated the potentialroles of two risk metrics, individual and societal risk, to support decision making about newflood safety standards. These risk metrics are already used in the Dutch major hazards policyfor the evaluation of risks to the public. Individual risk concerns the annual probability ofdeath 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 assessmentsas well as policy implications are discussed. Preliminary estimates of nationwide levels ofsocietal risk are presented. Societal risk levels appear relatively high in the southwestern partof the country where densely populated dike rings are threatened by a combination of riverand coastal floods. It was found that cumulation, the simultaneous flooding of multiple dikerings 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 policyare presented and discussed.

KEY WORDS: Flood risk; individual risk; loss of life; quantitative risk analysis; risk evaluation; societalrisk

1. INTRODUCTION

Flood protection is of paramount importance tothe low-lying Netherlands. The Dutch governmentis currently in the process of updating its flood riskmanagement policy. The Water Act of 2009 (pre-viously the Flood Defence Act of 1996) lays down

1Delft University, Faculty of Civil Engineering and Geosciences,Delft, The Netherlands.

2Royal Haskoning, Coastal & Rivers Division, Rotterdam, TheNetherlands.

3Jongejan Risk Management Consulting, Delft, The Netherlands.4HKV Consultants, Lelystad, The Netherlands.∗Address correspondence to Sebastiaan Jonkman, Delft Univer-

sity, Faculty if Civil Engineering and Geosciences, Stevinweg1, Delft, 2628 CN, Delft, The Netherlands; tel: 31615943962;[email protected].

the exceedance probabilities of the hydraulic load-ing conditions that the primary flood defenses shouldbe able to safely withstand. The policymakers in-creasingly voice the need for an integrated floodsafety policy, in which flood probabilities (other thanexceedance probabilities of loading conditions) andflood consequences are mitigated in conjunction.EU Directive 2007/60/EC, the report of the SecondDutch Delta Committee,5(1) and the recently pub-lished Dutch national water plan(2) all stress the needfor evaluating flood probabilities and consequencesin an integrated manner in a risk-based policy. In the

5The second Delta Committee advised Parliament and the Execu-tive about the long-term prospects for protecting the Netherlandsagainst floods.

282 0272-4332/11/0100-0282$22.00/1 C© 2010 Society for Risk Analysis

The Use of Individual and Societal Risk Criteria Within the Dutch Flood Safety Policy 283

year 2011 new flood safety standards will be defined,based on risk assessment.

Current flood safety standards are largely basedon the outcomes of cost-benefit analyses. In suchanalyses, the sum of the discounted investments inflood defense and the discounted expected value offuture losses is minimized to determine an optimallevel of protection.(3) Various intangible losses, in-cluding loss of life, are valued in monetary terms andincluded in the financial balance. To inform decisionmaking about new flood safety standards, a new cost-benefit analysis has been commissioned by the Dutchgovernment.(4) This focus on the economics of floodsafety seems understandable given the costs of floodrisk mitigation and the enormous economic impactof large-scale floods. On August 29, 2005, HurricaneKatrina struck the U.S. Gulf Coast. The levee systemprotecting New Orleans proved no match for the en-suing storm surge and large parts of the low-lying citywere flooded. With damages totaling U.S.$ 138 bil-lion Katrina is the costliest natural disaster to date.(5)

Floods on the scale of New Orleans are not unthink-able in the Netherlands, a country with broadly sim-ilar topographical characteristics as the Mississippidelta.(6)

Apart from economic losses, floods can cause se-vere societal disruption and loss of life: over 1,100people lost their lives due to Hurricane Katrina. TheDutch government has indicated that it will explic-itly and separately consider potential loss of life whendeciding on the stringency of new flood safety stan-dards.(2) In another policy domain, concerned withthe safety of those living in the vicinity of major in-dustrial hazards, loss of life is already explicitly takeninto account in the evaluation of risks to the pub-lic.(7) Two risk metrics are used: individual and so-cietal risk. Individual risk refers to the probability ofan individual at a certain location getting killed by anaccident, while societal risk concerns the probabilityof an accident with a large number of fatalities. Tosupport the formulation of a new flood safety policy,the Dutch government commissioned a study aboutthe opportunities for transferring the approach usedin the Dutch major hazards policy to the domain offlood safety.(8)

The emphasis on risk in the formulation of a newflood safety policy has led to a demand for quantita-tive estimates of the risks to life in all flood-proneareas in the Netherlands, the so-called dike rings.Ideally, such estimates would be based on detailedrisk assessments. Unfortunately, detailed risk esti-mates are only available for a limited number ofareas, see, for example, Reference 9 for an elabo-

ration for dike ring area South Holland. Detailedrisk analyses are currently being carried out for allflood-prone areas in the Netherlands in the floodrisk and safety (FLORIS) project, a study commis-sioned by the Dutch Ministry of Transport, PublicWorks and Water Management in cooperation withwater boards and provinces. The complete results ofthe FLORIS project will not be available until afterthe year 2011. Hence, well-founded risk estimates forall flood-prone areas in the Netherlands are unavail-able at this stage. In order to give a first and prelim-inary indication of the level of flood (fatality) riskin the Netherlands, it was therefore decided to usean approach that combines information from a lim-ited number of detailed risk analyses with other datasources and expert judgment.(10) Estimates of soci-etal risk have been presented for all dike rings in thecountry. As levels of individual risk are highly de-pendent on local flood conditions and topography,it proved troublesome to estimate individual risksthroughout low-lying regions without detailed infor-mation from flood scenario calculations. As such in-formation was unavailable, no nationwide estimatesare presented for individual risk.

