Roadsinfloodplains(1)FRANKVANSTEENBERGEN–

FVANSTEENBERGEN@METAMETA.NL

Costsandbenefitsoffloods• Costs:damagetoinfrastructureandhouses,lostlivesandcrops• Benefits:sedimentation providesfertilelands,flushingofstagnantwaterandpollutantsandopportunitiesforfloodbasedfarming

Goal:toadapttotheconstrainsthatfloodsimposeandtakeadvantageoftheirbenefits

Interactionsbetweenroadsandfloods•Roadsfragmenthabitatsandinterrupttheflowofwater,sediments,nutrientsandaquaticlife

• Roadscanactforinstanceas‘reservoirdam’tokeepwaterforirrigationinthedryseason

• Floodscausedamagetoinfrastructureincreasingmaintenancecosts

• Floodingononesideoftheroadanddryingoutontheother,alteringvegetationandassociatedspecies

RoadsinfloodplainsConstruction-Maintenance Drainage characteristics Erosion susceptibility Water Harvesting potential

Elevatedsubgradesandembankmentscanincreasecostsasconstructionmaterialinfloodplainscanbescarce.

Laminarflowsthroughfloodplainsrequirewidedrainagesystems,floodcontrolmechanismssuchasflapgatesorsurfacedrainageoutlets.

Waterloggingonbufferareasbetweenroadembankmentsandfloodplain.

Floodinterception– lessmoisture(sediments)ondownstreamareas.Thiscanhavemajorimpactonfloodplainagriculture

Shallowgroundwater–handdugwellsandmanualdrilledboreholesaswellasdugoutpondsandborrowpits

Roadsasfloodcontrolmechanisms• Roadssubgradesandembankmentsactasdikes,manyroadsintheNetherlandsarelaidonthetopofdikes.

• Inareaspronetoperiodicalfloods– roadsmayserveasfloodregulators.

• Roadslaidinfloodplainsoftenblockfloodflowfrontsdiminishingfloodareas.

• Thisscenariocouldberevertedifroaddesignprinciplesaremodified.

• Roadmaybeopentofloodflowsenabling“floodcorridors”includingbridgesandlongculverts,flapgatesandfusingsluicegates.

• Thusroadscanactasfloodregulators,controllingfloodanddrainagepatterns.

Roaddesigninfloodplains• Planninganddesignshouldtakeintoaccountthelocalfloodflowpatterns• Assesstheexistinghydraulicbehaviourofdrainagepaths,waterwaysandfloodplains,withparticularreferencetoknownandpotentialfloodlevelsinthevicinityoftheroadalternatives.• Identifyandassessthepotentialshort- andlong-termimpactsofconstructionandoperationonthequantityandqualityofsurfacerunoff,floodplaininundationandwaterwaywaterquality.

Damagetoruralroadbyflood

Importantquestionstobeaddressedduringplanningphase•Willtheprojectaffectanexistingwatercourseorfloodplain?

•Willtheprojectchangeeithertheroaddrainageornaturallanddrainagecatchments?

•Willtheprojectchangethenumberortypeofjunctions?

•Willearthworksresultinsedimentbeingcarriedtowatercourses?

•Willtheprojectallowdrainagedischargestotheground?

Roadembankmentgeometry•TherecommendedcrestlevelforNationalandProvincialroadsisthehighestrecordedfloodlevelplus0.5meters.Forregionalroadsthecrestlevelshouldcorrespondwithaminimumheightofthewaterleveloffloodswitharecurrenceof10yearsplus0.25meters.

• Forroadembankmentsupto4metershighaslopegradientof1:3providessufficientsafetyprotectionagainstthemacro-instabilitymechanismduringtheriseandfallofthewaterlevel.

•Investigatethegeotechnicalcharacteristicsofthetopsoilsandtakeadequatemeasuresinroaddesign,forexampleremovalofinappropriatetopsoils.

Resistancevsresiliencestrategies• Resistancestrategy:embankmentsandroadsareconstructedtoprotectrextractareasfromtheinfluenceoffloods

• Disadvantages:o pressureonthestructurecancausedamageandhighmaintenancecosts

o floodplainhydraulicsaredisrupted,whichnegativelyimpactsonthefloodplainecosystem

o waterqualityproblemsbecausedirtandpollutedwaterarenolongerremovedbythefloodwaters

Resistancevsresiliencestrategies• Resiliencestrategy:minimizingtheconsequencesoffloods,butatthesamemaintainthenaturalfloodplaindynamicsasmuchaspossible.

•Mightrequirehigherinitialinvestment,butlongertermcostsintermsofroaddamageandecologicalimpactswillbelower.

