May2013

TheFloodwayEncroachmentStandard:MinimizingCumulativeAdverseImpacts

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Acknowledgements

Alan R. Lulloff, P.E., CFM, Science Services Program Director - Association of State Floodplain Managers is the primary author of this report. DAS Engineering LLC conducted the analysis of the HEC-RAS hydraulic engineering models. Samuel Johnson developed the maps included in the report and provided editorial assistance.

The following members provided helpful comments on initial drafts of this publication: Larry Larson, Dave Carlton, Sally McConkey, Ed Thomas, Sivash Beik, Tim Trautman, and Dave Knipe.

Cover Photographs Top: Sept. 8, 2011. Rain associated with Tropical Storm Lee flooded a development that was allowed to encroach into the floodway of Lycoming Creek in Run, Pennsylvania. (AP Photo/Williamsport Sun-Gazette, Mark Nance)

Bottom Left: Nov. 6, 2006. A house swept away by flood waters being conveyed down the floodway of the Cowlitz River near Packwood, Washington in the aftermath of a Pineapple Express storm event. (The Seattle Times/Lewis County Sheriff's Office)

Bottom Right: Aug. 28, 2011. Emergency personnel search for anyone who may be occupying a building in the floodway of Ola Creek in Waitsfield, Vermont. Rain associated with Tropical Storm Irene caused floodwaters to rise and caused massive flooding across the region. (AP Photo/Sandy Macys)

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TableofContents

Acknowledgements .............................................................................................................. iIntroduction ......................................................................................................................... 1Background ......................................................................................................................... 1More restrictive State and local standards for mapping floodways .................................... 3National Flood Insurance Act of 1968 ................................................................................ 4

NFIA promotes floodplain land use management .......................................................... 4NFIA addresses cumulative impacts ............................................................................... 4Base Flood Elevations not increased to reflect surcharge amount ................................. 4SFHAs include wider floodways in States with higher Standards ................................. 5

Impact Assessment.............................................................................................................. 5Impact on the physical characteristics of the floodway .................................................. 5

Methodology ............................................................................................................... 5Results ......................................................................................................................... 6

Impact on flood damages ................................................................................................ 8Impact on existing and new development built at or above the BFE ......................... 8Impact on existing development constructed below the BFE ..................................... 9

Impact on floodplain natural resources ........................................................................... 9Discussion ......................................................................................................................... 11Summary ........................................................................................................................... 12References ......................................................................................................................... 13Appendix A - Mitigating the Impact ................................................................................. 14

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Introduction

Smallencroachmentsintoafloodplaininandofthemselvesmayhaveanegligibleimpactonfloodelevations.However,thecombined,incrementaleffectsofhumanactivity,referredtoas cumulative impacts, can cause significant increases in flooding. The National FloodInsuranceProgramattemptstoaddressthecumulativeimpactsofencroachmentsintothefloodplain through the use of a regulatory floodway. Federal minimum standards allowfloodways to be developed based on the concept of allowing some encroachments butlimitingtheincreaseinfloodelevationscausedbytheseencroachmentstoonefoot.Thisreportreviewstheorigin,useandestablishmentofregulatoryfloodwaysandprovidesanassessmentoftheimpactofencroachmentsbaseduponallowingaonefootincreaseinfloodelevationsduringtheregulatoryflood.Theassessmentevaluatedimpacton:

1. thephysicalcharacteristicsofthefloodway(floodwaywidth,watervelocityandareainundated),

2. damagestodevelopmentinthefloodplainand3. floodplainnaturalresources.

ThereportalsoincludesanappendixthathighlightsmeasuresStateandlocalgovernmentshaveincludedintheirfloodplainmanagementregulationstolessenthecumulativeimpactofencroachmentsintothefloodplain.

BackgroundEveryone lives in awatershed, that is, the land area that collects and feedswater into awaterway. Within these watersheds are areas that become inundated following heavyrainstormsorsnowmelt.Theseareas,calledthefloodplain,holdtheexcesswaterandallowit to be slowly released into the river system, to seep into groundwater aquifers, and betakenupbyplants.Floodplainsalsoofferaplaceforsedimenttosettleoutoffloodwaters,therebykeepingitoutofwaterwaysandcreatingfertilefloodplainvalleys. In the natural setting and in the absence of man, flooding occurs but there is no floodhazard. It isonlywiththeintroductionofhumanencroachmentintothefloodplainthatahazardisestablishedandtheproblemofflooddamagearises.Asmanmadedevelopmentisintroduced into the natural floodplain, it can become an obstruction in addition to beingvulnerable to flood damage. It may so encroach upon the watercourse as to retard itscapacitytopassfloodflows.Theimpactsonneighboringresidentsinacommunityandonanyadjacentcommunitiesareanincreaseinfloodcrestsandwiderareasbeingflooded.Historically disaster responsewas a State and local government responsibility. Followingseveralmajorflooddisastersintheearly1900stheU.S.CongressrecognizedthethreattolivesandpropertycausedbyunwisefloodplaindevelopmentandpassedtheFloodControlAct of 1936. The act directed federal efforts toward reduction of such losses by placingemphasisoncontroloffloodwatersthroughtheuseofstructuralworks.