The remainder of this article is organized asfollows. Section 2 provides background informationon the individual risk and societal risk metrics usedwithin the Dutch major hazards policy. The secondpart provides insight into methods for the quantifi-cation of individual and societal risks in the domainof flood safety and presents the results of nationwideestimates of societal risk (Section 3). The effects ofseveral measures on the risk levels are evaluated inSection 4. The final part (Section 5) then discussesinstitutional consequences of alternative uses of indi-vidual and societal risk criteria within a flood safetypolicy. Concluding remarks for the whole study arediscussed in Section 6.

2. RISK METRICS USED WITHIN THEDUTCH MAJOR HAZARDS POLICY

The Dutch major hazards policy deals with therisks to those living in the vicinity of major indus-trial hazards such as chemical plants and LPG-fuelingstations. The development of the Dutch major haz-ards policy was strongly incident driven, as wereEuropean efforts aimed at the prevention of majorindustrial accidents.(11) After a number of severe in-dustrial accidents, including the Bhopal accident in1984, which killed an estimated 3,000 people andseverely injured over 200,000,(12) a European di-rective was drafted concerning the prevention of

284 Jonkman, Jongejan, and Maaskant

major accidents: the 1982 Seveso Directive. Thiswas later replaced by the Seveso II Directive.(13) Itcovers a wide range of topics, ranging from plantsafety requirements to inspection and land-use plan-ning provisions. Because member states have hadconsiderable freedom in implementing the direc-tive, various types of major hazards policies can befound throughout the European Union. These canbe grouped into two broad categories:(14,15) effect-based approaches (e.g., Germany, France before theToulouse accident) and risk-based approaches (e.g.,the United Kingdom, France after the Toulouse ac-cident). While effect-based approaches use referencescenarios for evaluating risk acceptability, risk-basedapproaches consider a wide range of accident scenar-ios together with their probabilities. Under the latterapproach, accident probabilities are an integral andexplicit part of decisions on the acceptability of risks.The Dutch major hazards policy has remained firmlyrisk-based ever since its foundations were laid by theannex Premises for Risk Management in 1999.(16)

The cornerstones of the Dutch major hazardspolicy are (1) quantitative risk analysis, (2) individualand societal risk as risk-determining parameters, and(3) quantitative acceptability criteria for evaluatinglevels of individual and societal risk.(7,17,18) Individ-ual risk is defined as the probability of death of an av-erage, unprotected person who is constantly presentat a given location.

Individual risk criteria are reference levels forevaluating individual risks. The individual risk crite-ria were given a legal status in 2004 by the ExternalSafety Decree. These limits to individual risk pre-vent disproportional individual exposures. Permits

for property developments or plant modifications aredenied if vulnerable objects would then be locatedwithin the 10−6 contour (Fig. 1).

An individual risk criterion alone cannotprevent the too frequent occurrence of multifatalityaccidents. As shown in Fig. 1, the area affected byan accident can differ considerably from the areathat is defined by an iso-(individual) risk contour.When individual exposures are low, there couldstill be a chance that a single accident kills a largenumber of people. While a vast number of smallaccidents can go by largely unnoticed, multifatalityaccidents can shock a nation. Psychometric studieshave indeed shown that “dread,” or catastrophicpotential, is an important factor in explaining riskperceptions.(19) To prevent the too frequent occur-rence of large-scale accidents, societal risk criteriawere implemented in the Netherlands. Societal riskrefers to the probability of an accident with multiplefatalities. In general, it is graphically represented byan FN-curve that shows the exceedance probabilitiesof the potential numbers of fatalities (P (N ≥ n)) ondouble log scale (Fig. 2).

The Dutch societal risk criterion of 10−3/n2 perinstallation per year was initially developed for LPG-fueling stations. It was later applied to all Seveso es-tablishments. Similar societal risk criteria thus ap-ply to hazardous establishments of different charac-ter and size despite considerable differences betweenthe marginal costs of risk reduction in different cases(see Ref. 20 for further discussion).

Because the outcomes of quantitative risk anal-yses for low-probability industrial accidents aresubjected to considerable uncertainty, the position

Fig. 1. Individual risk contours around a hazardous establishment and the area affected by an individual accident scenario.


Fig. 2. The Dutch societal risk criterion for hazardous establish-ments and a fictitious FN-curve.

of risk contours varies by orders of magnitude,depending on engineering judgment and modelused.(21) Since the outcomes of quantitative risk anal-ysis (QRA) play an important role in the Dutch ma-jor hazards policy, such variability is troublesomefor licensing purposes (land-use permits and operat-ing permits). It was therefore decided to prescribethe use of a single risk model with little scope forinterpretation on the part of risk analysts.(22) This,however, limits the opportunities for tailor-made riskanalyses based on site-specific information.

3. NATIONWIDE ESTIMATES OFSOCIETAL RISK

3.1. Background: Flood Risk Assessment andRisk Metrics

Methods for the analysis of flood risk gener-ally include three main steps: (1) determination ofthe probability of flooding; (2) simulation of floodcharacteristics; and (3) assessment of the conse-quences.(23) The implementation of these steps inflood risk assessment studies in the Netherlands hasbeen described in detail in References 9, 24, and 25.Such methods for flood risk generally include the fol-lowing steps: (1) determination of the probability offlooding; (2) simulation of flood characteristics; and(3) assessment of the consequences.(9) Based on theresults of the risk analysis, different risk metrics canbe calculated, such as the expected number of fatal-ities or the expected economic damage. The risks tolife from flooding can also be quantified with the met-rics that are also used in the major hazards policy.Following the definition that is used in the Dutch ma-

jor hazards policy (see Section 2), IR is determinedfor a person who is constantly present at a given lo-cation. The calculated IR is a property of a certainlocation and it can be used for risk-based land-useplanning. Disasters that fall within the scope of themajor hazards policy, for example, explosions, toxicreleases, and airplane crashes, are generally charac-terized by very limited possibilities for evacuation. Itis therefore a reasonable assumption to ignore the ef-fectiveness of evacuation in these risk analyses. Thepossibilities for evacuation could be significant whenit comes to floods. It could thus be relevant to includethe effects of evacuation in the quantification of IRfor floods. An alternative definition of individual riskcould be used that refers to the probability of deathof a person at a location including the effectivenessof evacuation. In addition, the societal risk level forflooding can be quantified based on probability andconsequence estimates for a set of flood scenarios.The results can be shown by means of an FN-curve.

