2006FloodinElPaso,Texas

Fall2016

RicardoLugo

GeographicalInformationSystemsinWaterResources

Introduction

FromlateJulytomid-September2006thePasoDelNorteregion,consistingofElPasoCity, Southern New Mexico and Ciudad Juarez, Mexico experienced several record highprecipitationevents.Thiscausedseverelocalizedandwidespreadflooding.

ThePasodelNortemetropolitanareaconsistsofapproximately2millionpeopleofwhichElPasocomprised736,310duringthedisasteryear.ElPasoandciudadJuarezareseparatedbytheRioGrande.ElPasoislocatedintheChihuahuanDesertandannualaverageprecipitationisapproximately9inches.MostofthisrainfallswithinintheNorthAmericanmonsoonseasoninlatesummer. It isnotuncommon for it to fall in shortbursts thatcause flash flooding in themetropolitanarea.ElPasoisbuiltaroundtheFranklinmountainsandbackedonthesouthendby Cierra de Juarez mountain. Orographic effects around the mountain lead to significantvariationinstormimpactsfromonelocationtoanother.

The El Paso international airport rain gagemeasures 15.01 inches of rain during themonthsofJulythroughSeptember.FromJuly27toAugust4ththisgagemeasured6.6inchesofrain. Almost three fourths of it yearly aver average precipitation amount, with some areasreceiving almost 10 inches of rain in less than 8 hours (Rogash et al 2009). Three of the 16maximumprecipitationeventsinthepast130yearsoccurredduringthistimeperiodmakingthisstormeventofficiallythewettestmonsoononrecordforthePasodelNortearea(GillandCollins2010).

TheRioGrande thatcreatesanaturalborderbetweenElPasoandCiudad JuarezwasdammedatElephantButtereservoir(NorthofElPaso)in1916.ThisreduceditsbaseflowatElPasofrom945cfsto178cfs,areductionof82%.DuringthesevereraineventtheRioGrandeoverflowedseveraltimesinandaroundthecityandreacheditshighestlevelssince1912.Arroyosthathadbeendryforyearsreachedfloodstage(Rogashetal2009).

Rainfallamountswereover300%ofnormallevelsbasedon128yearsofdatacollection.AccordingtothenationalweatherservicetherainsintheweekfromJuly27toAugust4thwereakintoa100-150yearevent.Anestimated1,500homesweredamaged,includingdamagetoinfrastructureandbusinessesthedamagewasestimatedtobebetween200-450million(Collinsetal2012).Theareawasdeclaredafederaldisasterandalthoughnolossoflifeoccurredtherewassignificanthealtheffects.

TherainfallwasduetoanumberofmesoscaleconvectivesystemsthatdevelopedovertheSantaTeresaNationalWeatherServiceForcastOffice.Sincetheclimateoftheareaisoftenariditisnotuncommonforittoexperienceflashfloods.However,duringthiseventareaswereaffectedthatwereatleast100kmapartwithina12hourperiod.(Rogashetal2009).

The only national weather service rain gauge in the city is located at the El Pasointernational airport. Itsdatawasusedduring the2006evident for flood recurrence-intervalplanningfortheentirecity.Thefloodduring2006showedthatthiswasinsufficient.Itdidnot

provideusefulflashfloodassessmentsinneighborhoodswithatopographicallycomplexurbanarea,suchasthoselocatednearthewestsideoftheFranklinmountains.(GillandCollins2010).This suggested that orographic affectswerenot properly consideredwhendevelopingurbanplanningintheElPasoarea.

Data

Data for the study gage, EL PASO AT RIO GRANDE was obtained from USGS. NFIEGeodatabasewasusedtodeterminethecharacterizethesub-watershedandbasinswithinthestudy site, as well as to delineate the flowlines. The national land database was used tocharacterizethelandcoverwithinthesub-watershedandbasins.Precipitationdataforthesub-watershed and for the three rain gages within were obtained from National Oceanic andAtmosphericAdministration(NOAA).AlldatawasprocessedusingArcGISpro1.3.12016.

