Coastal-flood risk management in central Algarve: vulnerability and flood risk indices (South Portugal)

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EcologicalIndicators71(2016)302–316

ContentslistsavailableatScienceDirect

Ecological

Indicators

j ou rn a l h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e /e c o l i n d

Coastal-ﬂood

risk

management

in

central

Algarve:

Vulnerability

and

ﬂood

risk

indices

(South

Portugal)

A.M.

Martínez-Gra ˜

na

a,∗

_,

_T.

_Boski

b

_,

_J.L.

_Goy

a

_,

_C.

_Zazo

c

_,

_C.J.

_Dabrio

d

a_Department_of_Geology,_Faculty_of_Sciences,_Square_Merced_s/n,₃₇₀₀₈_Salamanca,_University_of_Salamanca,_Spain

b_Centre_for_Marine_and_{Environmental}_Research_CIMA,_Edifício_7,_Campus_{Universitário}_de_Gambelas,_Universidade_Algarve,₈₀₀₅_Faro,_Portugal c_National_Museum_of_Natural_Sciences,_Section_Geology,_Street_José_Gutiérrez_Abascal_n◦_2,₂₈₀₀₆_Madrid,_Spain

d_Department_of_{Stratigraphy,}_Faculty_of_Geology,_Complutense_University,₂₈₀₄₀_Madrid,_Spain

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received3March2016

Receivedinrevisedform12July2016 Accepted14July2016

Availableonline22July2016 Keywords: AVIIndex FRIIndex Floodrisk Vulnerability Coastalmanagement GIS

a

b

s

t

r

a

c

t

Thispaperpresentsananalysisofthevulnerability(AVIIndex)andhazardofﬂoodingbysealevelrise (FRIIndex)inthecentralAlgarve(SouthPortugal),betweenthecitiesofPortimãoandTavira,which isanareaofintenseurbanimpactandfastgrowingtourism.Thevulnerabilityindexwascalculated usingthefollowingparametricthematicmaps:lithology,geomorphology,slopes,elevations,distances, bathymetry,variationsofthecoastline,waveheightandactivity,variationsofsealevelandtidalrange. TheAVIIndexwasvalidatedbytheresultsobtainedfromtheanalysisoftheriskofﬂoodingfromtheFHI Indexappliedtoseveraltimehorizons(X0-present,X1-100years,X2-500years,X3-1000year,X4-Storm

andX5-Tsunami).ApplicationofGISandremotesensingtechniques,viz.spatialanalysis,interpolation

processesandgeostatisticalanalysis,permittedaregionalforecastingmodelofchangeinthemeansea levelandtheensuingconsequencestobeestablished.Analysisoftheobtainedresultsshowsanincrease inpotentialﬂoodzonesinpopulouscoastaltouristareaswithahighriskofexposureandasigniﬁcant spatialextentof8.84km2_only_in_Faro_{municipality.}_The_assessment_and_delineation_of_other_endangered

sectorscouldcontributetodesigningappropriatelong-termmanagementpoliciesforthecoastalof CentralAlgarve.

1. Introduction

Theparadigmof“Globalchange”isasubjectthathasattracted theattentionofthescientiﬁccommunityfordecadesandbecame atrulyhottopicafterthe1982RiodeJaneiroEarthSummit.Often climatechangeandglobalchangeareequated,andclimateand “globalwarming”arecommonlyusedasanallinoneexplanation forallsortsofchangesorprocessescurrentlytakingplaceatthe Earth’ssurface(Zazo,2015).

AccordingtothelatestreportbytheIntergovernmentalPanel onClimateChange(IPCC,2014),thewarmingoftheclimate sys-temisunequivocal.Since1950there havebeenunprecedented changesin theclimatesystems, whichcan beseenin boththe observationalhistoricalrecords,fromthelatenineteenthcentury, andwithpaleoclimaticrecordsspanningthelastmillennia.These

∗ Correspondingauthor.

E-mailaddresses:amgranna@usal.es(A.M.Martínez-Gra ˜na),tboski@ualg.pt

(T.Boski),joselgoy@usal.es(J.L.Goy),mcnzc65@mncn.csic.es(C.Zazo),

dabrio@ucm.es(C.J.Dabrio).

changesaremanifested,bythewarmingoftheatmosphereand oceans,decreaseinthemassofcryosphere,andbyanincreasein theconcentrationsofatmosphericgreenhousegases,amongother typesofprocesses.

Global studies of the current sea level indicate a sustained risethathasoccurredsincethelatenineteenthcentury,witha turnaroundandaccelerationinthesecondhalfofthetwentieth century.Thistrendcanbeseenintidegaugerecordssince1880, andhasbeenlargelyconﬁrmedbytheseasurfaceelevationdata recordedbyseveralaltimetricsatellitemissions:Topex-Poseidon, JasonI,andOSTM-JasonII(TooleyandJelgersma,1992;Churchand White,2011).Theavailableﬁguresobtainedfromthetidegauges pointtoarateofincreasearoundof2.8±0.8mm/year,whereas valuesprovidedbysatellitemissionsamountto3.2±0.4mm/year (Churchetal.,2013).