3.2. Nationwide Estimates of SocietalRisk: Approach

The results of detailed flood risk analyses are ex-pected after the year 2011, but policymakers wantedto gain insight into the severity of fatality risks wellbefore that date in the context of policy develop-ment. A study was therefore commissioned to esti-mate nationwide levels of societal risks.(10) A simpli-fied procedure was therefore designed to estimate so-cietal risks from floods on the basis of available data.It rests on the following assumptions:

1. In case of flooding, at least one person iskilled. The probability of flood thereforeequals the probability of at least one fatality.

2. The conditional distribution of the numberof fatalities is exponential. This implies thatlarger numbers of fatalities are increasinglyless likely in case of flood.

The first assumption seems relatively uncontro-versial as failures of primary flood defenses are low-probability, high-impact events. The second assump-tion is based on the observation that the three avail-able FN-curves that were derived from detailed riskestimates (9,26) match this assumption relatively well.Fig. 3 shows the comparison for the calculated FN-curve for one dike ring area (South Holland) withthe exponential approximation. The difference be-tween the expected value of the number of fatalitiescalculated on the basis of the actual FN-curve and


Fig. 3. Comparison between the calculated FN-curve for South Holland (black line) with the approximation by means of the exponentialdistribution (gray line).

the exponential approximation was negligible. Thestandard deviations differed by only 2.6%. Compar-ison of the approximation with the calculated FN-curves for two other areas presents a broadly similarpicture. Although the second assumption is obviouslycoarse, an assumption regarding the shape of FN-curves is unavoidable if we are to estimate FN-curvesfor all dike rings on the basis of currently availabledata.

The FN-curve with the exponential distributionis given by:

P(N ≥ n) = Pf · e− (n−ξ)σ for n ≥ ξ, (1)

where P(N ≥ n) = probability of n or more fatalities[per year]; Pf = probability of flood [per year]; ξ =threshold value that determines the starting point ofthe exponential distribution; and σ = standard devi-ation of the number of fatalities given flood [−].

The threshold value ξ equals the minimum num-ber of fatalities in case of flood. It has been assumedthat at least one person is killed in case of flood. Sincethe number of fatalities is an integer, both ξ = 0 and

ξ = 1 would match this assumption. Here, a value ofξ = 0 was chosen. In that case, the expected num-ber of fatalities given a flood equals the standard de-viation, i.e., μ = σ . Estimating FN-curves now re-quires only two figures per dike ring: an estimate ofthe probability of flood, and an estimate of the aver-age number of fatalities in a flood. The proceduresfor these estimates are presented in the next section.

Given the approach used, the outputs are rela-tively uncertain, since no detailed risk estimates areavailable to validate the outcomes for all dike rings.It is expected, however, that the approach will pro-vide useful insight into the levels of societal risk. Theapproach will be most accurate at estimating relativedifferences in societal risk levels between dike ringsas the relevant factors (strength of the flood defensessystem, number of inhabitants, potential for evacu-ation, and mortality) are taken into account in theestimates. Moreover, the procedures for estimatingthe failure probability and expected consequencesgiven flooding have been thoroughly reviewed as partof several earlier nationwide policy studies on floodrisks in the Netherlands.(27,28)


3.3. Input Information: Flood Probabilitiesand Fatalities

3.3.1. Flood Probability

For estimates of the flood probabilities, infor-mation was used from a large ongoing governmentproject, Water Safety in the 21st Century.(4) In thatstudy, flood probabilities were determined for alldike rings in the Netherlands. It was assumed that theflood defenses comply with the current safety stan-dards (based on probabilities of exceedance) and thatthis situation corresponds to the situation in the year2015. The resulting flooding probabilities per dikering are shown in Fig. 4. The probabilities are some-what lower than the existing safety standards as itwas assumed that the actual critical overtopping dis-charge that the dikes are able to withstand (5 l/s/m)is somewhat higher than the current design criterionfor overtopping discharge (1 l/s/m).(29)

3.3.2. Number of Fatalities by Dike Ring

The number of fatalities can be calculated ac-cording to:

N = NPAR FEXP(1 − FE)FD, (2)

where N = number of fatalities [−]; NPAR = num-ber of people in the dike ring [−]; FEXP = fractionof the total population in the flooded area [−]; FE =fraction of people evacuated [−]; FD = mortality, i.e.,the percentage of people killed by the flood [−].

In order to estimate the expected number of fa-talities given flooding, average values for these pa-rameters were estimated on the basis of availabledata and expert judgment. The number of people inthe dike ring (NPAR) approximately equals the num-ber of inhabitants. This quantity has been obtainedfrom national population data sets. Due to varia-tions in elevation and limitations in inflowing water

Fig. 4. Estimated flooding probabilities(the numbers in the areas refer to thenumbers that are included to indicatedike rings in the Flood Defence Act).