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However,majordisastersfromfloodingcontinued.Asaresult,therecamearealizationthatflood damage reduction programs needed to include not only levees, dams and otherprotectivestructurestocorrectexistingproblems,butalsomeasureswhichwouldpreventuncontrolleddevelopmentintofloodplains.The use of floodplain regulations to supplement flood control structures to reduce flooddamagepotentialandencouragewiseuseofthefloodplainwasinitiatedinthe1950sbytheTennesseeValleyAuthority(TVA)1.ThefirstfloodplainmapsintheU.S.weredevelopedbythe TVA in 1953. (Goddard, 1978) The floodplain regulations and associatedmapswereintendedtoencouragesoundlanduseinthefloodplain.Whendeveloping floodplainmaps, theTennesseeValleyAuthority divided the floodplain into the floodway (theportion of the floodplain with flowing water) and theflood fringe (backwater areas). The TVAs firstconsideration in designating floodways was fullconveyance floodways. All of the area inundated by theselectedflood(e.g.onepercentannualchanceflood)wasto be included, except those shallow areas andembaymentsintosmalldrainsorgullieswheretherewasponding but little if any flow (see Figure 2). In otherwords,themappedfloodwaywouldcomprisethosepartsofthefloodplainthathavemovingfloodwaters.However, the TVA received opposition on this concept.Issues associated with existing land use, developmentexpectations, and the physical constraints presented bysteepslopesoutsidethefloodplainintheTennesseeRivervalley prompted the TVA to adopt a less conservativeapproach.AccordingtoGoddard:The floodway was to be the channel and that portion of adjacent floodplainsnecessarytocarrytheselectedfloodwithoutincreasingfloodelevationssignificantly.Bygeneralacceptanceamongprofessionals significantlyhadcometobeconsiderednomorethanone(1)foot.Insteadofmappingfullconveyance floodwaysTVAmappednarrower floodways inwhichonefootofincreasedflooding(alsocalledsurcharge)wasallowed.Itwastobeaminimum 1 The Tennessee Valley Authority is a federally owned corporation in the United States created by congressional charter in May 1933 to provide navigation, flood control, electricity generation, fertilizer manufacturing, and economic development in the Tennessee Valley.

Figure 2 Shallow areas that do not effectively convey flood waters not included in the floodway.

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criterion intended as a regional standard, recognizing that there were urbanizing areaswhere the existing development, physical conditions or other elements might demand amorestringentevaluationandamuchsmallerrisemightbeconsideredmoreappropriate.(Goddard,1978)

MorerestrictiveStateandlocalstandardsformappingfloodwaysOnthispoint,theGoddardreportincludessummariesofinterviewswithStatesthatdidnotdeemtheonefootof increasedfloodingappropriate.ThereportlistsatotalofnineStatesthatbythemid1970shaddevelopedStatespecificminimumstandardsrelatedtomappingfloodways.Intheinterviewssummarizedinthereport,StatestaffprovidedtherationaleforthestandardtheStatehadadopted.Illinois interpreted significantly to mean anything greater than zero, but its practicalinterpretation is 0.1 foot for computer purposes2. Its rationale was that the overbankfloodplainofmostofthestreamsintheStateisquiteflat.Asmallincreaseinthefloodprofilecansignificantlyexpandthewidthofthefloodplain.Itseemedunreasonableeconomicallytoallowany significant increase in the flood stage that subjectspreviously safe structures tofloodwaters.Indianaalsoadopted0.1footasthemaximumsurchargeallowed,andindicatedtherearefew topographic restraints on development in Indiana, so there is no real need to viewfloodplainsastheonlydevelopablearea.The responses from Illinois and Indiana illustrate the opinion that allowing increases inflooding was unreasonable and unnecessary. The other States with a more stringentcriterionlistedintheGoddardreportwereMaryland,Michigan,Minnesota,Montana,NewJersey,OhioandWisconsin.Today States that have standards more stringent than the federal onefoot surchargecriterion includeWisconsin(0.01), Illinois (0.1), Indiana(0.1),Michigan(0.1),NewJersey(0.2),Minnesota(0.5),Montana(0.5),andColorado(0.5)wherethenumberinparenthesesis the increase in flood elevation (in feet) allowed. The first four States have adoptedfloodway standards intended to limit the impact to ameasurable amount. The remainingfour have adopted standards that are more than measurable amounts but are a lessersurchargethanthefederalonefootstandard.

2 The most common hydraulic engineering model being used at the time was the U.S. Army Corps of Engineers Hydraulic Engineering Center (HEC) HEC-2 water surface profile model. The output of the model provided calculated flood elevations to the nearest 0.1 foot. A number of States adopted a 0.1 foot standard with the intent of not allowing a measurable increase.