Thephysicalphenomenabehindtheriseoftheglobalaverage sealevelareprimarilyoceanthermalexpansionandthemeltingof glaciers.Tectonicsandsalinityonlyhavealocalinﬂuence(Table1). Thereisawidevariabilityinprojectionsoffuturesealevelrise, whichhavebeenestimatedas:21–48cm(Meehl,2007),50–135cm (Bindoff etal.,2007;Rahmstorf,2007),60–115cm(Vellingaand http://dx.doi.org/10.1016/j.ecolind.2016.07.021

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A.M.Martínez-Gra˜naetal./EcologicalIndicators71(2016)302–316 303

Table1

Contributionstothebalanceofsealevelsince1972(Churchetal.,2013).

Components 1972–2008(mm/year)1993–2008(mm/year) Tide-gauge(Total) 1.83±0.18 2.61±0.55 Tide-gaugeandaltimeter(Total) 2.10±0.16 3.22±0.41 1.Thermalexpansion 0.80±0.15 0.88±0.33 2.Glaciersandicesheets 0.67±0.03 0.99±0.04 3.IceofGreenland 0.12±0.17 0.31±0.17 4.Antarcticice 0.30±0.20 0.43±0.20 5.Terrestrialstorage −0.11±0.19 −0.08±0.19 Sumofcomponents(1+2+3+4+5) 1.78±0.36 2.54±0.46

Wood,2008),85–200cm(Pfefferetal.,2008),60–95cm(Koppetal., 2009), 80–190cm (Vermeer and Rahmstorf, 2009), 78–160cm (Grinstedetal.,2010),>100cm(KatsmanandOldenborgh,2011) fortheXXICenturyDuringtheperiod1901–2010,theglobalmean sealevelraisedanaverageof1.7[1.5–1.9]mm/year,aratehigher thanthatoftheprevioustwomillennia(Churchetal.,2013).

Theprojectionsofthechangeinsealevelataregionalscale, sug-gestthatitisverylikelythatintheXXIcenturyandlater,changing sealevelswillhaveapronouncedregionalpattern,with signiﬁ-cantdeviationsfromtheglobalaverage.IntheFifthAssessment Report(Conde,2015;Churchetal.,2013;IPCC,2014), itis pos-tulatedthatduringdecadalperiods,theseregionalvariationrates resultingfromclimatevariability,maydifferbymorethan100% fromtheglobalaverage.

There is growing public awareness of the impacts that climaticallydrivenenvironmentalchangesmayhaveonthe socio-economic sphere. For instance, The European Union’s directive 2007/60/EC(DOUE60,2007)waspromulgatedduetotheincreased frequencyofcatastrophicevents—213majorfloodingeventswith 1126deathsandthelossof52billionEuros(JonkmanandKelman, 2005).Currently,thereisaneedfora renewedassessment pol-icyandflood riskcontrolmeasuresinallofthememberstates, bothincoastalandinlandSettings.Tomanagetheriskoffloods, adetailedanalysisofthevariablesaffectingthecurrentsealevel riseisrequiredinordertodevelopareliablesimulationmodel,and mapping(Kurtetal.,2004,2011;Kulkarnietal.,2014).Such map-pingis,averyeffectivetool,thatiswidelyusedinplanningand environmentally-orientedlandmanagement.

TheAlgarvecoastlineisvulnerabletosealevelrise,andin partic-ularalongbeaches,deltas,tidalflatsandcoastalwetlands.Human activity in these areas, especially tourism, brings about addi-tionalchallengesintermsofincreasingvulnerabilityanddegree ofexposuretothehazard.Thereforeastudyinvolvingshortand medium-termfloodriskismostneeded.Widespreadfloodingof AlbufeirainNovember2015,duringthetorrential precipitation associatedtoastormsurgewasreportedtohavecausedmaterial lossesinexcessof10Meuro,andisaclearexampleoftheneed forpreventionplants.Likewise,estimatingthepossibleriseinsea level,whateverthetimescale,maypreventor,atleast,induce pro-tectionandmitigationmeasures,bothinstructuralandlanduse planningterms,aimedtominimizethepresumablesocialimpacts involved.Itisestimatedthat,worldwide,some200millionpeople liveincoastalareas,afigureexpectedtoriseupto600millionin 2100(NichollsandMimura,1998).

Theobjectivesofthisstudyweretoassessthedegreeof vul-nerabilitytochanges insealevelandtherisk ofﬂoodinginthe coastalsectorbetweenPortimãoandFaroincentralAlgarve(South Portugal)(Fig.1).Thestudyarea,includestwoclearly differenti-atedsectors:totheeasttheRiaFormosatidalﬂatslayingbehind theprotectingsandyspitsandbarrierislandsthatmigratedunder aneastward-movinglongshoredrift(Andradeetal.,2004);andto thewest,themostlyruggedcoastlineextendingbetweenthecities ofPortimãoandAlbufeira,withbeachesandurbanareasprotected byrockyoutcrops.

Vulnerabilitywascalculatedusingempiricalmethodsthat com-binedaseriesofintegratedfactorsfromparametricmaps,fromthe Algarvevulnerabilityindex−AVI-createdbytheauthorsbasedon Ojedaetal.(2009)changingsomeparametersfortheAlgarvearea usingGIS(ArcGisv10.3).Forthepurposeoftheﬂoodriskanalysis deterministicmethodswereusedbyassigningaprobabilityofsea levelrisebasedonavarietyofforeseeabletemporalscenarios(100 years,500years,1000years,stormsandtsunamis).