Fig. 5. Expected number of fatalitiesgiven flooding by dike ring.

a flood event will generally not flood the whole dikering area and thus not affect the entire population.The fraction of the population that is in the floodedparts of the dike ring (FEXP) is estimated based onavailable information on flood scenarios, elevation,and population.(27) The fraction of the people evac-uated has been determined from a nationwide studyon evacuation fractions per dike ring(30) in which traf-fic models have been applied to estimate the timerequired for evacuation. For the estimation on themortality (FD), flood events were grouped into threebroad categories. These are defined by the severity ofthe flood, water depths, and/or flow velocities, and towhat extent the flood is unexpected, with little or nowarning. The three categories are:

1. Dike ring with severe flooding (large wa-ter depths, high flow velocities, unexpected):mortality equals 1%, so FD = 0.01. This is theestimate used for dike rings along the coast.

2. Dike rings with some warning time but still ex-tensive flooding: mortality is assumed 0.7%;dike rings that are both influenced by sea andriver.

3. Dike rings with long warning time and tosome extent expected flood: mortality 0.5%;dike rings along rivers.

These estimates on event mortality have beenbased on available detailed risk studies for individ-ual areas.(26) Combining the above variables accord-ing to Equation (2) yields estimates for the expectednumber of fatalities given flooding. The results for alldike rings are shown in Fig. 5.

3.3.3. Results of Nationwide Estimates ofSocietal Risk

Based on the method and input data presentedin Sections 3.2 and 3.3, the FN-curves were estimated


Fig. 6. FN-curves for individual dike rings and the total FN-curve for flooding for the Netherlands.

for the dike rings in the Netherlands. The results areshown in Fig. 6. Each curve represents the FN-curvefor one dike ring and the number refers to the dikering numbers included in the Flood Defence Act (seeFigs. 4 or 5).

The national level of societal risk is shown as abold line and has been estimated by vertically addingthe FN-curves of the individual dike rings. This im-plies (as a first approximation) that floods in differ-ent dike rings are mutually exclusive events. This as-sumption is further discussed in Section 3.5.

As part of the investigation, the societal risk lev-els of different dike rings were analyzed and com-pared. For decision-making purposes it is relevant toprovide insight into the areas where societal risks arehighest and the areas that contribute strongly to thenational level of societal risk. Two dimensions, thatis, exceedance probabilities and consequences, deter-mine the level of societal risk of a dike ring. For rea-sons of presentation and communication, the level ofsocietal risk for each dike ring was characterized bymeans of a single value: the point where a tangentline with the calculated FN-line crosses the y-axis of

Fig. 7. Determination of the tangent line and constant C for a cal-culated FN-curve.

the FN-curve (see Fig. 7). The tangent line has thefollowing general formulation:

P(N ≥ n) = C/nα, (3)

where P(N ≥ n) = probability of exceedance of n fa-talities [per year]; C = constant that determines thepoint where the tangent line crosses the y-axis of theFN-curve [per year]; and α = steepness of the tangentline.


Fig. 8. C-values for dike rings in theNetherlands (indicating the relative levelof societal risk).

It should be noted that this formulation is ex-actly the same as the typical formulation of an FN-limit line(9)—see also Section 5. The tangent line thatis found for a certain area thus equals the limit linefor which the situation would just be acceptable. Thevalue of constant C thus represents the vertical posi-tion of the tangent line. A quadratic steepness of thetangent line (α = 2) was chosen because the sameslope is used within the Dutch major hazards policy.This is a risk-averse criterion and it is expected thatthis value will be used in further discussions aboutflood safety.

C-values were determined for the Dutch dikerings; see Fig. 8 for the results. For most of thedike rings, the events with fatalities in the range be-tween 100–10,000 fatalities determined the C-values.Table I provides more detailed information for theeight dike rings with the highest C-values.

From Fig. 8, it can be seen that dike rings in thesouthwestern part of the Netherlands have relatively

high levels of societal risk. This is due to the fact thatthese dike rings are densely populated, and that thereis limited time for evacuation as the coastal floodsthat threaten these areas cannot be predicted well inadvance. As part of the investigation, the relationshipbetween the C-values and the probability and conse-quence estimates was analyzed.

This shows that there is a weak relationship be-tween the C-value and the flooding probability esti-mates. A much stronger relationship exists betweenthe C-value and the consequence estimates. This isnot surprising as (1) the differences between theprobability estimates for most of the dike rings arerelatively small; and (2) the consequences have aquadratic influence due to the choice for a quadratictangent line. Further investigation did not reveal astrong, persistent relationship between the C-valueand the underlying factors that influence the conse-quences of floods, such as the number of inhabitants,the size of the area at risk, or the population density.


Table I. Overview of Dike Rings with the Highest Societal RiskLevels and C-values

ExpectedNumber

Flooding of C-Nr. Name Probability Fatalities Value

17 IJsselmonde 2.5 × 10−4 1,341 15014 Zuid-Holland 1.0 × 10−4 1,566 10322 Eiland van

Dordrecht5.0 × 10−4 393 39

34 West-Brabant 3.57 × 10−4 452 3920 Voorne-Putten 1.92 × 10−4 600 3616 Alblasserwaard &

Vijfheerenlanden5.0 × 10−4 332 28

21 Hoeksche Waard 5.0 × 10−4 332 2815 Lopiker en

Krimpenerwaard5.0 × 10−4 328 27

The estimates of the number of fatalities depend ondike ring specific combinations of these factors.

A sensitivity analysis was carried out using a dif-ferent steepness of the tangent line (α = 1). Thissteepness corresponds to an FN-criterion that placesequal weight on exceedance probabilities and num-bers of fatalities. This type of FN-criterion is oftenindicated as risk-neutral. With α = 1, the group ofthe eight dike rings with the highest C-values remainsthe same, although the ranking within this group be-comes somewhat different.

