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Park West CC(1)-CS 870818 (2) Federal Emergency Management Agency Washington, D.C. 20472 ~,' The Honor~le Lou Duggan Case #86-06-47R Mayor of Se City of Cop~ll Coppell, Texas ~ ~ L. ~ Dear Mayor Duggan: mis is in reference to a letter dated May 29, 1986, and technical data s~mitted by Mr. Ed Powell, P.E., foyer City Engineer for the City of Cop~ll, Texas. In his letter, ~ich was fo~arded to us by our Region VI office, Mr. Powell r~uested that the Federal Emergency Management Agency (FEMA) issue a conditional Letter of Map Revision for a proposed channel modification and bridge construction project along Grapevine Cre~ be~een Coppell Road and Interstate Highway 635. ~is proposed project involves realignment of the Grapevine Cre~ channel including construction of a grass-lined channel with the ~se width varying fr~ 70 to 200 feet, construction of a concrete drop s~ucture downstre~ of Interstate Highway 635, and const~ction of a bridge at Freeport Parkway. Technical data s~mitted in sup~rt of this r~uest included a report entitled "Conditional Letter of Map Revision R~uest for Grapevine Cre~ in Cop~ll, Texas," prepar~ by Albert H. Halff Associates, Inc. ~is report contained a description of methodologies used and hydraulic backwater models for existing and proposed conditions. Additional te~nical data were s~mitted on several occasions at our r~uest; all r~uired data to process Sis r~uest were received by March 18, 1987. During the course of review, we were i~o~ed Sat the channel modification between Cop~ll Road and Freeport Parkway and Se construction of Se Freeport Parkway bridge were completed. Mr. B. Anatole Falagan of Albert H. Halff Associates, Inc., s~mitted hydraulic analyses, ~ich reflected the completed portion of Se project and r~uested Sat FEMA review Sese analyses and issue a Letter of Map Revision. In reviewing the analyses, we noted that excessive velocities exist in the ea~nen cnanne~ u~n ~e do--stream end of the completed channel modifications and the Freeport Parkway bridge, ~ich could ~esult in severe erosion and under~tting of both ~e bridge structure and the h,u~led channel during flowing events. FEMA ~11 not perfo~ a revision at this time to Se effective Flood Insurance Study (FIe), Flood Boundary and Flo~way Map (FBFM), and Flood Insurance Rate Map (FI~) for the City of Cop~ll to reflect the completed portion of ~e channel modification and bridge construction project due to the unst~le conditions creat~ by the partially c~pleted project. Our regional office has been notifi~ of the stability problem wis ~e complet~ portion of ~e project, and will be contacting your community in an effort to resolve this problem. We have reviewed the data s~mitt~ for the entire channel modification and bridge construction project, and have determined Sat the project as a whole meets Se ~nimum flo~plain ~nagement criteria of the National Flood In- surance Pr~ram. ~e drop s~ucture Sat is Drooosed between the c~D]o~o~ channel modification and the ~eeoort Parkway brid~e would aD,ar to ~le_ channel conditions durin~ flo~ events. If Se entire project is c~pleted as proposed, a revision to ~e effective FIS, FBFM, and FI~ for the AUG. City of Coppell will be warranted. Base (100-year) Flood Elevations and floodway boundaries for Grapevine Creek would be revised as depicted in the aforementioned report. Please note that future revisions to the FIS, FBFM, and FIRM or restudies of the flood hazards in this area could modify this de- termination. This determination is based on the 100-year flood discharges computed in the effective FIS for the City of Coppell, and does not consider subsequent changes in watershed characteristics that would tend to increase flood dis- charges. The development of this project and other projects upstream could result in increased flood discharges, which, in turn, could result in in- creased 100-year flood elevations. Future restudies of your community's flood hazards, which would take into account the cumulative effects of development on flood discharges, could establish higher 100-year flood elevations in this area. This conditional Letter of Map Revision is based on minimum floodplain man- , agemen~ cz~erla esu~bil~L~d ~d~ une National Flood Insurance Program. City of Coppell is responsible for approving all proposed floodplain develop- ments, including this request, and for assuring that necessary permits re- quired by Federal or State law have been received. State and community officials, based on knowledge of local conditions and in the interest of safety, may set higher standards for construction or may limit development in floodplain areas. If the State of Texas or the City of Coppell has adopted more restrictive or comprehensive floodplain management criteria, these criteria take precedence over the minimum Program requirements. It should be noted that National Flood Insurance Program regulation 44 CFR 60.3(b)(7) requires communities to "assure that the flood carrying capacity within the altered or relocated portion of any watercourse is maintained." This provision is incorporated into your community's existing floodplain management regulations; consequently, upon completion of the project, re- sponsibility for maintenance of the modified channel will rest with your community. Upon completion of the entire project as proposed, your community may request a revision to the effective FIS, FBFM, and FIRM. The revision request should be submitted to our Region VI office and must include the data listed below: 1. "As-built" plans of the channel modification and bridge construction project, certified by a registered engineer. 