StudySite

ElPasolieswithinthePasodelNortesub-watershed(Figure1).Itcoversanareaof256.3m2. The sub-watershed is comprised of 17HUC 10Units, it extends 340miles along the RioGrandeRiver.Itirrigatesapproximately200,000acresoffarmlandandisimpactedbytheneedofnearly2millionpeople(includingCiudadJuarez).TheNFIEGeodatabasewasusedtoisolatethesub-watershed.WithinthePasoDelNortesub-watershedtherearetwodistinctbasins.TheMesillabasintotheNorthWestofElPasoandtheHuecobasintotheEastandSouth.

Figure 2 was used to evaluate the land cover of the sub-watershed (Figure 2b). Themajorityofthesub-waterhedisshrub,scrub,andgrass(~85%).ElPasoandLasCrucesmakeup6%indevelopedareaandagriculturearoundtheRioGrandemakesupanother6%.Thisaccountsforthe200millionacresoffarmlandthatthesub-watershedprovideswaterfor.

LookingcloseratjustthecityofElPaso(Figure3)itisclearthatmostofElPasoishighlydeveloped.Thishassignificantimpactsonrunoffasadevelopedareahasgreaterrunoff.Thisisparticularly trueof El Pasowere theharddry grounddoesnot absorb toomuchwater. Thiscombinedwiththedevelopedlandlendsitselftofrequentandsevereflashflooding.Theareathat is not highly developed is the Franklin mountains, and even then, there is significantdevelopmentinthelowerregionsofthemountain(Figure4),especiallyinWestElPasowhereamajorityoftheflooddamageoccurred.

Figure1.PasoDelNorteSubwatershed

Figure2.LandcoverageinPasoDelNorteSub-watershed.

Figure2b.PercentageoflandcoveragebytypeforPasodelNortesub-waterhsed.

0.0010.0020.0030.0040.0050.0060.0070.0080.0090.00

%

Landcover

Figure3.ElPasocitylandcoveragedata.

Figure4.10mContourforElPasocityandsurroundingarea.

StudyGage

USGSonlyhadonegagelocatedwithinthestudysitethatwasrecordingdataduringtheevent.Thisgage,RioGrandeatElPasoislocatedwithintheMesillabasin(Figure5)andwasusedtoobtaindischargedatefortheRioGrandeduringtheweekofJuly27-August6th.WithintheMesillabasintheRioGranderuns173kmandthereisatotalstreamlengthof1810km(Figure6).Figure7showsthepeakflowduringJuly27-August6th.Fromthistable, it isclearthatthebiggest impactwas seen fromAugust1-3rd.With the largestpeakonAugust2ndof7000cfs,almosttwoordersofmagnitudehigherthanbaseflowconditions.

Figure5.RioGrandeatElPasostagegaugelocatedintheMesillabasininPasoDelNortesub-watershed.

Figure6.StreamdelineationwithintheMesillabasin.

Figure7.PeakflowfortheRioGrandefromJuly27-August6th.(SourceUSGS)

RainGagesandPrecipitation

NOAAwasusedtoobtainraingagedataandsatelliteprecipitationdataforthearea.ThethreegagesusedforthisanalysiswereStateuniversityatLasCruces,SantaTeresaAirportandELPasoInternationalAirport(Figure8).TheprecipitationdatawasaddedtoArcGISprothenconvertedtoaraster.Thisrasterwasthenusedtoextracttheinformationforjustthesub-watershed.Figure9showstheprogressionofthestormprecipitationoverthesub-watershedduringthesametimeperiodasthestagegage(July27-August6th).Fromtheseimages,youcanseethatitcorrelateswiththeobservedpeakriverdischargefromFigure7,wherethebiggestdischargehappenedonAugust2nd.ThisisalsothedaythatElPasoExperienceditsheaviestrainfallwithsomeareasonthewestsideseeing6inchesofraininasingleday(Figure9,8-02).TheanimatedversionofFigure9islocatedatthefollowinglink:https://drive.google.com/open?id=0B-42Sf917H4SMVZSZ2hlSjV6VzA

Figure8.RaingagelocationwithinPasoDelNortesub-watershed.

Figure9.PrecipitationdataforfromJuly27toAugust6thinPasoDelNortesub-watershed.