3.3.4. Societal Risk and Cumulation

In the determination of the national level of so-cietal risk, the possibility that floods occur simultane-ously in different dike rings should be taken into ac-count. Experience from historical floods, both in theNetherlands and abroad, shows that multiple dikerings are likely to flood simultaneously under ex-

treme conditions. For example, in the big flood of1953, approximately 140 breaches occurred at ap-proximately 10 of the (current) dike ring areas. Inthe insurance industry, the effect of correlations be-tween individual loss experiences on the distributionof overall losses is called cumulation. It is relevant forthe estimation and evaluation of societal risks. Dueto the simultaneous flooding of multiple dike rings,the consequences of a single flood event will increase.

Fig. 9 shows the difference between the FN-curves for a simple system of two dike rings fora situation without cumulation (flooding of the ar-eas is mutually exclusive—left) and with cumulation(flooding of dike rings is fully dependent—right).Each dike ring has flooding probability P and con-sequences N. In case of perfect correlation, the con-sequences will be larger than in the base case wherefailures are assumed to be mutually exclusive. Notethat the probability of a flood is lower when failuresare perfectly correlated.

A full-scale analysis of cumulation would requirea complex probabilistic model that takes into ac-count the development of flood waves in a system,as well as the dependencies and interactions betweenfailures of dike rings.(31) As such a model was un-available at the time, the following simplified ap-proach was used to produce conservative estimatesof the effect of cumulation. The dike rings in theNetherlands are located in different hydrological sys-tems. Dike rings in the coastal zone can be floodedby storm surges, whereas the dike rings above thetidal limit are threatened only by rivers. The latterarea is referred to as the “bovenrivierengebied.” Thedike rings below the tidal limit are threatened byboth river and coastal floods and this area is indi-cated as the “benedenrivierengebied.” In total, thereare five hydrological subsystems in the Netherlands.Fig. 10 shows these different subsystems and the dikerings located within each subsystem. The effect of

Fig. 9. The effect of cumulation on theFN-curve.


Fig. 10. Dike rings in the fivehydrological subsystems in theNetherlands.

cumulation was estimated by assuming that all dikerings within a subsystem will flood simultaneously. Inthat case, the consequences at the national level areequal to the sum of the consequences for the individ-ual dike rings. This implies that the FN-curves for theindividual dike rings should be added horizontally.This is a somewhat conservative approach, and fur-ther studies are currently being carried out to refinethe estimates presented in this article.

Fig. 11 shows the FN-curves for each subsystem,and the total FN-curve that would be found by (ver-tically) summing the risks for the subsystems (boldline— thus assuming that flooding of subsystems aremutually exclusive). The analysis shows that the na-tional total for larger numbers of fatalities (morethan 500) is largely determined by the areas belowthe tidal limit (the “benedenrivierengebied”) that arethreatened by both coastal and river floods. The re-sulting national societal risk line (bold) is comparedwith the national societal risk line that would result

if cumulation would not be taken into account (thedashed gray line). It is clear that the curve with cu-mulation shows higher consequences. Especially if arisk-averse evaluation criterion is used, such as anFN-limit line with a quadratic steepness, cumulationcould significantly influence risk appraisal.

4. THE EFFECTIVENESS OF STRATEGIESFOR MITIGATING FLOOD RISKS

As part of the discussion on the role of individualand societal risk in the new flood safety policy it wasalso desired to have insight into the effects of sev-eral interventions on the level of risk. Broadly speak-ing, there are three types of strategies to reduce floodrisks:

1. Reducing flood probabilities. Measures in-clude dike strengthening, beach nourishment,


Fig. 11. FN-curve for the five subsystems from Fig. 10 (including cumulation within subsystems) and at a national scale (assuming floods indifferent subsystems are mutually exclusive events).

and widening of rivers to increase their runoffcapacity.

2. Reducing the consequences of floods. Mea-sures include flood proofing of buildings andinfrastructure, improving the opportunitiesfor evacuation (early warning, constructingshelters, etc.), safety zoning, and splitting dikerings into smaller compartments.

3. A combination of the above.

FN-curves and individual risk maps are usefultools for demonstrating the effects of a wide varietyof risk reduction measures on fatality risks. They canthereby be used to facilitate the evaluation of alter-native flood risk management strategies.

The effects of several interventions have beenanalyzed for one flood-prone area for which detailedrisk estimates are available. The estimates of the in-dividual and societal risk for this area, South Hol-land, are presented in Reference 9. These estimateswere used to illustrate the effects of several measureson fatality risks. The effects of interventions werequantified on the basis of available studies and ex-

pert judgment. These are rough assumptions and fur-ther quantitative (risk) studies are recommended toprovide more detailed insight into the effectivenessof risk reduction measures.

Fig. 12 shows the FN-curve in the referencesituation for the dike ring South Holland in theNetherlands(9) and the effects of two risk reduc-tion strategies. Strategy one concerns the reductionof the flooding probability by a factor of 10 dueto dike strengthening. For example, for riverine ar-eas this could be achieved by heightening dikes byabout 1 m and by widening the dikes. Because thefailure probability of every dike section is assumedto be lowered by the same factor, the FN-curveshifts downward without changing shape. The sec-ond strategy concerns the construction of compart-ment dikes within a dike ring. In that case the conse-quences can be significantly reduced, as the exposedarea and population are limited. However, there arecases in which compartmentalizing dike rings canalso raise fatality risks, as rise rates and water depthsin smaller compartments exceed those in larger com-partments. In this case it has been assumed that the


Fig. 12. The impact of the strategies (1) dike strengthening; (2) compartmentalization; and (3) improved evacuation on the FN-curve forSouth Holland (the numbers in the FN-curve indicate the strategies).

compartmentalization is highly effective and reducesthe loss of life by 50%. Reducing the consequencesof floods will cause the FN-curve to shift to the left.

