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This Issue

Katherine Flood – April 2006At around midnight on Thursday 6th of April, floodwaters of the Katherine River peaked at19.0 metres at the railway bridge gauge in the centre of Katherine. This was the secondmajor flood in just over eight years – the January 1998 “Australia Day” flood was much moresevere – and the third highest since systematic recording began in 1952.

Katherine has a population of approximately 10,000 and is located some 300 km south-eastof Darwin in the Northern Territory. The township is situated on the banks of the KatherineRiver, a tributary of the Daly River. The catchment area to Katherine is about 8,200 km2.The river topography, including the area around Katherine, generally consists of an incisedmain channel with a wide flat floodplain on either side. The exception is Katherine Gorge(Nitmiluk National Park), which begins about 30 km upstream of Katherine and extendsupstream for a further 25 km or so. Within the gorge the river is confined by vertical rockescarpments 40 to 60 metres high.

Flood DetailsComparison with design flood levels at the gauge suggests the April 2006 flood had a returnperiod of just over 20 years (see Table 1). The most affected areas were west of the river andalong the river north and south of the main part of town, particularly on the road to KatherineGorge. Only the fringes of the Town Centre were affected. Figure 2 shows the modelledextent of inundation for the ARI 20-year design flood. The recent flood appeared to cover aslightly larger area but was remarkably similar to the design flood; because of the manyvariables involved, real floods don’t always exactly match design floods. The photos inFigures 1 and 3 were taken when the flood was 20 cm below its peak.

Figure 1: Aerial view looking south across the Katherine River towards the Town Centre at2:42pm Thursday 6th April; i.e. looking approximately from top to bottom of Figure 1. (Source:Department of Defence; photo taken by Leading Aircraftman Steve Duncan from aHornet F18.)

Katherine Flood - April2006

Natural DisasterLosses and ClimateChange: An AustralianPerspective

Many modelling studies have concludedthat bushfire risk will increase under highcarbon dioxide emission scenarios. Table1 lists two relevant statistics on the annualbushfire loss frequency calculated betweena given start year and 2003. The first rowshows that the likelihood of some homedestruction remained approximately stableat 60%. The second row lists the likelihoodof having more than 25 homes destroyedwithin a single week; in other words, theannual probability of having a significantevent loss. Both statistics have remainedremarkably constant despite largeincreases in population and improvementsin fire fighting technology. To argue thatglobal increases in air temperature, etc.and the increasingly lengthy urban-forestinterface have been exactly compensatedby improved fire management techniqueswould stretch credulity.

While none of the above evidence isunambiguous, we see little confirmationthat natural hazards are increasing infrequency and destructiveness in theAustralian context. We conclude that, todate at least, societal changes – wealthand population - remain the main driversof the increasing inflation-adjusted cost ofnatural disasters in this country. How longthis continues remains to be seen.

References

Crompton, R. 2005. Indexing the InsuranceCouncil of Australia Natural Disaster EventList. Report prepared for the InsuranceCouncil of Australia, Risk Frontiers.

Gray, W.M. 2003. Twentieth CenturyChallenges and Milestones, In Hurricane!Coping with Disaster edited by RobertSimpson, American Geophysical Union,Washington DC 2003.

McAneney, K.J. 2005. Australian bushfire:Quantifying and pricing the risk toresidential properties, pp. 13-22. In R. JMorrison, S. Quin and E.A. Bryant [eds.]Proceedings of the Symposium onPlanning for Natural Hazards – How CanWe Mitigate the Impacts? 2-5 February2005, University of Wollongong.

Figure 1: Aggregate insured losses indexed to 2004 dollars for all events inthe IDRO database for 12-month periods ending 30 June. Four earthquakesand one tsunami have been eliminated. Tropical cyclone losses have beenreduced by 50% to roughly allow for improvements in building codes.

Figure 2: Combined frequency-severity of tropical cyclones that have crossedthe east coast of Australia during five-year periods. Only events having acentral pressure less than or equal to 995hPa were included.

Table 1: Statistics of bushfire loss probabilities (Source: PerilAUS, RiskFrontiers) calculated between the start year and 2003. Data in the first column(1900) has been adjusted to account for missing data (McAneney 2005).

This year’s Risk Frontiers Seminar Series will be held on 21st September, 2006 at the AGL Theatre, Museum ofSydney (cnr Phillip and Bridge Streets) from 2.30-4.30pm followed by nibbles and drinks in the foyer.Further details and registration forms will be forwarded shortly and will also be available on our website: www.riskfrontiers.com.

Seminar Series 2006(see back for details)

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Start Year 1900 1926 1939 1967 1983 1990

Annual probabilityof a loss 57% 54% 49% 59% 62% 64%

Annual Probabilityof a major event 40% 43% 41% 38% 38% 36%

Dominatedby 1974BrisbaneFloods (TCWanda)

1974 TC Tracy

1990 Sydney Hailstorm1983 AshWednesdayFires

1999 SydneyHailstorm

Figure 2: ARI 20-year flood extent for the Katherine town area (extract from NT Department of Natural Resources,Environment and Arts, Computed 5% AEP Flood Extent Map, September 2005).