2. Materialandmethods

2.1. Vulnerabilityanalysisforcoastalﬂooding

ThevulnerabilitywasassessedbymeansoftheAlgarve vulnera-bilityindex—AVI-,similartothatusedbytheUSGeologicalSurvey (Hammar-KloseandThieler,2001)appliedtotheAmericanAtlantic coast,PaciﬁcandGulfofMexico,andalsovalidatedintheSpanish Andalusiancoastneartheareaofthepresentstudy(Ojedaetal., 2009).Thisindexwasadaptedandmodiﬁedaccordingtothe intrin-sicparametersofthestudyarea,consideringtenfactorsthatmade uptheindexAVIequation(Eq.(1))andareexplainedbelow: AVI =√Fl×Fg×Fs×Fh×Fd×Fb×Fc×Fw×Fsl×Ftr/10 (1)

2.1.1. Lithologicfactor(Fl)

Thisfactorcreatedaparameterwhichindicatedtheresistance ofrockunitsagainstmarineerosion.

Fromageologicalpointofview,twodifferentareas(Manuppella etal.,2007)arerecognizedalongtheAlgarvecoastalfringe:a north-ernareawithcarbonateMesozoicformationsandasouthernone, wherediverselyconsolidateddetritalsedimentsofCenozoicage predominate.Theseterrainsareeasilyandimmediately differen-tiatedbytheirtopographicrelief.Theoldestmaterialscorrespond totheLateTriassicevaporiticmarlsthatevolvedintosaltdiapirs under the cities of Faro and Loulé. During theJurassic period, fossiliferous carbonateformationswithabundantmarinefossils weredeposited, withan erosionaleventintheMiddle Jurassic. Cretaceouscarbonateslayunconformablyontop.N-Sfaults pro-motedtiltingofblocks.DuringtheMiocene,biocalcarenites(Pais et al.,2012)withabundantmarinefossils and sandstoneswith interbedded glauconiticsilts accumulated, heavilydeformedby theundergoingdiapirism.FivePlio-Pleistocene(MouraandBoski, 1999;Mouraetal.,2009)fluvialtomarineunitsaccumulatedon akarstifiedsurfaceofMioceneage(PereiraandCabral,2002).In ascendingstratigraphicorder,theseare:Falesiafeldspathicsands, Montenegro burrowed sands of Montenegro, Quarteira orange sands,Ludoyellowsands,andGambelaspebblysands.Thereare also gravel terraces and fluvial channel-fills of Pleistocene age coveringJurassiclimestones.FinallyduringtherecentHolocene, coastalsandsaccumulatedasbeachesanddunesystems,barrier islands,andsiltsastidalflatsandtidalmarshesofthesheltered channelsofRiaFormosa.Terrestrialdepositsaccumulatedin allu-vialriverchannel,floodplainsandlowterraces.

Fortheanalysisofthelithologicalfactor,thegeologicalmaterials aregroupedintoﬁveclassesaccordingtothe“hardness”againsta possiblearrivalofthesheetofwater.Then,themostrecent uncon-solidatedmaterials(grainsand,gravel,etc.)havelessresistance totheeffectofthewatersurfacemeaningsectorsmore vulnera-bletoanadvanceoftheseathanthemostconsolidatedlithologies: basalts(value1),limestonesandmarls(value2),aremoreresistant toascendingsealevelthanbiocalcarenitesandsandstones(value 3),conglomerates,clays andsilts(value4)and sandandgravel (value5)(Fig.2).

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304 A.M.Martínez-Gra˜naetal./EcologicalIndicators71(2016)302–316

Fig1. LocationofthestudyareawithintheGulfofCadizandmainlocalities.

Fig.2. Lithologicalmap(basedonManuppellaetal.,2007)andvulnerabilitymapafterlithologicreclassiﬁcation.

2.1.2. Geomorphicfactor(Fg)

Wedistinguishedthreemajorunits:mountainousterrain, liai-sonunitsandthoseresultingfromthegreaterorlesseractivityof coastaldynamics.Themountainousterrainincludethemountains andhillscarvedinPaleozoicandMesozoiccarbonates,grewacke, shaleandbasalttothenorthwhicharethemainsourceareasof sed-iment.Externalgeodynamicagentsareresponsiblefordismantling,

givingrisetovariousgeomorphologicalformations,suchas allu-vialfans,glacis,endorheicareas,whichconstitutetheliaisonunits betweenthemountainsand hillsandthecoastalenvironments. Thecoastalunitconsistsofspitbars,beaches,dunesystems, off-shoreshelf,cliffs,marshes,etc.Beachesanddunesystemsoccupy asigniﬁcantpartofthespitthatformsthebarrierislandwherethe buildingboomhasdestabilizedtheerosion/sedimentationbalance,

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Fig.3.Geomorphologicalmappingandvulnerabilityreclassiﬁcation.

causingalossofsandincoastalfronts(Mouraetal.,2006;Rodrigues etal.,2012).Thespatialdistributionofthegeomorphologicalunits andassociatedsurfaceformations,allowsestablishingdegreesof resistanceaccordingtofuture stagelocationunderthesea sur-face,showingthedegreeofdisaggregationofeachformation.We assignedvulnerabilityvaluesfrom1to5,where1meansverylow vulnerabilityand5meansveryhighvulnerability.Accordingtothis, weassignedavalue1tomountainsandhills,2toalluvialfans,3to glacis,4toﬂuvialandmarineterracesandendorheicareas,and5 toallcoastalsedimentaryenvironments:beaches,dunes,spitsand channels(Fig.3).