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NationalFloodInsuranceActof1968

NFIApromotesfloodplainlandusemanagementInanattempttoreducedevelopmentinhighriskfloodareas,CongresspassedtheNationalFloodInsuranceAct(NFIA)in1968.TheActcreatedtheNationalFloodInsuranceProgram,presently administered by the Federal EmergencyManagement Agency (FEMA). The Acthadthreemaingoals:tobetterindemnifyindividualsforfloodlossesthroughinsurance;toreduce future flood damages through State and community floodplain managementregulations;andtoreduceFederalexpendituresfordisasterassistanceandfloodcontrol.Akey provision of the 1968 Act prohibits FEMA from providing flood insurance in acommunity unless the community adopts the flood elevations and flood hazard maps3developed by the Federal government and enforces floodplain regulations that meet orexceedthosestatedintheAct.NFIAaddressescumulativeimpactsTheNationalFloodInsuranceActalsoincludesaprovisionintendedtolimitthecumulativeimpactsofencroachments into the floodplain.CommunitiesareprovidedFlood InsuranceRateMaps(FIRMs)thatcanincludearegulatoryfloodway.Section59.1oftheNFIAdefinesthe regulatory floodwayas the channelof a riverorotherwatercourseand theadjacentlandareasthatmustbereservedinordertodischargethebasefloodwithoutcumulativelyincreasing the water surface elevation more than a designated height. The designatedheightisestablishedintheCodeofFederalRegulations(CFR)Title44,Part60,Section60.3(d)(2)asbeingnomorethanone(1)footatanypoint.When floodways are not mapped, communities must require landowners submittingproposalstoanalyzetheimpactoftheirproposedencroachmentonconveyancetoensuretheonefootsurchargecriterion isnotexceeded.Withamappedfloodwaythisanalysis isconductedaspartoftheFloodInsuranceStudy,thusnoadditionalanalysisregardinglossofconveyanceisneededwhendevelopmentinthefloodfringeisproposed.

BaseFloodElevationsnotincreasedtoreflectsurchargeamountTheprimarypurposeofFloodInsuranceRateMaps(FIRMs)istoratethefloodinsurancepremiums for existing development at risk. FIRMs also identify where communities canallownewdevelopmentnotyetconstructed.FEMAspositionisthatthemapsmustreflectconditions existing at the time the studywas conducted, not projected future conditions 3 The base flood adopted by the NFIP is the onepercent annual chance flood. The base flood, which is alsoinformallyreferredtoasthe100yearflood,isthenationalstandardusedbytheNFIPandallFederalagenciesforthepurposesofrequiringthepurchaseof floodinsuranceandregulatingnewdevelopment.Thelandareacoveredbythefloodwatersofthebasefloodisthebasefloodplain.OnNFIPmaps,thebasefloodplainiscalledthe Special Flood Hazard Area (SFHA). The SFHA is the area where the NFIPs floodplain managementregulationsmust be enforcedby the community as a conditionof participation in theNFIP and iswhere themandatoryfloodinsurancepurchaserequirementapplies.

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thatcouldbeprojected.Consequently,basefloodelevations(BFEs)reflectedontheFIRMSdo not reflect the impact of encroachments within the floodway fringe. This is due toconcernsthatitwouldbeinappropriatetorequireownersofexistingdevelopmenttopayflood insurance premiums associatedwith a future risk thatmay nevermaterialize. TheBFEsthatareestablishedreflectexistingdevelopmentconditionswithintheSpecialFloodHazard Area, not the flood elevations that will result when the new development in theSFHAoccursandthefloodfringeisfilled.ThusFIRMsaredualpurposemaps,usedbothfor:

1. establishingfloodinsuranceratesforexistingdevelopmentatrisk,and2. guidingnewdevelopmentsothatitisreasonablysafefromflooding.

FEMAdoesnotgenerateaseparatemap for insuranceandone formanaging future floodrisk.SFHAsincludewiderfloodwaysinStateswithhigherStandardsSection60.1(d)oftheNFIPregulationsstatesthatanyregulationsadoptedbyaStateoracommunitywhich aremore restrictive. . . are encouraged and shall take precedence (overnationalminimumstandards).IfaStatehasestablishedmorestringentregulationsforamaximumallowableriseinwatersurfaceelevationsthroughlegallyenforceablestatutesorregulations,FEMAsupportsthesehigherstandardsbyusingthemincomputingregulatoryfloodwaysinthatState.However,while some communities in the remaining 42 states have established and enforce morestringentstandardsassociatedwithfloodways,theNFIPmapsforsuchcommunitiesusuallydepictregulatoryfloodwaysbasedonthestandardonefootsurcharge.(FEMA2009)ImpactAssessmentTheassessmentpresentedinthisreportfocusesontheimpactsofestablishingregulatoryfloodwaysbasedontheonefootsurchargecriterion.Theassessmentevaluatedimpacton:1) the physical characteristics of the floodway (floodwaywidth, water velocity and areainundated), 2) damages to development in the floodplain and 3) floodplain naturalresources. Impactonthephysicalcharacteristicsofthefloodway

Methodology

For this assessment, the engineering modeling and floodway mapping for eight riversegments insixstateswereanalyzed todetermine thecumulative impactsofestablishingregulatoryfloodwaysbasedontheonefootsurchargecriterion.ThestreamswerechosenbasedontheavailabilityofupdatedHECRAS4models,withthegoalofincludingstreamsof 4 HEC-RAS is an engineering modeling package developed by the U.S. Army Corps of Engineers. The letters stand for Hydrologic Engineering Center River Analysis System.

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different sizes and gradients from various U.S. regions. The characteristics of these eightstreamsareincludedinTable1below.