The third strategy concerns the improvement ofthe opportunities for evacuation. The effectivenessof such a strategy is shown in Fig. 12. FN-curvesare depicted for two situations: the reference situa-tion and a situation in which the probability of suc-cessful evacuation and the evacuation rate are rela-tively high. Every evacuation rate has been assigneda probability by means of an event tree (see Ref-erence 9 for details). As shown by Fig. 12, the tailof the FN-curve cannot be avoided by efforts to im-prove early warning or disaster preparedness. This isbecause the probability of a failed evacuation staysnonzero.

As shown by Fig. 12, FN-curves provide a usefulbasis for comparing the effectiveness of alternativeflood risk management strategies. Cost-effectivenesscan be evaluated by relating the costs of measures tothe reduction of the risk to life. The reduction of theflooding probability will push the FN-curve down-ward. Such an intervention will be especially attrac-tive when a limited number of weak links in the flood

defense system can be strengthened at relatively lowcost. Through compartmentalization, the number offlood victims in the Netherlands could realisticallybe reduced by up to 50% (the effectiveness of com-partmentalization could be lower depending on localcircumstances; note that compartmentalization canalso increase fatality risks). This strategy would gen-erally require considerable investments in new com-partment dikes within flood prone areas. Compart-mentalization becomes less cost effective when theprobability of failure of the outer ring (the primaryflood defense) becomes smaller. This is because thecompartmentalizing flood defense only pays off whenthe outer ring fails. Improving the opportunities forevacuation and emergency management can reduceloss of life but the probability of events with many fa-talities cannot be reduced to zero as evacuation andemergency response might fail.

Similar exercises can illustrate the effect of al-ternative risk management strategies on individualrisks. Fig. 13 (left) shows the level of individual risk inthe base case. In total, about 1.9% of the flooded area(0.7% of the total dike ring area) is characterized byrisk levels higher than 10−6 per year. However, the


Fig. 13. Individual risk for South Holland (base case—left, from Ref. (9) and the effects of reduction of the flooding probability by a factorof 10 (right).

levels of individual risk for most of the other partsof the area are lower because (1) the probability offlooding is relatively low, (2) flood mortality is typi-cally substantially less than one, and (3) not all floodscenarios affect similar parts of South Holland. Re-duction of the probability by a factor of 10 leads toa reduction of individual risk; see Fig. 13 (right). Al-most all the areas that had risk levels higher than 10−6

per year are now eliminated. The IR-level remainshigher than 10−6 per year in 0.1% of the flooded area.

In these IR-plots, the effectiveness of evacuationis not taken into account. It is possible to create anindividual risk map for South Holland that takes intoaccount the effectiveness of evacuation. However,the maximum evacuation rates that can be achievedfor this area are generally limited to 20% to 30%of the population. This is due to the large amountof time required for evacuation and the limited timeavailable.(32) In addition, the individual risk map usesa logarithmic scale so that the differences betweenindividual risk maps without and with evacuation arevery limited.

Although the effects of measures will depend onlocal characteristics of the dike ring and the floodhazard, it is expected that risk reduction by means ofdike strengthening and compartmentalization will beof a similar order of magnitude for other areas. The

effectiveness of evacuation will differ more betweenregions. For areas threatened by river floods, it is ex-pected that evacuation will be more effective thanfor a coastal area. The reason is that river floods canbe generally predicted longer in advance than coastalfloods.

5. POTENTIAL USES OF INDIVIDUAL ANDSOCIETAL RISK CRITERIA IN THEDUTCH FLOOD SAFETY POLICY

5.1. Evaluating Individual and Societal Risks

As also discussed in previous sections, loss of lifewill play a role in the design of new safety standardsfor flood defenses in the Netherlands. Loss of life canbe evaluated from two distinct perspectives:(33−35)

1. The individual perspective: the safety of a par-ticular individual.

2. The societal perspective: the probabilities oflarge numbers of fatalities.

A limit to individual risk can be used to guaran-tee a minimal safety level to every individual livingbehind a primary flood defense. The introduction ofsuch a basic safety level has also been proposed bythe Second Delta Committee.(1) The proposed indi-vidual risk limit equaled 10−6 per year (including the


Fig. 14. A fictitious FN-curve and different FN-criteria.

effectiveness of evacuation) for the provision of floodsafety to those living behind primary flood defenses.The proposed value is similar to the individual risklimit used in the Dutch major hazards policy (withthe exception that evacuation is not considered inthis field). Further research is needed to see whethersuch a stringent safety standard would also be feasi-ble for the Dutch flood safety policy, given the mea-sures that would have to be taken.

FN-curves show the probability distribution ofthe number of fatalities and can be used for the eval-uation of fatality risks from a societal perspective. Tofacilitate the evaluation of FN-curves, criterion linescould be defined: an FN-curve should, in principle,not exceed the criterion line. An FN-criterion is de-fined by three variables: (1) its base point (the ex-ceedance probability of 1 fatality), (2) its slope, and(3) its probability and/or consequence cutoff. Fig. 14shows the different constraints that could make up anFN-criterion.

A theoretical link between expected utility the-ory and FN-criteria might be hard, if not impossi-ble, to establish(20,36) and the use of FN limit-lineshas been criticized on that ground.(37) Yet FN-criteriahave proven themselves in the Dutch major haz-ards policy as practical tools for the evaluation ofthe probabilities of large-scale accidents. The FN-criteria used in the Dutch major hazards policy havea quadratic steepness, meaning that the exceedanceprobability of 10 times as many fatalities should be100 times lower. This has been motivated by publicaversion to large numbers of fatalities. A recent studyon flood events found that individuals are more con-cerned about “low probability/high damage” floodevents than reflected by the annual expected value offlood damages.(38) It should, however, be noted that

FN-curves with different slopes are used in differentcounties.(39)

The stringency of societal risk criteria could,among others, be based on cost-benefit considera-tions on risk-risk comparison across domains.(34) Rel-ative levels of societal risk within one domain couldalso be evaluated by comparing levels of societal riskin different objects (e.g., dike ring areas) as a basisfor prioritization (see Section 3 for examples).