2.1.3. Slopefactor(Fs)

Theslopeofthegroundlargelyinﬂuencestheinundationduring ariseofsealeveleithersporadic(tsunamisandstorms)or per-manent.Inclinationalsocontrolsthevelocityofwithdrawalofsea waterfacedtoapotentialﬂoodingbyinlandwaters.Lowerslopes increasetherateofdisplacementoftheshorelinetowardsthesea (PilkeyandDavis,1987).

Toproduceparametricmapsofslopes,wegenerateda Digi-talTerrainModel—DTM-,inwhicheachpixelhasthevalueofthe height(DigitalElevationModel),joiningthelidarmodelofyear 2011(coastal strip150mfromthecoastlineinland)where spa-tialresolutionis2m,andthecontinentalDigitalTerrainModelof year2013,whereresolutionis25m.Thisproducesamapofslopes expressedinpercentages,interpolatingwithaspatialresolutionof 1m,byweightingthevulnerability,takingintoaccountthatlower slopesaremorevulnerablebecausepenetrationofseawateris eas-ier.Themapshows5intervals:0–1%(value5),1–2%(value4),2–4% (value3),4–6%(value2)and>6%(value1)(Fig.4).

2.1.4. Heightfactor(Fh)

Theheightfactorisoneofthemostimportantwhenassessing thecurrentrisksrelatedtoariseofsealevel.Weconsiderthelimit

at10m,aheightconsideredunattainableunderthecurrent estima-tionsofpotentialsealevelriseinthenext100years(IPCC,2014). Areaswithelevationabove10mareassignedlowlevelsof vulner-ability,incontrastwithareasclosetoelevation0m,particularlyif laterallyrelatedtotheshore,withmaximumlevelsof vulnerabil-ityassignedtothecoastalfrontsofbarrierislands,estuariesand beaches.Vulnerabilitydecreasesgraduallywhenmovinginland. Wehaveconsideredthe10mhightoanalyzetheentirecoastline includingmoredistantlandofthecoastline.Valuesof vulnerabil-itywerereclassiﬁedusingthevaluesofelevationintheDTM.The heightvaluesforeachpixelare:0–2m(value5),2–4m(value4), 4–6m(value3)6–10m(value2)and>10m(value1)(Fig.5). 2.1.5. Distancefactor(Fd)

Closelyrelatedtoheightandslopefactors,thedistancefactor considersthelineardistancebetweenthepresentcoastlineand ahypotheticalcoastlineplacedatelevation10m,whosecontour wasderivedfromdigitalprocessingoftheDTM.Thedistance fac-torestimatesthecapacityofrisingseawatertoadvanceinlandfrom thepresentAlgarvecoast,providedthatthereisspatialcontinuity withthesea.Thisparameterwascalculatedusinganextensionfor ArcGIScreatedbytheAmericanUSGS“DigitalShorelineAnalysis System”—DSAS-.Withthistool,wecalculatedthelineardistances betweenthe10mcontourwithrespecttothe2014coastline. Dis-tanceswerecheckedbymeansofnestedvectorbuffervalidating techniques, and a raster layer wasgenerated whose data have beenreclassiﬁedbasedonthestandarddeviation,withintervals 0–700m(value5),700–3000m(value4),3000–5000m(value3), 5000–9000m(value2)and>9000m(value1)(Fig.6).

2.1.6. Bathymetryfactor(Fb)

Knowledgeofthecontinentalshelfgivesinvaluableinformation forunderstandingtheactionofwaves.Aswavesapproachthecoast, theirshapechangesoncethewaterdepthissmallerthanhalfthe

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Fig.4.Slopemap(in%)andreclassiﬁcationintermsofvulnerability.

Fig.5.Mapofelevations(inmeters)andreclassiﬁcationaccordingtovulnerability.

wavelengthofwavefronts.Then,frictionwiththebottom progres-sivelyreducesvelocitywhereaswavecrestscontinuepractically

unaffected.Eventuallywavescollapseandbreak(Barrera,2005)are changingtheirmorphologytoacriticalpointwherethedistanceto

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Fig.6.Mapofdistancesinmeters.

thebottomisequaltohalfthewavelengthofthewavefrontsand brushwithbackgrounddestabilizeswaves(watervelocityinthe ridgeishigherthanthebottom)andbreak(Barrera,2005).

Thebathymetryfactordeterminestheeaseofagreaterorlesser marineingressioninthecoastalsector,sothatdeeper prograda-tionishigherthaninshallowwaterswithlessrunup,wherethe depthislessthanhalfthewavelengthcausingtheswashofthe waves.Sincewehavenodataregardingthewavefronts,thisfactor providesinformationontheeffectofwavesonthebeachandspits. Bathymetricmapstakeintoaccountpointsandbathymetriccurves suppliedbytheHydrographicServiceofthePortugueseNavyand theOceanographicInstituteofSpain,complementedwithdigital depthvaluesinthechannelsofRiaFormosa.Theresolutionofthe resultingbathymetrymapsis2mperpixel.

Theweightandreclassiﬁcationwasmadetakingintoaccount that vulnerability will be high in shallow depths because it enhancestheactionofbreakingwaves.Theintervalsare:depths less than 1.5m (value 5), 1.5–3m (value 4) 3–10m (value3), 10–20m(value2)and>20m(value1)(Fig.7).Waterdepthsinthe Algarvearerelativelyhigh,withvaluescloseto200minfrontof thebarrierislandsystemofFaro,whichisconsideredless vulner-able,whileshallowerwatersinPortimão-Albufeirayieldmedium tohighvaluesofvulnerability.