Streamsselectedforcasestudies

RiverSystemDrainage

Area(SqMi)Discharge(CFS)

StreamGradient(Ft/Mi)

ReachLength(Mi)

NumberofCrossSections

PineCreek(WI) 15.1 2,284 6.9 1.98 46PattersonCreek(WA) 21.6 820 4.2 9.18 74StevensBranch(VT) 66.8 14,790 23.6 15.6 199SugarRiver(WI) 46.6 2,335 1.4 3.85 29FourMileCreek(NC) 18.8 4,750 8.3 4.55 60CypressCreek100(TX) 110.2 27,258 2.7 7.06 35CypressCreek172(TX) 7.9 2,585 9.3 4.48 25PlumCreek(CO) 319 38,720 37.6 6.79 65

Table1StudyReachCharacteristics.The hydraulic models associated with these streams were analyzed to quantify thecumulativeimpactofallowingencroachmentsthatcauseone(1)footofincreasedflooding.The regulatorymodel for each streamwasmodified to create two hydraulic engineeringmodelsforeachstudyarea.Onemodelreflectsafullconveyancefloodway.Theothermodelhasencroachmentstationsinsertedthatreducethewidthofthefloodwaybasedupontheonefootsurchargethreshold.

Results

Amountoffloodwaywidthreductionandvelocityincrease

RiverSystem

Avg.Decreaseinfloodwaywidth(%)

Velocityw/oencroachments

(ft/sec)

Velocityw/encroachments

(ft/sec)

Averagevelocityincrease(%)

PineCreek(WI) 59 2.28 3.69 62PattersonCr(WA) 68 1.11 1.61 45StevensBranch(VT) 39 4.98 5.82 18SugarRiver(WI) 50 1.57 2.07 32FourMileCreek(NC) 43 2.58 3.25 26CypressCr100(TX) 48 1.92 2.46 22CypressCr172(TX) 64 1.18 1.71 45PlumCreek(CO) 32 7.15 8.31 16Average 50 2.85 3.62 33

Table2ComparisonofFullConveyanceandOneFootSurchargeFloodways.

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Encroachmentsessentiallypinchinthesidesofthefloodplainandforcemorewatertowardthe center of the stream. Thismakes the area available to convey floodwaters narrower.Sincethevolumeoffloodwaterpassingthroughthisarearemainsunchanged,constrictingfloodwatersthroughasmallerarearaisesfloodelevationsandincreasesthevelocityofthefloodwaterspassingdownstream.Table2on thepreviouspage showshowmuch the fullconveyancefloodwayisnarrowedandhowmuchthevelocityisincreased.On average, in evaluating the stream conveyance, encroachments that caused one foot ofincreasedflooding:

reducethewidthofthecrosssectionavailablefortheconveyanceoffloodwatersonaveragebyonehalf(50%)and

increasethefloodwatervelocitiesonaverageby(33%).

Increasingtheflowvelocitycanbeproblematicinthatitincreasesturbulenceandthereforeincreases thestream'sability toerode.Aswatervelocity increasessodoes thesizeof thelargest pieceof sediment that floodwaters can carry.By increasing the velocity ofwatermovinginthechannel,theflowingwatercanscourthestreambedanddeepenthechannel.Thismeansthebanksarehigherandoftenmoreunstableresultinginincreasedstreambankerosionandmoresedimententeringthestream.Increasedsedimentationmakesitdifficultforsomefishtofeedandspawn,andtheincreasedvelocityofthestreamdrivesoutfishthatcannottoleratefastmovingwater.As the floodway is constricted and flood elevations rise the area inundated by the sameflood event increases. For two of the streams analyzed topographic data was availablewhichallowedthegenerationoftwosetsforfloodinundationmaps.ThepercentofincreaseintheareainundatedisshowninTable3below.

RiverSystem Increaseinareainundated(%)PineCreek 6SugarRiver 14Average 10Table3Comparisonofareainundatedfortwoscenarios

Twostreamsdonotprovideenoughdatatoextrapolatethenumbersbuttheydohighlightthat with onefoot surcharge floodways people that build structures just outside thefloodplainwill likely find those structures impacted as the allowable encroachments intothefloodplainoccur.

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ImpactonflooddamagesTheU.S.ArmyCorpsofEngineers5hasdevelopedflooddepthdamagerelationshipsbasedonreviewsofdamagesassociatedwithhistoricfloodeventsforthemostcommontypesofbuildingconstruction.Figure3belowistheflooddepthtodamagerelationshipforasinglefamilyresidentialstructurewithnobasement(acommontypeofresidentialconstruction).

Impact on existing and new development built at or above the BFE

As indicatedpreviously,whileNFIP regulatory floodway limits the cumulative impacts ofencroachments to one foot, the associated base flood elevations (BFEs) are not raised toreflectthatincrease.StructuresconstructedattheBFEcouldexpecttoexperienceincreasedflooding as newdevelopment is constructed in the floodway fringe. Table 4 (on the nextpage)isderivedfromfigure3toprovidetheincrementalandcumulativeincreasesinflooddamageassociatedwitheachadditionalfootofflooding.

5 The U.S. Army Corps of Engineers has collected and analyzed actual flood damages to various types of properties and developed flood depth to damage curves as part of their HEC-FDA (Hydrologic Engineering Center Flood Damage Analysis) program.