The NT Department of Planning and Infrastructure’s RegionalManager for Katherine estimated that 300 dwellingsbetween Katherine Gorge and downstream of the town wereaffected by at least over-ground flooding. He furthersuggested that approximately a dozen commercialpremises suffered water damage. Katherine Town Councilcosted damage to municipal roads at $1.2M. The publiclibrary was flooded and $64,000 worth of books destroyed.At the end of May the Territory Insurance Office had received122 home building claims totalling $3.32M, 124 contentsclaims for $1.87M and 26 commercial property claims for$2.86M.

About 1,200 people registered at evacuation centres at threeschools in East Katherine between Wednesday 5th April

and Sunday the 9th. Most people had returned home byMonday morning and most businesses had reopened bythen. Hospital patients and retirement home residents wereevacuated to Tindal RAAF base and cared for by the RedCross.

Heavy rainfall in the Katherine River catchment can begenerated by decaying tropical cyclones or monsoonal lowpressure systems. The rainfall that produced the April floodwas primarily due to the latter; although steady and heavyrain throughout March had helped saturate the catchment.The rainfall was quite contained and short-lived, affectingonly the upper reaches of the main river rather than thetributaries. The maximum recorded rainfall was 195mm inthe 24 hours to 9am on the 5th of April in the uppercatchment.

Comparison with January 1998 floodIn contrast, rainfall from Ex-Tropical Cyclone Les thatproduced the January 1998 flood was heavier and moreextensive. Katherine received 374mm in 48 hours and fallsof between 300mm and 500mm were recorded across thewhole catchment and adjacent catchments for the sameperiod. One gauge recorded more than 700 mm in 48hours. The widespread nature of the rainfall meant thetributaries made a significant contribution to the streamflow through Katherine in 1998; the tributaries did not playa significant role in the recent flood.

Figure 3: Aerial view of theintersection of Katherine Terrace(running left to right, bottom ofcentre) and Lindsay Street (underwater, running bottom to top, rightof centre) at 2:42pm Thursday 6thApril. (Source: Department ofDefence; photo taken by LeadingAircraftman Steve Duncan from aHornet F18.)

Equally important was the different ground water levelsduring the two floods. Stormwater run-off in Katherineusually finds its way into the aquifer via the porous “karst”limestone geology. In 1998 the regional water table wasvery close to the surface so this mechanism didn’t operate,compounding the impact of the river flooding. In April thisyear the water table was lower so run-off from local rainfallescaped quickly.

The January 1998 flood measured 20.4m at the gauge andis the largest flood on record. The return period has beenestimated at 155 years (Water Studies 2000).Paleohydrologic analysis of sediments in Katherine Gorgeindicate that two other floods of about the same size haveoccurred in the last 600 years.

The 1998 flood covered virtually the whole area shown onFigure 2 except for East Katherine. 1,170 houses andapartments and 500 businesses were inundated, including

every government office and business in the Town Centre(Skertchly and Skertchly 1999).

ReferencesSkertchly A. and Skertchly K. 1999, The Katherine-Dalyflood disaster 1998, Australian Journal of EmergencyManagement, 14(10): 31-36, 50.

Water Studies Pty Ltd 2000, Katherine River Flood Study,Report no. 02/2000D for Department of Lands, Planningand Environment, 77pp.

AcknowledgementsWe gratefully acknowledge information provided by thefollowing people: Dale Demarco at TIO; Colin Beard atthe Bureau of Meteorology; Steve Duncan at Departmentof Defence; Graham Newhouse and LakshmanRajaratnam at NT Government.

Over the last century some 95% of Australian building lossesattributable to natural hazards were caused bymeteorological events. This suggests that Australia mightbe more sensitive to changes in global climate than someother jurisdictions. Here we add our two pennies worth tothe current debate.

Recently Risk Frontiers undertook an indexation of theAustralian Insurance Disaster Response Organisation’s(IDRO) database of insured losses (Crompton 2005). Ourinterest here is to see whether any observable signal remainsafter the original losses are adjusted for changes in inflation,population and wealth. Figure 1 presents the results andshows no obvious trend over the last four decades. Whilefurther modelling is needed to move beyond thisbenchmarking exercise, it is hard to dismiss the role thatthe increasing number and value of insured assets haveplayed in driving up insured losses.

Let’s look now at tropical cyclones by themselves.According to Gray (2003), when normalised for coastalpopulation, inflation, and per capita wealth, tropical cyclone-spawned damage in the US rises by a factor of four foreach successive increase in Saffir-Simpson intensitycategory. Assuming that this same relativity holds true forAustralian conditions, Figure 2 plots a relative potentialdestructiveness index for landfalling cyclones on the eastcoast.

By concentrating on the damage potential of the hazard,Figure 2 makes allowance for Australia’s low populationdensity and large physical separation between populationcentres. It assumes a spatially uniform exposure. Againwe see no obvious increase or decrease in potentialdestructiveness over the time periods represented inFigure 2.

Natural Disaster Losses and Climate Change:An Australian PerspectiveRyan Crompton, John McAneney and Roy Leigh

Table 1: Design flood levels and recent events for theKatherine River (after Water Studies 2000)

Event Peak Flood Height(m Gauge Height)

20-year ARI 18.9April 2006 19.0March 1957 19.350-year ARI 19.6100-year ARI 20.0January 1998 20.4Probable Maximum Flood 23.1