2.1.7. Coastalfactor/exchangerateforcoastalwaterfront(Fc) Thestudyoftheevolutionofthecoastlineanditsmorphological variationswascarriedoutusingoverlayandproximitytechniques implementedinGIS,byrestoringthesuccessivepositionsof coast-lineduringthelast59years(1956–2015).Thisanalysisisbased on thesuperposition of aerial photographyof 1956 (American Flight-USAirForcecoverage,greenlineinFig.8)ataresolutionof 1m,orthoimagesofthe2005PortugueseOrthophotographywith aresolutionof50×50cm(redlineinFig.8)andﬁnallythe geo-referencedaerialimagesArcGISOnlineViewer2015(yellowline

inFig.8).Thepositionofsuccessivecoastlineshavebeendigitized andanalyzedbymeansofUSGS—DSAS-extensionforArcGISv10.3, intermsofratesofretreatoraccretion(m/yr)duringthisperiod. Highvaluesofgrowth(progradation:coastlinemovestemporarily oncoastalzone)meanhighvaluesofvulnerability,whilenegative ratesindicatelow valuesofvulnerability (coastlinerecedesand seabackwardsinside),withthefollowingintervals:<−2m/year (value5),−2to−1(value4)−1to1(value3),1–2(value2)and>2 (value1).Overall,averagevaluesarepresented(between−1and 1m/year)forthecoastalretreatinalltheAlgarvecoast(Fig.8). 2.1.8. Swellfactor.Averagerateofsigniﬁcantwave(Fw)

Theswellfactorindicatesthemaximumvaluesofmean signif-icantwave(averagewaveheightsconsideringthehighestwaves in the geodatabase considered) that affect thecoastline of the Algarve.Thehistoricalaveragedatahavebeenobtainedfromthe websiteoftheStatePortsofSpain(http://www.puertos.es/es-es/ oceanograﬁa/Paginas/portus.aspx)anddifferentauthorsdescribe theconditionsofmaritimeagitationandwavesofsomeports,such asFaroandSines(Costaetal.,2001).Weanalyzedvarioustypesof networks:REDCOS(networkofcoastalbuoyslessthan100mdeep andnearharbourfacilities),WANA(fromsimulatedtimeseriesof parametersofwindandswelldata),SIMAR44(simulateddatatime seriesofatmosphericandoceanographicparameters),andREDEXT (networkofdeepwaterbuoys,morethan200mwaterdepth).The studiedtimeseriesincludestheintervals1958–2015(52years), 1983–2012(29years),and1992–2015(23years).Dataforeach stationareorganizedinﬁleswiththeparameterstobeconsidered (Fig.9A–C).

SurfrateshavebeenintroducedintoGISinordertostatistically processinformationandmakeinterpolations.Therationalefor tak-ingtheaveragesigniﬁcantwaveinsteadtheextremewasthatit representsbetterthemoreprobablewavestatesanditgives impor-tancetorepresentativenessascomparedwiththetotalratherthan

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Fig.7. Bathymetricmaps(inmeters)andreclassiﬁcationofvulnerability.

Fig.8.MapofcoastlinevariationanalyzedwithDSAS,forthe1956–2015interval,superpositionofaerialphotography(greenline1956;redline(2005andyellowline (2015).(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

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Fig.9. Top:Historicdataofstationswithdatabuoys(reddots),simulatedbymodelsinpoints(greendots)andtidegauges(yellowdots).Bottom:examplesofbuoysﬁles (A),SIMARnetwork(B)andREDEXTnetwork(C).(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

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Fig.10.Waveheightfactormap(inmeters)forcentralAlgarve.

focusingonsporadicepisodes.Weselectedthehighestvaluesofthe meansigniﬁcantwavesbecauseourstudyisdirectedtotheriskof ﬂoodingthatwouldbestatisticallymoreprobable(likelytooccur), evenifislessfrequent.Wetookintoaccountthelongesttemporal seriesforeachmeasurementpoint(device).Vulnerabilityruns par-alleltowaveheight,sincehigherwavesmeanastrongereffecton thecoast,andassignedvaluesare:3.5–4m(value1),4–5m(value 2)5–6m(value3)6–7m(value4)and7–8m(value5).Accordingto theestablishedparametersofwaveheight,vulnerabilityvaluesfor centralAlgarvecoastwouldbehigh(between3.5and5m)(Fig.10). 2.1.9. Sealevelfactor(Fsl)

Thisfactorexaminestherelativesealevelchangeusing histor-icaldataofmeansealevel(Borregoetal.,1995;Boskietal.,2008; Delgadoetal.,2012;Sampathetal.,2014)collectedbytidegauges alongthenearbySpanishcoast:Huelva,CadizandTarifa(Fig.9), whichcontinuouslyevaluateandrecordtheaveragelevelseaand tidegaugedataofPortugal(LagosandCascais)recordsbetween 1908and1987and1882–1987(DíasandTaborda,1988;Antunes andTaborda,2009;Antunes,2011).Forthispurposewe georefer-encedandgeneratedwithArcGISthegeodatabaseoftheclosest tidegauges:gaugesofHuelva3.4and5from1996to2015data, tidegaugeBonanza2atthemouthoftheGuadalquivirriverwith datafrom1992to2015,andTarifaharbourtidegaugewithdata from2009to2015.ThenetworkofstationsisREDMAR(Fig.9C).