RESIDENTIAL STRUCTURE DEPTH DAMAGE 1 STORY NO BASEMENT

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Water Depth From Ground Level

Structure Damage %

Figure 3 Flood Depth-Damage Curve for a Single-Story Residential Structure with No Basement. (Source: U.S. Army Corps of Engineers Floodplain Inventory Tool (CEFIT))

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Accordingtotable3, inthecaseofasinglefamilystructurewithnobasementbuilttotheBFEflooddamageswouldincreaseto23.3%ofthevalueofthestructurewhenthereisonefootoffloodingabovethefirstfloorofthestructure.

Table4AmtofdamagesperonefootincrementofincreasedfloodingImpact on existing development constructed below the BFE

ExistingdevelopmentconstructedbelowtheBFEisbydefinitionmorevulnerabletoflooddamages.Anadditionalfootoffloodingwillincreasethedamagesincurred.Asanexampleforaonestorystructurewithnobasementwheretheamountoffloodingincreasesfrom4to5feetduringabasefloodeventthedamageswouldincreasefrom47%to53%.Thereisalsothepotentialforotherimpacts,whichinclude:

Limitationofaccesstofloodedproperties.Themaximumdepththroughwhichmostvehiclescansafelydriveistwofeet.Therefore,increasingflooddepthsfrom1.5feetto2.5 feet couldmeanahomeownermaynotbeable to safely reachhighgroundduring a flood event. In addition, emergency vehicles could be prevented fromrespondingtoahousefireormedicalemergency.

Possiblecompromiseoffloodproofingmeasures.Homeownerscanattimespreventtheirstructuresfrombeinginundatedifthreatenedbyonetotwofeetoffloodingbyusingsandbags.Anadditionalfootormoreoffloodingcouldoverwhelmasandbageffortordryflooding.

ImpactonfloodplainnaturalresourcesDespiterepresentinganaverageoflessthantwopercentofthesurfaceareaofwatersheds,floodplains provide 25 percent of all the services and benefits to terrestrial ecosystems.(Opperman,Lusteret.al.2010)

Flood Damages associated with Flood Depth One Story Residence With No Basement

Depth

Incremental Increase in Flood Damages

Cumulative Increase in Flood Damages

-2 0.0% 0%-1 2.5% 2.5%0 10.9% 13.4%1 9.9% 23.3%2 8.8% 32.1%3 8.0% 40.1%4 7.0% 47.1%5 5.9% 53.2% 6 5.4% 58.6%7 4.8% 63.2%8 4.0% 67.2%9 3.3% 70.5%10 2.8% 73.2%

0 0%

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OneofthestreamsanalyzedforthisassessmentwastheSugarRiverinsouthernWisconsin.For this case study, two floodways were mapped. One floodway map was based on thefederalminimumstandardof one (1) foot of allowable increase in floodingdue to lossofconveyance.Theotherwasthefullconveyancefloodway.In order to assess the impact of cumulative allowable encroachments on ecosystemservices based on the federal onefoot surcharge standard, the wetlands component ofecosystem services was evaluated for this stream segment. Wetlands mapping wascomparedwiththetwodifferentfloodwaymapsdevelopedfortheSugarRivercasestudyinDaneCounty,Wisconsin(seeFigure4).Results:ThetotalacresofwetlandslyingwithinthetwoSpecialFloodHazardAreasforthistwomilesegmentoftheSugarRiverwere:

SFHAwithoutencroachments(95acres),and Onefootsurchargeregulatoryfloodway(58acres).

Basedonthisanalysis,37acresofwetlands(18.5acres/streammile)wouldpotentiallybeimpactedifencroachmentswereallowedintothefullconveyancefloodwaytothelimitofaonefoot flooding increasedueto lossofconveyance.Whenusingaonefootsurchargeformapping the floodway, these 37 acres would be identified as flood fringe, wheredevelopment is generally allowed. Note: This does not necessarily apply to all stream

Figure 4 Sugar River, Dane County, WI floodplains and wetlands.

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segmentsinawatershedorallwatershedsinthecountry.Wetlandsmoreoftenoccuralongstream segments in the lower portion of a watershed. In the arid west there are fewerwetlandsthanintheeastandmidwest.Intheseareas,othertypesofhabitat(e.g.floodplainforests)mayinsteadbetheresourceimpacted.