FlowDirection

Byanalyzingtheterrain inArcGIS, it ispossibletodeterminetheflowdirectionoftheprecipitationalongwiththeareasthataremostlikelytofloodduringheavystormevents.Figure10showsthepercentagedropoftheElPasoarea.Fromthisfigure,youcanseethattheeastsidehasasharperdropwithlesselevatedarea.Thewestsideincontrasthaslesspercentagedropbuthasalargerareawithdiversetopographiceffects.Whencomparingittofigure4Italsoshowsthat this area is further developed than the east side. Figure 11 shows the flowdirection ofprecipitation using the D8 model along the franklin mountains. Figure 12 can be used todeterminethedirectionofflowonthemountains.Ontheeastside,thedirectionispredominantlytowards1andonthewest,sidethedirection is towards8and16.This indicatesthatontheeastsidetheflowisgenerallytowardsalessurbanizedareathanonthewestside.Again,thisaccountsforthegreaterdamageseenonthewestsideofElPaso.Wecanthenlookatthelowestpointsonthesurroundingareatoseewherewaterwouldtendto localize in(Figure13).Thissuggestthattheseareasaremorepronetoflooding.

Afterthefloodingin2006FEMAreassessedthefloodingpotentialofelPasoandupdateditsfloodszonemapforthearea.ComparingthegeneratedimageinArcGISthatconsidersthetopography,flowlinesandlowestsinksitcanbeseenthatthetwoimagesareverysimilar(Figure14&15).

Figure10.PercentagedropfortheFranklinmountainsinElPasocity.

Figure11.PrecipitationflowdirectionforElPasocityandtheFranklinmountains.

Figure12.D8flowdirectiongrid.

Figure13.10mcontourmapofElPasocityshowingdevelopedareasanddeepestsinks.

Figure14.FlowlinesandsinksinElPasocity.Figure15.FEMAupdatedfloodzonesinElPaso

Summary

Littledatawasgatheredduringthe2006ElPasofloodduetothelackofsufficientraingages,stagerecordersandstreamgagesinthewatershed.Sincetheseverestormevent inElPasoUSGShassetupmorestagegagesandraingagesintheareatobetterassesfloodriskandtobeabletogiveaccurateflashfloodwarning.Weatherstationplacedinkeyareascouldprovideaccuratemeasurementsandappropriatewarningsinordertominimizelossoflifeandproperty.UsingArcGIS,itwaspossibletoseethatwestElPasoislocatedinamoretopographicallycomplexareawithalargerpercentageoflandthatisurbanized.Furthermore,onthewestsidewehavemoreflowlinesandalargerareathataccumulatesrainfall.ThisresultedinthewestofElPasoreceivinggreaterdamagetobusinessesandhomes.ItcouldbepossiblethatorographiceffectswerenotproperlyconsideredduringurbandevelopmentinElPasoTexas.Thisresultedinflashfloodriskbeingunderassessedinspecificneighborhoodsandincreasingthecostofdamage.

References

Collins,T.W.,Jimenez,A.M.,&Grineski,S.E.(2013).Hispanichealthdisparitiesafteraflooddisaster:Resultsofapopulation-basedsurveyofindividualsexperiencinghomesitedamageinElPaso(Texas,USA).JournalofImmigrantandMinorityHealth,15(2),415-426.FederalEmergencyManagementAgency,2007.FloodFrequencyDeterminationElPasoCountyandIncorporatedCommunities,Texas.

National Oceanic and Atmospheric Administration. Climate Data Online Data Tools. Source:https://www.ncdc.noaa.gov/cdo-web/datatoolsRogash,J.,Hardiman,M.,Novlan,D.,Brice,T.,&MacBlain,V.(2009).Meteorologicalaspectsofthe2006ElPasoTexasmetropolitanareafloods.NationalWeatherDigest,33,77-101.ThomasE.GillandTimothyW.Collins,2010.Differential ImpactsofFlashFloodingAcrossthePasodelNorte.SouthwestHydrology.

USGS Current conditions for USGS 08364000 Rio Grande at El Paso.http://nwis.waterdata.usgs.gov/tx/nwis/monthly/?search_site_no=08364000&agency_cd=USGS&referred_module=sw&format=sites_selection_links