04 Flood Resilience InitiativesAugust 2019This publication was prepared by the AustralianTransport Assessment and Planning (ATAP) SteeringCommittee and approved by the Transport andInfrastructure Senior Officials’ Committee. TheCommittees cannot accept responsibility for anyconsequences arising from the use of this information.Readers should rely on their own skill and judgmentwhen applying any information or analysis to particularissues or circumstances.ISBN: 978-1-925843-15-6INFRA4022© Commonwealth of Australia 2019August 2019For enquiries or feedback about this publication pleasecontact:ATAP Steering Committee SecretariatAustralian Transport Assessment and PlanningGuidelinesCommonwealth Department of Infrastructure andRegional DevelopmentGPO Box 594 CANBERRA ACT 2601Email: atap@infrastructure.gov.auThis publication is available from the website atap.gov.au04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines iContentsFlood Resilience Initiatives…………………………………………………………………………………………………………11. Introduction………………………………………………………………………………………………………………………….22. Overview………………………………………………………………………………………………………………………………32.1 The ATAP assessment model………………………………………………………………………………………….32.2 Benefits of improving flood resilience………………………………………………………………………………..32.2.1 Monetised benefits………………………………………………………………………………………………32.2.2 Non-monetised or difficult to monetise benefits……………………………………………………….42.3 Uncertainty in estimating benefits …………………………………………………………………………………….43. Estimating monetised benefits ……………………………………………………………………………………………..63.1 Estimating user benefits………………………………………………………………………………………………….63.1.1 Waiting time for facility to re-open …………………………………………………………………………73.1.2 Trips diverting around a closed facility……………………………………………………………………83.1.3 Trips cancelled or postponed…………………………………………………………………………….. 103.1.4 Use of alternative modes of transport such as air transport …………………………………… 113.1.5 Accommodation costs………………………………………………………………………………………. 113.2 Injury or loss of life at flooded roads ……………………………………………………………………………… 123.3 Cost reductions to transport agencies……………………………………………………………………………. 124. Non-monetised or difficult to monetise benefits…………………………………………………………………. 155. Areas for future research…………………………………………………………………………………………………… 17References …………………………………………………………………………………………………………………………….. 18FiguresFigure 1 Diverting route ………………………………………………………………………………………………………………..9Figure 2 Annual repair cost by AEP ……………………………………………………………………………………………. 13TablesTable 1 Inputs to estimation of waiting time benefits (road example) ………………………………………………….8Table 2 Inputs to estimation of diverting trip benefits (heavy vehicles) …………………………………………….. 10Table 3 Repair costs by ARI and AEP …………………………………………………………………………………………. 13Table 4 Estimation of average annual repair costs ……………………………………………………………………….. 1404 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines ii04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 304 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 1Flood Resilience Initiatives At a glance This guidance covers the assessment of flood resilience initiatives. Flooding that causes transport facilities to be closed to traffic can give rise to a range of costs fortransport users, transport agencies and sometimes for the community more broadly. The principal costs to users consist of: waiting for facilities to be re-opened; diverting aroundclosures; use of alternatives transport modes; trip cancellation; and accommodation costs. For transport agencies, the principal cost is damage repair after flooding. This guidance describes approaches to estimating these costs in the base and project cases. Some impacts of facility closure are not readily monetised. Examples are discussed. 04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 21. IntroductionThis part of the Guidelines considers the appraisal of flood resilience initiatives in transport. Such initiativesare also referred to elsewhere by the terms flood immunity initiatives and flood mitigation initiatives, as theyrelate to transport.Its primary purpose is to illustrate how the ATAP assessment model can be applied to flood resiliencetransport initiatives. In doing so, it aims to assist practitioners required to undertake assessments of suchinitiatives.The impact on flooding is most frequently reported in relation to roads, and the associated road closures.However, flooding can also affect other transport mode (rail, airports, cycling and walking). It can also affectboth people transport and freight transport. The guidance here is therefore generic rather than being modespecific, or specific to traffic type. It can be applied in any modal, multi-modal or trip type situation. Thediscussion elaborates across various situations. The examples given here relate to roads. Examples acrossother modes can be added in future as relevant data is obtained.Flood resilience initiatives aim to reduce the impacts of flooding on transport facilities and services, transportusers and transport agencies. Flood resilience initiatives could include: Raising or replacing bridgesRaising pavements, tracks or pathsRe-building roads, tracks or paths on a new alignment. Sometimes, the same rain event can cause flooding at multiple