AnothersourceofdataisthePermanentServiceforMeanSea Level—PSMSL-,locatedinLiverpool,UK.Thisorganizationcollects, analyzes,interpretsandpublishesdatarelatedtothechangesinthe averageglobalsealevel,recordedbytidegauges.Inadditiontothe existingnetworkoftidegaugescurrentlyoperativeinvarious coun-triesthisServiceincorporatesdatafromtidegaugesthatworked inthepast,addinglongtemporalseriestothedatabase.Datafrom theREDMARnetworkareincorporatedperiodicallyininternational datacenters,oneofwhichisPSMSL.Asthereareseveral

representa-tiveseries,wemadealinearinterpolationusingArcGISv10.3from theinformationprovidedbythegaugesindicatedabove.Foreach timeseries,weobtainedanaveragevalueofsealeveloscillations whichsubsequentlywedividedbythenumberofyears.Thisyields arateforeachtidegauge.Theaveragerateofsealevelriseforthe studyarea,accordingtoinformationprovidedbylocaltidegauge (1992–2015)rangesbetween1.71and1.89mm/year.Interpolating andreclassifyingtheintervalinthecentralAlgarvecoastobtained valuesofthecoastwiththefollowingweightinterval:1.29–1.34 (value1),1.341–1.46(value2),1.461–1.58(value3),1.581–1.70 (value4)and1.701–1.89(value5)(Fig.11).

2.2. Extremetidalrangefactor(Ftr)

Thetidalrangeanalyzesthedifferenceinheightbetween suc-cessivehighandlowtides.Dependingonthegeographicallocation andlocalconditionsthetidalrangecanvaryfromafewcentimeters toseveralmeters.Coastswithtidalrangesbelow2mareconsidered microtidal(MasselinkandShort,1993).Weanalyzedthevaluesof theneareststationsoftheREDMARnetwork(Fig.9)toknow pre-ciselyfluctuationsinthetidalrange.Subsequently,weconducted alinearinterpolationtoobtainsuchdataandhazardmappingas ameasureforfuturepreventionappliedtocoastalmanagement. Becausetheultimategoalof thestudyistoknow the vulnera-bilityandtheriskofflooding,wehavetakenthemaximumtidal rangesrecordedfortimeseriesasrepresentinghighs,althoughin othercasesthemeantidalrangescouldbeconsideredas represen-tative.Thetidalrangeobtainedinthestudyareaisabout2–4m (Borregoetal.,2000),asexpectedinthismesotidalAtlanticcoast (Fig.12),withvalues between120cm(intheStraitofGibraltar sector)and420cm.Thereclassificationofvulnerabilityvaluesare: 120–180.1cm(value1),180.2–240.1(value2),240.2–300.1(value 3)300.2–360.1(value4)and360.2–420(value5).

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Fig.11.Mapoftheaverageriseofsealevelfactor,expressedinmillimeters,inGulfofCadiz.

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2.3. Analysisofcoastalﬂoodinghazard

Thisanalysisisbasedontheﬂoodhazardindex−FHI-, generat-inginthispaperanovelcontributionwithisnewindex,whichtakes intoconsiderationdifferentscenariosofriseandfallofsealevel fromestimates,assumingthattherestofquantitativevariablesin thearea(tidalrangeandwaves)remainconstant.

Recentstudies(Churchetal.,2013;Holgateetal.,2013;IPCC, 2014)based upon satellitealtimeterdata, calculatedan annual increasein sealevel riseof3mm/year betweenfrom1993and 2011.However,iftheseratesremain;theriseofsealevelwould reachvalueswellabovetheestimatedbyotherauthors,inﬂuenced bythermalexpansionandchangesinsalinity(MarcosandTsimplis, 2008;Marcosetal.,2011).Fromthewaves,tidalrangeandsealevel parametersofstudiedintheanalysisofvulnerability,weestimated thecoastalﬂoodhazard in differentscenarios,considering that thesearemeanvaluesforthepast25years(meanofthetimeseries oftheavailablestations).Theabsoluteriseinmetersiscalculated fromminimumandmaximumrise(mm/year)rates.

Thestudyareaispartofazoneofcollisionbetweenthe Euro-peanandAfricanplates,whichcangeneratetsunamis.Thisisan area, where thedepth of thebasin acts as anaggravating fac-tor(Larioetal.,2010,2011).Alsoinsurroundingareasthere is evidenceofrecordsinQuaternarysedimentaryseriesofextreme eventsoftsunamitype,similartothe1755event(Lisbon earth-quake)whensealevelroseupto8mabovethecurrentlevel.That wasthecaseoftheeventrecordedinConiltownintheprovinceof Cadiz,wherethesmallﬁshingvillageofConiletewascompletely destroyedandneverreconstructedagain(Luqueetal.,2002).Other geologicalandhistoricalrecordsoflesserenergeticstormevents causingsuddenrisesofsealevelarecommoninthearea.These records,whetherlandformsor deposits,proverises ofsealevel understormconditionsup to2minelevation.The chronologi-calsequenceofoccurrenceofevents,asexpected,doesnotshow cyclicitybuttheagesofthesecatastrophiceventshavebeendated: 365BC;1680;1804;1860and1875(Luqueetal.,2001).