Societal risk can be quantified and evaluatedfor areas of different sizes, for example, for a city,a province, a dike ring, a hydrological subsystem,or an entire country. The boundaries of such areascould be aligned with institutional boundaries. Var-ious government actors are responsible for influenc-ing (mitigating or creating) flood risks, most notablythe national government, provinces, municipalities,and water boards. Each of these entities has its ownterritorial boundaries. An alternative would be toalign the boundaries of the areas for which societalrisk is evaluated with those of the natural system,such as the boundaries of hydrological systems (e.g.,Rhine, Meuse, coast) or dike rings (note that theboundary of a dike ring does not necessarily coincidewith the territorial boundary of a water board). It iscurrently being proposed to evaluate societal risk atthe dike ring level because: (1) current safety stan-dards are also defined per dike ring and the futuresafety standards will likely have the same format;(2)

(2) measures taken at a dike ring will mostly affectthe risk within that ring; and (3) current risk studiesare mainly focusing on separate dike rings. An alter-native could be to consider the risks within a hydro-logical subsystem (see Fig. 10). It seems reasonableto assume that multiple dike rings within one sub-system could flood during the same extreme event,but that it is unlikely that dike rings from differentsubsystems will be flooded simultaneously (see alsoSection 3.5). An assessment of the risks at subsystemlevel will allow for a proper evaluation of the effectsof cumulation. An assessment of the risks at the na-tional scale would provide insight into the threat ata national level. By combining information for thedifferent spatial scales, those areas and subsystemscould be identified that give the highest contributionto the overall national level of risk.

5.2. The Status Given to Individual and SocietalRisk Criteria

Individual and societal risk could play a numberof different roles in a flood safety policy, depending


partly on the status that is given to the yardsticks forjudging the acceptability (or tolerability) of individ-ual and societal risks.

Under the least binding alternative, individ-ual and societal risk are merely used for agenda-setting and/or policy evaluation purposes. Policy-makers consider (past or potential changes in) lev-els of individual and societal risk when making pol-icy choices. There is no formal rule stipulating a needfor action when some predefined level of individualor societal risk is exceeded.

A second option would be to define criteria orreference values for evaluating individual and/or so-cietal risks, but to allow exceedances when thereare strong reasons for doing so. Decision rules thatseem reasonable in some cases might lead to grosslydisproportionate outcomes in others. Allowing forflexibility can reduce the unintended social cost ofrules and regulations, but it can dramatically increasetransaction cost (the cost of decision making). In theDutch major hazards policy, there is no legal limitto societal risk, only a reference value. Exceedancesof this reference value (as well as increases in soci-etal risk below the reference value) have to be prop-erly motivated by competent authorities. The Exter-nal Safety Decree lays down the basic elements thathave to be considered by competent authorities, andjurisprudence has led to further refinement of thedefinition of a “properly motivated” decision.

Under the third and most stringent regime, thegovernment lays down legal limits to individual andsocietal risks. These limits would then have a similarstatus as the exceedance probabilities that are cur-rently laid down in the Flood Defence Act. Whenprevention would be the basis for the flood risk man-agement policy the government could also definemaximum flood probabilities, based on considera-tions related to individual and societal risks. For ex-ample, for each dike ring, the probability of floodwould have to be reduced until the level of societalrisk would be lower than the limit line (see also Fig.14). Limits to individual and/or societal risk wouldeffectively find their way into the legal limits to floodprobabilities.

It should be noted that the Dutch central gov-ernment is responsible for living up to the Water Act(previously: Flood Defence Act). In this respect, thefunctioning of the Water Act is rather different from,for example, the External Safety Decree (see Section2) that lays down the rules of the game that individ-uals, firms, and local governments have to play by. Iflocal governments fail to observe the rules laid down

in the External Safety Decree, their decisions can bechallenged in court. But in the domain of flood safety,the government plays the role of the provider of apublic good. Flood safety standards effectively definethe government’s (public’s) level of ambition whenit comes to the provision of flood protection. If cen-tral government fails to meet the standards laid downin the Flood Defence Act out of budgetary consider-ations, the government cannot be successfully chal-lenged in court. This is because budgetary decisionsare legitimized through the workings of the politicalsystem, the same system that legitimized the WaterAct. Experience has, however, shown that legally de-fined flood safety standards can be highly effective insetting government priorities, as they provide a clearbasis for the evaluation of the safety of flood defensesand make it virtually impossible for failures to meetpredefined safety standards to go by unnoticed.

5.3. Options for Using Individual and Societal Riskin the Dutch Flood Safety Policy

As outlined in the previous sections, individualand societal risk can play different roles in shapingand implementing a new flood safety policy in theNetherlands. Key policy choices concern:

1. The status of individual and/or societal risk:agenda-setting/policy design, reference val-ues, legal limits (see Section 5.2).

2. The stringency of individual and/or societalrisk criteria: when risk criteria are imple-mented (as either reference values or legallimits), how stringent should they be (Section5.1)?

3. The strategy for reducing flood risks: reduc-ing flood probabilities, reducing the conse-quences of floods, a combination of both (Sec-tion 4).