sites along a corridor, route or link. Wherethis is the case, there will be interdependence between flood initiatives. Such interdependent initiatives mustbe appraised as a package of initiatives. Other than for localised trips, reducing flooding risk at any one sitewill produce benefits only if all sites on the link are suitably upgraded.Flood resilience initiatives can be difficult to appraise because of a lack of data describing how usersrespond to flooding-induced closures. Often appraisers will need to make assumptions about user behaviour.Those assumptions should be made clear in the appraisal report. Where flood resilience benefits are a largeproportion of total initiative benefits, sensitivity testing of alternative behavioural assumptions are desirable.Users of this part of the Guidelines should familiarise themselves with the following related ATAP Guidance: Part F3: Options generation and assessment, Chapter 3 Part T2: Cost–benefit analysis.Chapter 2 provides an overview, reminding the reader of the ATAP assessment model and its applicabilityhere, listing benefits of flood resilience initiatives (monetised and non-monetised) and discussing someuncertainties in benefit estimation.Chapter 3 discusses in more detail the various monetised benefits of flood resilience initiatives and illustratestheir calculation. It discusses a road immunity improvement as a specific example of a resilience initiative.Chapter 4 provides a discussion of some non-monetised or difficult to monetise benefits.Chapter 5 suggests some areas for future research.04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 32. Overview2.1 The ATAP assessment modelThe ATAP assessment model outlined in Section 3.3 in ATAP Part F3 provides the basis for assessing allinitiatives, including flood resilience initiatives. The model consists of: Clarification of relevant jurisdictional goals, transport system objectives and targets – It is important to beclear about which of these relate to flood resilience from early in an assessmentConsideration of strategic merit / alignment – The degree of strategic alignment of flood resilienceUse of cost–benefit (CBA) and the Appraisal Summary Table (AST). The AST provides the mechanismfor presenting all the appraisal results—monetised and non-monetised—in a single location The ATAP appraisal methodology recognises that all benefits and costs—monetised and nonmonetised—are relevant to the appraisal of initiatives. It facilitates this through use of the AST, in whichmonetised and non-monetised benefits and costs are presented side-by-sideThe AST also includes quantitative and qualitative impact descriptions—these are necessary inputs tocalculating monetised and non-monetised benefits, costs and impacts. Presentation of these inputs canalso be of assistance to the decision-maker. Non-monetised impacts that are non-quantifiable can onlybe described in qualitative terms Bringing together all aspects of the assessment into a Business Case (see ATAP Part F4).2.2 Benefits of improving flood resilienceThe benefits of improved flood resilience are classified according to cost reductions to: transport users;transport agencies; and external parties.2.2.1 Monetised benefitsThis paper largely addresses monetised benefits1 of improving flood resilience which include changes in: User costs of flooding:– Waiting time for facilities to re-open (includes dry-back period for heavy vehicles)– Use of diversion routes, including travel time and vehicle operating costs– The costs of trip postponement or cancellation– Use of alternative modes of transport such as air transport– Injury or loss of life of those attempting to traverse flooded infrastructure1 Flood-induced property damage costs could be affected (increased or decreased) by flood resilience initiatives from time to time, butchanges in property damages are more likely to arise from broader flood resilience initiatives such as levees and dams. This is a veryspecialised area of economic analysis that is outside the scope of these guidelines, but interested readers could refer to BTRE (2002)and NRM (2002).04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 4– Accommodation costs. User costs associated with ‘normal’ (dry weather) operation2 Agency costs:– Saturation of pavements, embankments, tracks and paths– Washaways of pavements, tracks, paths and tracks, including cleanup costs– Damage to bridges and culverts including embankments, including cleanup costs.As explained in ATAP Part T2: Costs are calculated separately for the base and project casesBenefits are equal to the incremental difference between base case costs and project case costs. 