TheFloodRiskIndex−FHI-(Eq.(2))considersthreeparameters thattakeintoaccounttheinteractionbetweensealevelriseandthe physiographicfeaturesofthecoastalstrip,fordifferentscenarios inourstudyarea.Theproposedscenariosare(Table2):

vFHI= Fw×Fsl×Ftr(Xn) (2)

X0scenario(present):Representsthehazardofﬂoodingbased ondatacollectedfromtidegaugesandbuoysinthevicinityofthe studyarea,basedondataofthepast25years.

X1scenario(100years):Representsthehazardofﬂoodingtaking intoaccountthedatacollectedonstageX0calculatedforthenext 100years.

X2scenario(500years):Estimationofcurrentsealevelrise sce-nariofor500yearsbyextrapolatingX1Astage.

X3scenario(1000years):Estimationofcurrentsealevelrise scenariofor1000yearsbasedonpredictionsX1Astage.

X4 scenarioExtreme events:Storms(X4)andtsunamis (X4): probabilitiesoflargeeventssuchasstormsortsunamisandtheir impactonthecoastareadded.

3. Results

3.1. CoastalvulnerabilityinthePortimão-Farosector

Fromthe“AVI”indexcalculatedthevulnerabilityofthecentral sectoroftheAlgarve,forascenarioX₃_,equivalenttosealevelrisein 1000years,fromthefactorsthatinﬂuencesealevelrise.Particular attentionwaspaidtothevarioussectorsofthecoastalstrip.The

higherthevalueoftheAVIindex,thehighervulnerabilitytorising sealevel(Fig.13).

ValuesofvulnerabilityareveryhighfortheFarosector,while intheareaofAlbufeiraandPortimãothevulnerability indexis medium-low.Thedistributionofareasofsealevelriseisconsistent withtheriskanalysisthatwillbeexplainedbelow,althoughthis islessintenseandmoreconservative.Highvulnerabilityalso con-centratesonsectorswithhightouristpopulation(Faro,Portimão, OlhosdeAgua-WQuarteira,Pera_._._..).Notethehigh vulnerabil-ityoftheInternationalFaroairport,hotelsandsomenearbycities (Tavira,Olhão....)

3.2. RiskofcoastalﬂoodinginPortimão-Farosector

Wehaveprojectedseveralfuturescenariosofsealevelriseon amapbasedonthepresent(2015)orthophoto,wherethe differ-entdegreesofriskofcoastalfloodinginthecoastalstripcanbe observed(Fig.14)andthemostdenselypopulatedareasareshown. Themapofhazardsshowsthatthecoastalstrip ofthemore advancedwaterfront(barrierislandsofFaro)istheone experienc-ingthewiderandgreaterpenetrationofseawater,whereasthose areasunprotectedbybarrierislands(Albufeira)arelessaffected. Obviously,thetopographicallydepressedareas,asisthecaseof rivermouthsarethemostaffectedbyarisingsealevel,withmore inlandpenetrationofseawater(PortimãoandOlhosdeAgua).In someplaceswithvitalinfrastructures,suchasFaroInternational Airport,ortouristicconcentrations,suchasOlhosdeAguaand Por-timãohighfloodhazardisobserved.Alongtopographicallyhigh sectorssuchasAlbufeira,flood bandsarerestrictedto environ-mentsclosetothepresentseafront:beaches,dunesystems,for example(Fig.15).

Toassessthedegreeofriskexposure,severalparametersare analyzed: surface, number of inhabitants in each municipality, populationdensity,topographicalheightofthecitieanddistance fromthetownto thewaterfront.Fromthemap of landuseof theEuropeanprojectCorine(Fig.16,Table3),theanthropicareas (neighborhoods, industrial areas, infrastructure....) are deter-minedand,usinggeospatialtools,thepopulatedareasvisibleon thepresentaerialphotographyaredelimited,obtainingthesquare kilometersof urbanizedarea.Algebraof layersallowseasy cal-culationoftheexposedpopulation.Finally,linkingthedensityof populationofeveryzonetothefloodriskineachsector,theexposed populationforeveryscenarioiscalculated.Thisdirectmethodisthe mostusefulforauthoritiesinchargeofplanningtheriskofcoastal flooding,becausetheycanquicklyupdatethethematiclayers,as theprocessisa simplesuperposition.OtherGIStechniquesuse indirectmethodssuchasmultivariatemethodswithspatial ana-lystToolssuchasISOclusterunsupervisedclassification,oralso maximumlikelihoodclassification.Theseindirectmethodsmust firstestablishspectralsignaturesfromdigitalimagelevelssetby atechnicianusingabasicsupervisedclassificationmadeby unsu-pervisedclassificationextrapolatedtotherestoftheimage.Itsuse ismorecostlyandlessflexibleandslowerthanthedirectmethod proposedinthispaper.

The resultsof exposuretoof flooding hazard in thecentral Algarve for differentscenarios deducedfrom modeling of data obtainedandpotentialurbanizedareasaffected,indicatethatthe coastalsectorwithwiderurbanizedareaexposedtohighhazardof seafloodingis7.46km2_in_surface,_between_Albufeira_and_Olhos_de Agua,whereapopulationdensityof612.9inhabitant/km2_induces thehighestriskoffloodingforpopulationinthecentralAlgarve: 4572peoplemaybeaffected.Inaddition,Faroisthelargest sur-faceareaexposedtotherisk offloodingbysealevel rise,with 8.784km2_and_an_exposed_population_of₂₈₃₁_people._It_is_worth tonotethatsomesectorsofFaro,suchastheairportareaandparts

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Table2

Stagesandparametricvaluesofabsoluteandtotalestimatedriseinsealevelasthepresentstudy.