DiscussionGoddards1978reportstatesthatthe floodwaywastobethechannelandthatportionofadjacentfloodplainsnecessarytocarrytheselectedfloodwithoutincreasingfloodelevationssignificantly.Further,Bygeneralacceptanceamongprofessionals [intheTennesseeValleyatthattime]significantlyhadcometobeconsiderednomorethanonefoot.According to Goddard, the number one (1) did not suggest an accuracy or degree ofguidance that a fractionor fractionsof a footmight connote. It related,practically, to theengineeringjudgmentappliedinhydrologicandhydrauliccomputations.Unfortunately, what Goddard failed to recognize is the difference between absolute andrelativeaccuracy. It is indeedcorrect that floodplainstudieshaveadegreeofuncertainty,especially in early studies. However, what is being addressed when assessing proposedencroachments isnot theabsoluteaccuracyof thestudybut therelative increase in floodelevations that an encroachment into the floodplain will cause. This increase can bemeasuredwithagooddealofprecisionandisrelatedmoretothesurveyingmethodsusedtodeterminetheamountoftheencroachmentthantotheengineeringjudgmentassociatedwiththestudymethodology.Surveystandardsaresuchthatmeasurementstowithinafewcentimeters(aboutoneinch)arecommon.Asindicatedearlier,theStateofIllinoisinterpretedsignificantlytomeananythinggreaterthan zero, but its practical interpretation is 0.1 foot for computer purposes. The mostcommonhydraulic engineeringmodel beingused at the timewas theU.S.ArmyCorpsofEngineersHydraulicEngineeringCenterHEC2watersurfaceprofilemodel.Theoutputofthemodelprovidedcalculatedfloodelevationstothenearest tenthofafoot.AnumberofStates (Illinois, Indiana, Michigan and Wisconsin) adopted a 0.1 foot standard with theintentofnotallowingameasurableincrease.Inthe1980stheU.S.ArmyCorpsofEngineersmodified the engineeringmodelingprogram so that the output provided calculated floodelevationtothenearesthundredthofafoot(0.01foot).Consequently,Wisconsinmodifieditsstandardtoallownomorethan0.01foottomaintainitsrestrictiononanyencroachmentthatwouldcauseameasurableincreaseinflooding.The MerriamWebster definition of the word significant is having or likely to have aninfluence or effect; also: of a noticeably or measurably large amount. This assessmentdemonstrates that the one (1) foot surcharge standard does indeed have a significantimpact.Itdecreasesthecrosssectionalareaavailablefortheconveyanceoffloodwatersonaveragebyonehalf. It increases flowvelocitiesbyonethird.Whenevaluating the impacton flood damages to structures, the Corps of Engineers flood depthdamage curvesdemonstratethatstructuresbuilttotheBFEcansustainsignificantdamagesthatamounttoover20%ofthevalueofthestructure.

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Summary

Thecombined,incrementaleffectsofhumanactivityinthefloodplain,knownascumulativeimpacts,cansignificantlyincreasefloodriskandflooddamagesandposeaseriousthreattothe environment. While most encroachments are seemingly insignificant in and ofthemselves, numerous encroachments from one or more sources can have significantcumulativeimpactsovertime.Itisimportantthatthecumulativeimpactsofencroachmentsintothefloodplainbeaddressedwhenpermittingnewdevelopmentinthefloodplain.FEMAdefinestheregulatoryfloodwayasthechannelofariverorotherwatercourseandtheadjacent land areas that must be reserved in order to discharge the base flood withoutcumulatively [emphasis added] increasing the water surface elevation more than adesignatedheight.Reservingaportionofthefloodplainforthedischargeoffloodwatersisin and of itself a good thing. Doing so not only ensures that encroachments into thefloodplain do not cause increased flooding, but it helps keep human development out ofareaswherestreamvelocity,flooddepthsandcurrentscanbeathreattolifeandsafety.While the federal standarddoesestablishanupper limit, setting thedesignatedheightatone (1) foot perpetuates an upward trend of increased flood damages. Allowing thedesignatedheighttobeanythinggreaterthanzeroisproblematic.This typicalmethod of establishing a floodway implicitly allows levees to be constructedalonganentirereachofariverattheouterlimitsoftheregulatoryfloodway;allowspartsofthe natural conveyance and storage to be filled; and allows any type of residential ornonresidentialdevelopmentaslongasfloodelevationsareatorabovetheBFE.(AIR,2007)Impactsassociatedwiththefederalminimumstandardformappingfloodwaysbasedupontheonefootrisecriterioninclude:

thatnewdevelopmentisallowedwithintheSpecialFloodHazardAreathatwillincreasefloodingonexistingdevelopment,

BFEsarenotincreasedtoavoidnewdevelopmentalsobeingplacedatrisk,and encroachmentsareallowedthatcanbedetrimentaltothenaturalandbeneficial

functionsofthefloodplain.Not all streams in theU.S. currently havemapped Special FloodHazard Areas. PresentlytherearealittleoveramillionmilesofstreamswithmappedSFHAsandanother2millionmilesintheNationalHydrographyDatasetwithnomappedSFHA.OfthemillionplusmilesofstreamswithmappedSFHAsabout20%includemappedfloodways.(ASFPM2013)However, automated modeling and mapping tools are making floodplain studies lessexpensive.Opportunitiesexisttogreatlyincreasetheamountofmappedfloodwaysoverthenext decade. Adopting a standard that reduces the allowable cumulative impacts ofencroachments into the floodplain while holding accountable those parties that causeimpacts could help reverse the upward trend in flood damages and disaster costs andimprovetheeconomicresiliencyofcommunitiesthroughouttheU.S.Appendix A is a summary of practices that have been implemented by States andcommunitiesdemonstratingmeasuresthatcanbetakentoreducethecumulativeimpactofencroachmentsandholdthoseresponsibleaccountable. .