2.2.2 Non-monetised or difficult to monetise benefitsFacility closures caused by flooding can give rise to a range of costs that are quite difficult to estimate eitherbecause of the complexity of flood events or because the data sources and the range of impacts are diffusedacross potentially large areas of impact. Such costs, and hence benefits of flood resilience, can include: Loss of perishable goodsLoss of access to essential servicesChanges in emergency services costsExcess inventory costs. The above list is not exhaustive but provides an indication of the possible cost reductions likely to occur fromimproved flood resilience. See section 3 for further discussion.2.3 Uncertainty in estimating benefitsEstimation of the benefits of flood resilience upgrades is characterised by an abnormally high degree ofuncertainty for the following reasons: Users have inadequate information about the expected duration of closures due to flooding. What mightbe a reasonable course of action when viewed in retrospect – whether to postpone a trip, wait at a floodsite or take an alternative route – might not be obvious at the time because users lack information abouthow long a facility or service will be closed and whether alternative routes are likely to be closed as well2 Flood resilience initiatives may contain elements that affect user costs when the subject facility is not flooded. For example, a roadflood resilience initiative may be a new bridge with wider approaches and smoother surface than in the base case. These featurescreate benefits for users in normal (dry weather) conditions. For road cases, these benefits can be estimated using the parametervalues in ATAP PV2.04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 5 Typically analysts will only have link-based trip making data (that is, AADT, traffic composition, the lengthof the closed section and the length of available diverting routes). Complete accounting for the benefitsof avoiding trip postponement and trip diversion is only possible with trip origin-destination informationFlooding can be the consequence of weather events that affect whole networks simultaneously or as a rolling event. Examples include cyclones in northern Australia and east coast lows that can bring highlevels of rainfall along much of the eastern seaboard. During these weather events, users are morelikely not to travel because of the uncertainties about closures and other weather-related risks. On theother hand, users who are travelling at the time a flood event unfolds may be trapped on flooded facilitiesfor long periods of time Benefit estimation relies on hydrological analysis to determine the average annual time of closure. Thatinformation will not be available for all sites that are subject to flooding.Some hydrological analyses incorporate consideration of climate change, sometimes via scenario testing. Bearing in mind that the horizon for climate change will tend to be longer than the normal 30 yearanalysis period for initiatives, appraisers will need to judge the extent to which climate change scenarioswarrant consideration by reference to both the average annual time of closure (AATOC) and the annualduration of closure (ADC). If for example the climate change scenario is for AATOC to increase by 1hour to 25 hours, it might not be worth consideration. However a doubling (for example) of the ADC from4 hours to 8 hours due to increased severity of rain events with climate change would be worthconsidering depending on the probability that the hydrologist attaches to the climate change scenario.04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 63. Estimating monetised benefitsUsers whose route is closed by flooding are generally faced with three (safe) options: Wait at the flood site Divert via an alternative routePostpone their trip.The use of other available modes of transport – although this is less likely. For example, considerflooding road closures. For non-emergency trips, air transport is an option for those trips that have not commenced when the road closure occurs. Air transport will be option for emergency trips but becausemedical evacuations and drops of essentials (water, food and medical supplies) are rare relative tonormal road-based trip making volumes the diversion costs would generally not be worth estimating.Each of these options incurs different costs, which become benefits of a flood resilience initiative.3.1 Estimating user benefitsUser costs associated with flooding-induced closures will be lower when diverting routes of suitable qualityare available (e.g. suitable surface width, passenger type, strength, roughness, alignment) provided theadditional travel time on the diverting route is shorter than the average annual time of closure (AATOC).Major benefit categories typically estimated in appraisals are: Changes in costs associated with waiting at the flood siteChanges in the cost of diverting around the flooded location including travel time, vehicle operatingcosts, fares and any other costs involvedChanges in costs associated with trip postponement or cancellation‘Normal’ (dry weather) benefits associated with the capacity and surface improvements that may beincluded in a flood resilience initiative3 Flood resilience improvements may reduce but not entirely eliminate these costs.3 Note that in estimating dry weather user costs the base case and project case expansion factors need to be expressed as net of theAATOC. For example if the base case AATOC is 1 day, the base case dry weather expansion factor is 364 (365 – 1 days). If theproject case AATOC is 0.5 days, the project case dry weather expansion factor is 364.5 (365 – 0.5 days).04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 73.1.1 Waiting time for facility to re-openWaiting time is time spent waiting at a flood site