Based on the three strategies for reducingflood risks, three policy alternatives were defined(Table II). According to current policy docu-ments,(1,2) the focus will rest on limiting the risks tolife and the economy through prevention (the firststrategy: the reduction of flood probabilities). In ad-dition, the possibilities for reducing the consequencesof floods will be investigated through land-use plan-ning for existing vulnerable facilities and to guide thedevelopment of future land use.

The three strategies for reducing flood risks havedifferent institutional implications. If, for instance,the decision were made to mitigate risks through a


Table II. Overview of Options for a Risk-Based Flood Defense Policy

Strategy forReducing FloodRisks, byReducing

Characteristics andImplications Standard/Policy Instruments Measures Parties Involved

(1) Floodprobabilities

Consequences are given,increase of consequencescompensated by smallerflooding probability

Prevention, flood probabilityreduced (determined fromcost-benefit analysis andevaluation of fatality risks)

Dike strengtheningRoom for rivers

Flood defensemanagementauthorities

(2) Consequences Flood probability is given (e.g.,based on CBA), policyfocuses on reducing theconsequences

Thereby possible limitationsfor spatial planning andeconomic growth

Steering development ofconsequences, e.g., byzoning

Spatial planning, zoning,flood-proof construction,shelters and mounds,evacuation

Land-use planning& emergencymanagementauthoritiesGrowth of consequences could

be limited by requiring aso-called stand-still of risks(i.e., no increase of risksallowed)

(3) Probabilities andconsequences

Case-by-case evaluation ofmeasures to reduce theprobability or consequences,e.g., based on effectivenessand side effects. Can lead tocomplex and therefore timeconsuming and costlydecision-making procedures

Cost-benefit analysis,evaluation of fatality risks

Dike strengtheningRoom for rivers zo-ning, flood-proof

construction, sheltersand mounds, evacuation

Flood defensemanagement,land-useplanning, &emergencymanagementauthorities

combination of flood prevention and safety zoning,rules would have to be laid down to ensure that localgovernments do not allow or develop spatial plansthat lead to increases in potential damages (notethat the interests of an individual local governmentand the central government need not be perfectlyaligned).

Designing rules and regulations that steer the be-havior of local governments and property developersinto a direction deemed desirable by central govern-ment would not just require answers to questions ofa purely technical nature (e.g., how to ensure that thejoint behavior of individual entities does not lead toexcessive societal risks?), but also to questions of apolitical and institutional nature (e.g., how to dealwith conflicts of interest between local governmentsthat wish to minimize restrictions on spatial plans anda central government that wishes to minimize invest-ments in flood prevention?).

6. CONCLUDING REMARKS

The Dutch government is currently in the pro-cess of updating its flood risk management policy.One of the novelties being considered concerns arole for fatality risks in evaluating flood risks. Fatal-

ity risks already play an important role in the Dutchmajor hazards policy. Individual risk limits are usedthere to provide a minimal safety level to those liv-ing in the vicinity of major industrial hazards; soci-etal risk criteria are used to prevent the too frequentoccurrence of large-scale accidents. This article re-viewed the potential for using individual and societalrisk for flood risk management. Techniques are avail-able for quantifying these metrics when it comes tofloods(9) and it has been shown that individual andsocietal risk can be used for comparing the effective-ness of alternative risk mitigation strategies and forappraising flood risks.

Preliminary nationwide estimates of societalrisks from floods in the Netherlands indicate that thelevels of societal risk are expected to be relativelyhigh in the southwestern part of the country wheredensely populated dike rings are threatened by bothriver and coastal floods. These areas therefore de-serve specific attention when it comes to flood riskmitigation.

It was found that the so-called cumulation ofrisks could have important consequences for thenational level of societal risk. Cumulation meansthat different dike rings flood simultaneously dur-ing the same storm surge or high river discharge.


Conservative assumptions have been used in thisstudy to model potential cumulation within a hy-drological subsystem. Further research is needed todetermine the actual level of dependence betweenfloods in different areas.

From a purely technical standpoint, little standsin the way of using individual and societal risk forthe evaluation of flood risks. But evaluating risks anddesigning a flood safety policy are not purely techni-cal exercises. Key policy choices concern the statusof individual and/or societal risk criteria (or the deci-sion not to formulate criteria), the stringency of suchcriteria, and the chosen strategy for mitigating floodrisks.

Obviously, loss of life is only one of the vastnumbers of consequences that together make up thepersonal and social impact of a large-scale flood.As loss of life is often considered to be one of themost important consequence types, a separate eval-uation of fatality risks constitutes a significant im-provement over an approach that focuses on the eco-nomics of flood safety alone. The presented approachcould therefore also be relevant for other (Euro-pean) countries, as most flood risk assessments fo-cus on potential economic damage rather than lossof life.

It is emphasized that the results presented in thisarticle are preliminary and based on a combination ofavailable information from risk assessments and ex-pert judgment. Further detailed risk studies are nec-essary to come to a more detailed and accurate un-derstanding of the level of fatality risks from floodsthroughout the Netherlands. The outcomes of theFLORIS project are expected after the year 2011.These outcomes will include estimates of the spatialdistribution of individual risks.

The outcomes of individual and societal risk es-timates provide an important input for the discussionon acceptable safety levels. As part of this discus-sion, it could be further investigated if and how stan-dards for individual and societal risk levels could beused. Calculated risk levels can be compared with ex-isting frameworks for deciding on risk acceptability,such as Reference 33. As a basis for decision making,further studies are needed that provide insight intothe effectiveness and costs of different risk reductionstrategies.

ACKNOWLEDGMENTS

The authors want to thank Prof. J. K. Vrijling(Delft University), Dr. Matthijs Kok (Delft Univer-

sity and HKV consultants), and Alex Roos (Rijkswa-terstaat) for their contributions to this research.

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