ScenarioFactor Fw Fsl Ftr Total

Min Max Min Max Min Max Min Max

PresentScenario—X0 3.5 5 0.042 0.047 4.0 4.2 7.54 9.24 100yearsScenario—X1 3.5 5 0.12 0.14 4.0 4.2 7.62 9.34 500yearsScenario—X2 3.5 5 0.63 0.70 4.0 4.2 8.13 9.90 1000yearsScenario—X3 3.5 5 1.26 1.40 4.0 4.2 8.76 10.60 StormScenario—X4 3.5 5 2m 4.0 4.2 9.50 11.20 TsunamiScenario—X5 3.5 5 8m 4.0 4.2 15.50 17.20

Boldrepresentstheﬁnalvalues.

Fig.13.MapoftheCoastalVulnerabilitycentralAlgarveasthe—AVI-index.

Table3

Parametersforthecalculationofexposuretoriskofﬂooding.

MunicipalityFactor Area(Km2₎ _{Inha-bitants.} _Density_of

population (Inh/Km2₎

Cityelevation abovesealevel(m)

Distanceto coastline(Km) Urbanizedarea exposed(Km2₎ Inha-bitants exposed Portimao 182.06 55614 305.47 2 3 5.52 1686 Pera 9.15 4867 531.9 7 0 2.42 1287 Albufeira-Olhosde Agua 140.91 38966 612.9 8 0–2 7.46 4572 Quarteira 37.78 16131 427 3 0 0.39 166 Faro 202.57 64560 318 12 0 8.84 2831 Olhao 130.90 42272 322.9 8 0 6.52 2105 Tavira 764.4 69824 91.34 28 0 3.88 354

oftheoldtownneartheharbour,wouldbeﬂoodedinatemporally nearscenario(scenarioX1)

4. Conclusions

Theresultsprovidevaluableinformationonthedegreeof vul-nerability and the risk proﬁleof the Algarve in the event of a potentialriseinsealevelwhetheritisgeneratedbyaprogressive riseorduringextremeevents.

ThevulnerabilityhasbeencalculatedfromtheAlgarve vulner-abilityindex−AVI.Themapsofvulnerabilityandriskhighlightthe highriskofﬂoodingposedbyanyrise,howeversmall,intheAlgarve coast.Theurbanizedareaaffectedbyﬂoodriskisabout35km2_,_but

theﬁgureishigherifcropandnotpopulatedareasareincorporated. ThepopulationsubjectedtoﬂoodingriskincentralAlgarvecoastal areasamounttosome13,000people,withahigherexposureinthe sectorsofAlbufeira,OlhosdeAguaandFaro.

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Fig.14.FloodHazardMapforthecentralAlgarvecoastlineunder—AVI-indexfor1000yearsScenario:X3.

Fig.15.DetailoftheﬂoodhazardforsomelocalitiesonthecoastalstripofcentralAlgarvefor1000yearsScenario:X3.

Theshorttimeseriesavailablefromtidegaugesandbuoys, cou-pledwiththecontinuouschangeof estimatesof trendsforthe centurymakethesubjectofcurrentsealevelriseinamost

con-troversialone. Thebroadspectrumoffutureprospects, coupled withtheuncertaintyofthepast(theoldestmeasurementsfrom tidegaugesdatefrom1883)leavethesedimentaryrecordasoneof

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Fig.16.Top:CORINEmap,showingthedifferentlandusesandassociatedgeodatabase.Bottom:exampleofcalculationatriskofﬂoodingfrombuilt-upareas(polygonswith blackstripe)ontheﬂoodedareasfordifferentstages(maplegend15)intheareaofOlhosdeAgua.

thefewwitnessesofthechangeinsealevel,whichinturnisrather unprecisebothtemporallyandspatially.

The methodology used here is self-validating because the affectedareas calculatedby themethodof vulnerability(based on empirical methods of parameters of the study area) match withthoseobtainedbythemethodofrisk(basedondeterministic methods,usingtemporaltrendscenarios).Manyforecastsonthe quantificationofthecurrentsealevelrisegloballydonotconsider theswellfactorandtidalrange,thusinvalidatingtosomeextenta directtranspositionofthesevaluesoveraspecificarea.Forthis rea-son,localstudies,suchasthepresent,whichaddressandevaluate therisksoffloodingundertheworst-caserisingscenariosimulated, aremuchmorereliable.

Finallythismappingisapreventivemeasuretominimizethe riskinthiscoast,withlargedevelopmentsand resorts,and vis-itedbyherdsoftouristsfromallaroundtheworld.Usingthistool, managementactorscandelimitatethesectorsinneedofstructural

measuresaimedtominimizeshort-andlong-termimpactsofsea levelrisecausedbynaturaltendencyorbyextremeevents.The Algarvecoastisahighlyvulnerableareaowingtothe characteris-ticsofthephysicalenvironmentandahighriskofﬂoodingbecause ofitsgeographicalposition.

Acknowledgment

Ministry of Economy and Competitiveness: BTE, CGL2012-33430/BTEandCGL2012-37581/BTE.

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