for a facility to re-open while not engaging in any alternativeactivity. Cost per hour of waiting could be estimated using the standard values of time stated in ATAP PV2.Waiting time generally does not increase proportionately with closure times as more users can be expectedto elect not to travel or travel to their destination via an alternative route as the time of closure lengthens.Closure times can vary for different vehicle types. Restrictions may be placed on heavy vehicles until thesurface has sufficiently dried and the strength of the surface has returned. This is known as the dry-backperiod.Average annual time of closure and average duration of closureTwo variables are critical in the estimation of benefits associated with waiting time at flood sites (and inestimation of benefits of reduced diversion costs): The average annual time of closure (AATOC) The average duration of closure (ADC).The method for calculating AATOC is set out in Austroads (2013); see also Weeks (2012). The AATOC iscalculated between the lowest flood height that causes a facility to be untrafficable and the height of theprobable maximum flood (PMF).AATOC is the average number of hours a facility is closed per year (TMR, 2011). AATOC is calculated asthe expected value of the probability distribution of flood closure times.The average duration of closure (ADC) is the average number of hours a facility is closed per flood event.The ADC is calculated using the following formula:ADC = Total Hours Closed annually ⁄ Number of Flood Events annuallyIn general terms, the longer the ADC, the greater the likelihood that users will divert around the closure siteor postpone their trip. The longer the practicable diversion route, the more likely it is that users will postponetheir trip or wait at the flood closure site. Unfortunately, little if any research information is available tosupport the assumptions that appraisers will need to make about the proportions of users who postpone theirtrip, wait at or divert around a closure site. Anecdotal information from local police and emergency servicespersonnel or local media reports may assist but if relied on should be appropriately qualified in the appraisalreport.Waiting time benefit estimationIf for example the ADC equals 16 hours, a relatively high proportion of users could be expected to postponetheir trip or find an alternative route should one be available. On the other hand with an ADC of 2 hours,most users could be expected to wait at the flood site for the facility to reopen unless the additional diversiondistance was relatively short.Consider as an example a road closure, with three user types: private car, commercial car and heavy vehicleroad users. There is an even rate of vehicle arrivals at the closure site. The assumptions applied in theexample are as follows:04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 8Table 1 Inputs to estimation of waiting time benefits (road example) Base caseProject case% of road users at a road closure who: Wait Divert Not travel/ postpone trip15%80%5%15%80%5%AATOC8 hours4 hoursValue of travel time: Private cars Commercial cars Heavy vehicles$/vehicle hr 2013$25.48$63.22$48.17$/vehicle hr 2013$25.48$63.22$48.17Average annual daily traffic Private cars Commercial cars Heavy vehicles (Note)10005003001000500300Route length150 km (diversion route)100 km (normal route)Net diversion distance50 km Notes:1. Value of time source is ATAP Part PV22. Using 6 axle semi-trailers as an example.In this example, the base case annual cost of waiting for private vehicles is as follows: Waiting cost year= 1 000 vehicles * 15% *8 hrs AATOC * 8 hrs/24 hrs * $25.48/hr * 0.5= $5 096 Because the net diversion distance is only 50 km, it is reasonable to assume that only a small proportion ofusers will choose to wait at the flood site.With an AATOC of 8 hours, one-third of a day’s traffic would be affected by the closure (i.e. 8 hours/24hours). The factor of 0.5 reflects the even rate of arrival of vehicles at the flood site, i.e. the vehicle thatarrives at the flood site at the instant the road is closed experiences the full closure delay; the vehicle thatarrives at the instant the road is reopened experiences no delay. This adjustment factor assumes an evenrate of arrivals over the period of the closure.Similar calculations apply for the other vehicle types and for the project case. Note however that withreduced closure times in the project case, the ADC is likely to be lower, which will influence assumptionsabout user behaviour. If for example closure time reduced from 8 hours in our example to 1 hour, most if notall users could be expected to wait at the flood site rather than divert or postpone their trip.3.1.2 Trips diverting around a closed facilityDiverting trips are those that use an alternative route when a flood closure occurs. If the diverting routearound a flood closure site is short, trip origin-destination information will not be needed. If the expectedflood closure period is protracted and the diverting route perhaps hundreds of km in length, origin-destinationinformation will need to be sought or inferred.04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 9Take for example a heavy vehicle trip between