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References

AmericanInstitutesforResearch.2007.EvaluationoftheNationalFloodInsuranceProgram.ASFPM. 2013. FloodMapping for theNation A CostAnalysis for theNations FloodMapInventory.Federal Emergency Management Agency. 1978. The Floodway: A Guide for CommunityPermitOfficials.CommunityAssistanceSeriesNo.4.FederalEmergencyManagementAgency.2009.GuideforCommunityOfficials.Goddard,James,et.al.1978.OriginandRationaleofCriterionUsedinDesignatingFloodways.MecklenburgCounty,NorthCarolina.ConsiderationofUnsteadyandSteadyStateModelingApproachestoProduceFloodplainInsuranceRateMaps.January2008.Opperman, Jeffrey, Luster, Ryan, et. al. 2010. Ecologically Functional Floodplains:Connectivity,FlowRegimeandScale.JournaloftheAmericanWaterResourcesAssociation46(2):211226.U.S. Army Corps of Engineers. 2008. Hydrologic Engineering Center Flood DamagesAnalysis(HECFDA)tool.

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AppendixAMitigatingtheImpact

The following is a summaryof the impacts associatedwith some currentpractices in themappingoffloodwayswithsomebestpracticeshighlightedthathavebeenimplementedbyStatesandcommunitiestohelpmitigatethoseimpacts.

1. Currentpractice: ThedefaultprocessinStatesthatdonothavehigherstandardsforfloodwaysistomapfloodwaysbasedontheonefootrisecriterion.OutsidetheStates with higher standards for mapping floodways, FEMA does not routinelyrecognizecommunityhigherstandardswhenmappingfloodways.Issue: Section 60.1(d) of the NFIP regulations indicates that any regulationsadoptedbyaStateoracommunitywhicharemorerestrictive...areencouragedandshalltakeprecedence(overnationalminimumstandards). Further, according to Section 60.3(d)(2), communities must select and adopt aregulatory floodway basedon theprinciple that thearea chosen for the regulatoryfloodwaymustbedesigned tocarry thewatersof thebase flood,without increasingthewatersurfaceelevationofthatfloodmorethanonefootatanypoint.BestPractices:a. MinnesotahasaState floodwaymappingstandardthat limits thesurchargeto

0.5 feet. It also has a Regulatory Flood Elevation (RFE) that adds one foot offreeboard to the BFE. The RFE is used for siting new development in thefloodplain. The State encourages communities to adopt a more restrictivefloodwaystandard.Iftheyadoptafloodwaywithnosurchargeallowed,theRFEfor newdevelopment is theBFEplus one foot of freeboard. If the communityadoptstheStateminimumstandardthatallows0.5feetofsurcharge,theStaterequires the surcharge be added to the RFEs established. Therefore, in thisinstancetheRFEwouldbetheBFEplus1.5feet.

Note: To be consistent with NFIP requirements associated with requests forincreasingBFEsafterfloodwayshavebeenestablished,Minnesotalimitssurchargestoareaswherenoexistingresidentialstructuresareimpacted.b. The State of Wisconsin requires communities that allow development to

increase flooding on neighboring property to obtain easements from allimpactedpropertyowners.

Discussion: Aprocessinwhichacommunityselectsthesurchargeused(onefootorless)tomapthefloodway,inconjunctionwithacorrelatedincreaseintheheightto which new development is constructed, helps to mitigate the impact. It alsoenablesthecommunitytobecompliantwiththerequirementthatnewdevelopmentbe reasonably safe from flooding. It helps ensurenewdevelopmentwouldnot bedamaged by the allowable increase in flood elevations when development in thefloodway fringe occurs. Furthermore, requiring easements from the impactedpropertiesincommunitiesthatoptforamappedfloodwaythatincludesasurchargehelps to ensure that impactedproperty owners are aware of the impacts and areproperlycompensatedtooffsettheassociateddamages.

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2. Current practice: Once floodways are established, encroachments into thefloodway often are not analyzed based on the concept of equal degree ofencroachment.Issues: The intent in mapping regulatory floodways is to address cumulativeimpacts of encroachments into the floodplain.Once established, if the cumulativeimpactsassociatedwithencroachmentsarenotanalyzed,thebasictenetassociatedwithmappingfloodwaysisundermined.Best Practices: Minnesota and Wisconsin conduct State engineering reviews ofproposed encroachments into the floodway. Both States require that an equaldegree of encroachment analysis be conducted to assess cumulative impacts ofproposedencroachments.Discussion: In order to address the cumulative impacts of any proposeddevelopmentinthefloodway,thedevelopmentneedstoalsobeanalyzedbasedonanequaldegreeofencroachmentengineeringanalysis.Forexample,ifonestructureis proposed 100 feet into the floodway, the engineer will assume that futurestructures in the area will also be allowed to encroach into the floodway to thisdegree.Theengineerwill thenblockoutthisareainmakingtheanalysis.Figure5shows that the engineer assumesmore obstruction than is created by the singleproposed structure. This assumption is based on the legal difficulty a communitywould have in denying similar proposals. The equal degree of encroachment ruleprovidesauniformlegalbasisforgrantingordenyingaproposeddevelopmentandall futuredevelopments.Therefore,anyproposedencroachment intothefloodwayshould include a revised floodway analysis based on the equal degree ofencroachmentconcept.