• Importanttolocatebridgesandculvertsatthebestpossiblelocationsandrecommendsolutionsthathaveonlysmalladditionalconstructioncosts.

Waterspreadingweirs• Exampleofresiliencestrategy

• Temporaryfloodsareroutedoutofthedryriverbedssoastoinundatethesurroundingarea

• Therivercrossingsaswellasembankedroadsleadingtothemactasfloodspreaders

• Dropstructuresandcrossdrainageisprovidedsoastoensuretheirstability

Exampleofresilienceapproach:RoomfortheRiverintheNetherlands• Createssafetyagainstextremeriverfloodsbywideningrivercrosssectionstolowerfloodlevels.

• Situatingthedikesfurtherawayfromtheriver,orloweringtheriverforelands,toreducevelocitiesandwaterlevelsbyprovidingspaceforthewatercourse.

•Requiresproperlydesignedflow-throughstructuresthatcanwithstandthehighflowvelocities.

Approach Pros Cons

Resistancestrategy • Betterprotectionagainstfloods• Reductiondamageinhighdensity• areas

• Fragmentationoffloodplainsandhydraulicchangesandimpactonflood-relatedfunctions

• Downstreamimpacts• Moreexpensivetoprotectroads

againstdamage

Resiliencestrategy • Lessfragmentationfloodplainsandhydraulicchanges

• Lessdamagetoroads• Long-termbenefitstoboth

financialinvestmentindevelopmentandbiodiversityconservation

• Increasedawarenessofdangersoffloodplainliving

• Morecostlyduetoconstructionthrough-flowstructures

• Reducedaccess(lowerroads)• Moreneedforintegratedplanning

andmanagement

Integratedapproach• Floodmitigationmeasuresupstreamsuchasprotectiondikes,drainagefromprotectedareas(irrigationschemes,cities,towns)orflushingoffloodwaterfromreservoirswillaffect floodconditionsdownstream.

• Land-usedevelopmentandconstructionofroadsandthrough-flowstructureschangethenaturalfloodflowpatternsandincreasefloodriskstoadjacentareasaswell.

Conclusions• Roaddevelopmentsinafloodplainsystem,requireadifferentapproachtoplanningandtechnicaldesign,comparedtoroaddevelopmentsinareasthatarenot(regularly)inundated

• Needforharmonisationofstandardsandguidelinesandinclusionofthespecificsofdevelopingroadsinadynamicfloodplainsystem

• Searchforawin-winsolutionwhererobusttechnicaldesignsleadtobenefitsforvariousconstructioncost,infrastructuremaintenance,ecology,naturalresourcesandvulnerabilityofpopulation.

Designrecommendations1. Tailoralignmentanddesignsolutionstothespecificfloodplainhydraulicand

ecologicalsituation

2. Thenumberanddimensionsofflowthroughopenings(bridgesandculverts)shouldbesuchthatinterferencewiththenaturalhydraulicsofthefloodplainintermsofextentanddurationisminimal

3. Scourprotectionnearbridgesandotherflow-through openingsinordertopreventdamagetotheabutmentsandeventuallythestructureitself

Recommendationsfortechnicaldesignguidelines1. Updateandreviewthedesignstandardsandguidelinessoroaddesigners

havebetterguidanceandbest-practiceexamplesofhowtodevelopfloodproofedandenvironmentallyfriendlyroads

2. Incorporateahydraulicanalysisordeterminationofthefloodhydraulicsandloadsonroadstructures

3. Makecostsanalysisforthedifferentoptionsofslopeprotection,costsofflowthroughstructures(bridgesandculvertsetc)andquantifytheoptionsintheintegratedapproach

Researchandcapacitybuilding• Improveknowledgeofthefloodplainsystemintermsofinteractionsbetweenfloodplainhydraulicsandbasindevelopments,functionsofthesystem,criticalthresholdstomaintainthesefunctionsandvaluesofthefunctions.

• Investineducation,trainingandtechnicalsupporttointroduceand/orstrengthenpracticeofintegratedplanningofroaddevelopmentandrehabilitation.

Generalrecommendations• Applyanintegratedplanningapproachthatconsiderstheconsequencesofthedevelopmentthroughoutthesystemincludingenvironmentalandsocialimpacts.

• Applyafloodplainsystem’sapproachinwhichnotonlylocalimpactsofroadsbutalsoregionalandcumulativeimpactsareconsidered.

• Providesufficientcoordinationbetweenroaddevelopmentandrehabilitationplanningandothersectorplanning.

• Assesspossibletransboundary (provincial,national,international)impactsofroaddevelopmentandrehabilitation