Melbourne and Brisbane. If truck drivers had sufficientwarning of a flooded road closure at Goondiwindi on the NSW/QLD border they could take a diverting routefrom Moree about an hour south of Goondiwindi, via the Gwydir and New England Highways to Brisbane.The net diversion distance would be approximately 90 km. On the other hand if truck drivers did not getsufficient warning, they could divert just south of Goondiwindi, taking the Bruxner and New EnglandHighways to Brisbane. The net diversion distance would then be approximately 170 km, an additional 80 km.Sometimes, the diverted trips will need to travel a lot further and may include some backtracking, asillustrated in the mud-map in Figure 1. The letters A, B, C and D denote specific towns. A flood prone bridgeexists near D and a flood free bridge is available downstream. Consider trips from origin A to destinations Band D. The normal dry weather route is along route AB and the diverting route is through town C. For tripsbetween A and D, the diverting route would be ACBD, including backtracking between B and D. Whereinformation is available about backtracking trips it should be used but, more likely than not, the informationwill not be available.Figure 1 Diverting routeDiverting route costs are a combination of travel time costs, vehicle operating costs, fares and any othercosts involved.Continuing with the example presented in Table 1, further information is presented in Table 2:ABCDFlood prone bridgeFlood free bridge04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 10Table 2 Inputs to estimation of diverting trip benefits (heavy vehicles) Base caseProject caseRoute length150 km (diversion route)100 km (normal route)Net diversion distance50 kmAverage speed80 km/h80 km/hVehicle operating cost (VOC) (Note)$1.30/km$1.30/kmValue of travel time (heavy vehicles)(Note)$48.17$48.17 Note: See notes for table 1.The diverting route cost for heavy vehicles in the base case (BC) is therefore: Diverting route VOC/yearBC= 300 vehicles*80%*8hrs/24hrs*150km*$1.30/km= $15 600= 300 vehicles*80%*8hrs/24hrs*150km/80 km per hr*$48.17= $7 225Diverting route travel time cost/yearBC The longer the diverting route, the more likely that the diverting route will contain segments that are nothomogenous in terms of route and traffic characteristics. Where this is the case, broadly similar segmentscould be averaged to reduce the computational demands (see ATAP PV2 for parameter values).Reductions in diversion distances and kilometres of travel will also reduce crash and environmental costs.Once the annual savings in vehicle kilometres of travel have been estimated, crash and environmentalbenefits can be estimated using parameter values in ATAP PV2 and PV5.In our example the net diversion distance is 50 km (the difference between the lengths of the diverting routeand the normal route). In reality, diversion distances could range between several kilometres and hundredsof kilometres.CyclingDiversion is usually more problematic for cycling trips. There are often less alternative options available forpeople on bicycles as it can be much more difficult to travel a longer way around the flood location. Detoursmay often include obstacles (such as stairs or hills) that can be difficult for more diverse bicycle riders suchas elderly and those with larger cargo bicycles to navigate.3.1.3 Trips cancelled or postponedUsers postpone their trip until a flood closure clears because they believe (based on the informationavailable to them) that not travelling is more productive or less costly than the alternatives. Withoutbehavioural information it is impossible to know the proportion of ‘not travelling’ users who can find a similarlyproductive replacement activity and those who cannot. For someone prevented by facility closure fromattending a special event (such as a family wedding or a once-off international sporting event or concert),finding an equally valued replacement activity would be difficult. A simplifying assumption is that ‘nottravelling’ users are evenly distributed between the two extremes of:04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 11 Not having an alternative equally valued activity, in which case they incur the full cost of waiting, andBeing able to undertake an equally valued activity until flood waters clear, in which case they incur nowaiting cost. For users who cannot find an alternative and equally valued substitute activity, the cost of trip postponement(in the base case or the project case) could be approximated as the time of closure multiplied by the relevantvalue of time according to trip purpose.Returning to the example in section 2.1.1 in which 5% of users are assumed to postpone their trip, the basecase cost of trip cancellation/postponement for commercial car users would be as follows:Trip cancellation/postponement costBC = 500 vehicles * 0.05 * 8 hours/24 hours * 8 hours * $63.22/hr * 0.5= $2 107.Similar calculations are carried out for the project case except but possibly with differences in the proportionof trips cancelled/postponed and in the AATOC.3.1.4 Use of alternative modes of transport such as air transportExcluding emergency services uses, air transport as an alternative to road travel will be an