Figure 5 Using Equal Degree of Encroachment principles in evaluating development proposals. (Source: The Floodway: A Guide for Community Permit Officials(FEMA))

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3. Currentpractice: Wherea floodplainengineeringstudyhasbeencompletedandflood elevations have been established but floodways have not been mapped,development proposals often are not required to conduct a cumulative impactsanalysis.Issues:AccordingtoCFRTitle44,Part60,Section60.3(c)(10),communitiesmustrequirethatnonewconstruction,substantialimprovements,orotherdevelopment(including fill) shallbepermittedwithinZonesA130andAEon thecommunitysFIRMuntil a regulatory floodway isdesignated,unless it isdemonstrated that thecumulative effect of the proposed development, when combined with all otherexistingandanticipateddevelopment,willnotincreasethewatersurfaceelevationofthebasefloodbymorethanonefootatanypointwithinthecommunity.BestPractices:a. TheStateofMinnesota requiresnewdevelopmentproposals inareaswithout

mappedfloodwaystoevaluatethecumulativeimpactofsimilardevelopmentbymapping a floodway as one of the first steps in the process. Minnesota hasespeciallysuccinctguidanceonevaluatingforcumulativeimpacts:Compute the floodwaynecessary to conveyor store the regional floodwithoutincreasingfloodstagesmorethan0.58foot.Alesserstageincreasethan0.5'shallbe required if, as a result of the additional stage increase, increased flooddamageswould result. An equal degree of encroachment on both sides of thestreamwithinthereachshallbeassumedincomputingfloodwayboundaries.

b. North Carolina has established flood elevations for all mapped Special Flood

Hazard Areas in the State. While all of these mapped SFHAs have floodelevations developed with an engineering model, many of them do not havemappedfloodways.Tomakeiteasierforcommunitiestoimplementfloodplainzoningintheseareas,NorthCarolinahasestablishednonencroachmentareasfor all streams lacking mapped floodways. Nonencroachment stations havebeen determined at each modeled crosssection by a hydraulic modelingtechniquesimilartotheprocessusedtodetermineregulatoryfloodwaywidths.However, thewidthshavenotbeenoptimizedbyrepeatedrunsof themodel.Therefore, the state requires that these surcharge values at individual crosssections be less than themaximum value of 1.0 feet allowed by 44 CFR 60.3(d)(2). North Carolina limits the surcharge to 0.7 feet unless the developerprovides an engineering study that includes amapped floodway. In thisway,communities can allow development landward of these nonencroachmentareas and alsomeet the requirements in44CFR60.3 (c)(10) associatedwithSFHAsthathavefloodelevationsbutnofloodwaysidentified.

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4. Currentpractice: FEMAsGuidelinesandSpecifications includeguidanceonhowto develop a onefoot rise floodway based on loss of storage. However, whenmappingregulatory floodwaysevaluating the lossof floodstorage isnot standardpractice.Issues: The floodplainprovidesavaluable functionbystoring floodwaters.Whenfillorbuildingsareplacedinthefloodfringe,floodstorageareasarelostandfloodheightswillincreasebecausethereislessroomforthefloodwaters.Thecumulativeimpacts of encroachments in the floodplain can actually bemore significant thanidentified in this assessment.As an example,MecklenburgCounty,NorthCarolinaanalyzed several watersheds in their jurisdiction to determine the cumulativeimpact of fill in the floodplain fringe on downstream flood depths due to loss offloodplain storage. The results of the Mecklenburg County study indicated thatbased on the standard NFIP regulations (maximum surcharge of one (1) foot),continuedinfillingofthefloodplainfringecouldresultinfloodelevationincreasesofupto2.3feet.(MecklenburgCounty,2008)Referring back to Table 3, flood damages on a singlestory residence with nobasementwouldamounttosome35%ofthevalueofthestructureifinundatedby2.3feetoffloodwater.BestPractice: DuPageCounty, Illinoishas adopted floodplain regulations statingthat any placement of fill, structures, or other materials above grade in thefloodplainshallrequirecompensatorystorageequaltoatleast1.5timesthevolumeoffloodplainstoragedisplaced,andshallbeprovidedatthesameincrementalfloodfrequencyelevationasthefloodstoragedisplaced.

5. Current practice: The NFIP regulatory floodwayand any floodway that iscalculated using a hydraulic engineering modelis a hydraulic concept that isdesigned to prevent unacceptable increases in flood levels due to encroachmentsintotheNFIPSpecialFloodHazardArea.Theconceptdoesnotaddresshydrologicchanges that could increase flood levels such as the loss of floodplain storage,increaseinimpervioussurface,orchangesinprecipitationpatterns.Issues:OneoftheshortcomingsofFEMAFIRMsisthatthemappedfloodzonesarenotbasedonfuturelanduseconditions. BestPractice: CharlotteMecklenburg,NC To estimate the potential impacts ofland use changes, CharlotteMecklenburg Storm Water Services assessed thehydrologicconditionsofseveralwatershedsusingbothexistinglanduseandfutureland use. The general conclusion of the land use change pilot study is that flooddepthswould increase significantly formanyof thewatersheds. If the communitycontinuedtoregulatefloodplaindevelopmentunderhydrologicconditionsbasedonthe existing land use, a significant number of buildings would be expected to beconstructed in areas that would become floodplain when the watersheds are restudied in future years. As a result, CharlotteMecklenburg has developed Floodplain Land Use Maps (FLUMs) based on projected future land use conditions, which are used for new development.