efficient optiononly in extreme events, for travellers who have ready access to an airport and for travellers who have notcommenced their trip by road. Analysis of this option is unlikely to be worthwhile.3.1.5 Accommodation costsSometimes users will need to wait out a facility closure. This will involve two costs: waiting time (see section3.1.1); and commercial accommodation such as a hotel or motel room, apartment, cabin or caravan parkingsite.If data is available from previous events about the numbers of users affected and their distribution betweendifferent types of accommodation, costs can be calculated as follows:Accommodation cost = Closure nights * Rooms occupied/night * $/ room nightNote that in estimating the number of room nights, unit nights or site nights attributable to flooding, allowancewill need to be made for average room/unit/site occupancy rates (persons per room/unit/site). Occupancyinformation should be available from commonwealth or state tourism agencies or from Australia Bureau ofStatistics visitor accommodation collections.Accommodation costs are applicable only when flooding causes a net increase in accommodation costs. Forusers on a fixed duration holiday, extra nights in a flood affected town are more likely to be offset by fewernights spent in other towns. On the other hand, some users will be forced to stay in commercialaccommodation rather than at home or in a hotel/motel rather than at a caravan park site. In those instancesthe incremental accommodation costs attributable to the facility closure should be calculated for the baseand project cases.It is very likely that appraisers won’t have the advantage of behavioural information other than anecdotalobservations of local police, emergency services, local government or commercial accommodation owners.Where this is the case, sensitivity testing of alternative assumptions would be desirable.04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 12Flood events, particularly large ones will produce a range of circumstances, including ones in which usersbacktrack to find accommodation or become trapped by the side of the facility unable to find accommodation.Appraisers need to use judgement in interpreting what user behaviour information is available, but anyassumptions made should be described in the appraisal report.3.2 Injury or loss of life at flooded roadsAccording to Coates (1999), approximately 2 213 people have died in Australia as a consequence of floodingsince 1788 of which 63% have been vehicle related. The average number of vehicle fatalities due to floodingwould therefore be around seven per year. Peden and Quieroga (2014) reported that in the ten years to2012 there were 125 reported drowning deaths in Australian rivers, creeks and streams of which 54% (68)were vehicle related, equal to around 7 per year.In the ten years to 2011 there were 12 920 road fatalities in Australia, on average around 1,300 per year(see BITRE 2018). Flooding-related road deaths are therefore equal to approximately 0.5% of all roadfatalities.Relative to the size of the road toll, the number of flood-related fatalities is small and likely to be widelydispersed. A further complication is that low probability, higher impact events are more likely to producewashaways incidents and fast moving streams in river valleys will be more dangerous than slow movingfloodplain inundation. Quantification of the impacts and monetisation of these benefits is unlikely to beworthwhile unless the flood site is a known blackspot for washaway-related injuries or fatalities. Otherwisethis category of benefit is best treated qualitatively in the appraisal report.3.3 Cost reductions to transport agenciesFlooding events can cause a variety of damage to existing infrastructure ranging from washing away ofsurfaces to the collapsing of bridges. Repairs to infrastructure can be costly and disruptive. Upgradedinfrastructure that is stronger or raised above likely flood levels can reduce these impacts.Calculating cost reductions to transport agenciesIn this section we use the example of improving the flood immunity of a road to illustrate improved resilience.A simple method of estimating agency cost savings attributable to improved flood immunity is to firstcalculate agency costs for a range of flood severities at a given site (see NRM 2002).We start with some definitions:ARI (annual recurrence interval – years) – is the average, or expected, value of the periods betweenexceedances of a given rainfall total accumulated over a given duration (Bureau of Meteorology, 2010)AEP (annual exceedance probability) – is the probability that a given rainfall total accumulated over a givenduration will be exceeded in any one year (Bureau of Meteorology, 2010). Engineers Australia (2015) definesAEP as ‘the likelihood of at least one occurrence of a given size or larger in any one year. This is usuallyexpressed as a percentage.’ AEP is calculated using the following formula:AEP = 1 – exp(–1 ⁄ARI)The stronger a rainfall event, the longer the ARI, and the lower the AEP.04 – Flood Resilience InitiativesTransport and Infrastructure Council | Australian Transport Assessment and Planning Guidelines 13PMF – is the probable maximum flood. Note that PMF