2. A written description of the methodology used to determine hydrologic and/or hydraulic parameters, if different from the effective FIS and FIRM. 3. Revised water-surface profiles of the 10-, 50-, 100-, and 500-year floods reflecting "as-built" conditions, including a zone determi- nation. a. The methodology and starting parameters for the revised profiles should be consistent with the present effective FIS, i.e., same discharges and hydraulic model, unless the parameters have been superseded by more current and technically superior data and analyses. (FEMA approval should be obtained before deviating from the effective FIS parameters.) b. Since only a portion of the existing profiles is being revised, the upstream and downstream portions of the revised profiles should coincide with the effective FIS profiles, i.e., hydraulic calculations should be continued upstream and downstream of the revised area until water-surface elevations coincide with those in the effective FIS. 4. Two floodway hydraulic backwater models. The first should be a duplication of the original baseline model used in the effective FIS. This is required to ensure that the original data has been duplicated correctly. The second model should incorporate the completed project and include any other channel modifications or encroachment that have occurred in the floodplain since the original floodway was delineated. If, however, additional cross sections are used in the second model to provide a more detailed analysis of the completed project and its effects on flood hazards, an intermediate model incorporating the additional cross sections should also be submitted. This model must reflect floodplain conditions as they existed at the time that the original floodway was delineated and, therefore, it is important that any cross sections added to the original model describe those con- ditions. In addition, any improvements to the original modeling technique may be incorporated into this intermediate model. This model will then become the new baseline model and will be used to accurately measure the effects of the completed project. a. The methodology and parameters for the revised floodway should be consistent with the effective FIS, i.e., equal conveyance reduction to establish encroachment limits, unless changes as specified in item 3a have been approved by FEMA. b. Since only a portion of the floodway is being revised, it must tie into the effective FIS floodway by duplicating the results of the original baseline model at cross sections upstream and downstream of the project. c. The revised floodway must carry the waters of the base (100- year) flood without increasing the water-surface elevations of that flood by more than 1.0 foot over the original baseline model at any point. If additional cross sections have been incorporated, then revised floodway elevations also may not exceed base flood elevations calculated in the new baseline model by more than 1.0 foot. In all cases, the revised floodway elevations may not exceed revised base flood elevations by more than 1.0 foot. 5. Delineation of the 100- and 500-year flood boundaries, the 100-year floodway boundary, and the locations and alignment of cross sections and flow line used in the hydraulic model. a. This information should be shown on a map of suitable scale and topographic definition to provide reasonable accuracy. b. All items should be labeled for easy cross-referencing to hydraulic model and summary data. 6. Source data and engineering documentation for the previously mentioned items, as well as a bibliographic list of other sources of information used. We have enclosed documents entitled Conditions and Criteria for Map Revisions and Conditions and Criteria for Floodway Revisions, which further describe the nature and extent of the material needed to support a request to revise an effective FIS, FBFM and FIRM. Compliance with the criteria outlined in these documents will expedite F~A's review process, thus allowing the effective FIS, FBFM, and FI~ for your community to be revised as appropriate, in a timely manner. Should you have any questions regarding this matter, please do not hesitate to contact the Chief, Natural and Technological Hazards Division of the Federal Emergency Management Agency in Denton, Texas, at (817) 898-9127 or members of our Headquarters staff in Washington, D.C., at (202) 646-2754. Sincerely, ~ ~icks M Chief, Risk Studies Division Federal Insurance Administration Enclosures cc: Mr. B. Anatole Falagan Mr. Patrick Lee Acker, Albert H. Halff Associates, Inc. Ms. Shohre Daneshmand, Civil Engineer, City of Coppell CONDITIONS AND CRITERIA FOR ~ REVISIONS A. Introduction This document describes the conditions and criteria for revising a Flood Insurance Study (FIS) under Part 65 of the National Flood Insurance Program (NFIP) regulations. The Federal Emergency Management Agency (FEMA) recognizes that FISs may require revision due to new information becoming available or through physical changes in flood plains. This document defines the situations that fall under Part 65 of the NFIP regula=ions and describes the procedures for correcting or updating Flood Insurance Rate Maps (FIRMs) and Flood Boundary and Floodway Maps (FBFMs). B. Definitions A Letter of Map Amendment (LOMA) issued under Part 70 Of the NFIP regu- lations indicates whether or not a structure(s) or proposed structure(s) is located in a Special Flood Hazard Area (SFHA) as shown on a FIRM or Flood Hazard Boundary Map (FHBM). A LOMA removes a property from the SFHA and thereby makes inapplicable the Federal requirement for the purchase of flood insurance. For more information regarding LOMAs, refer to "Condi- tions and Criteria for Letters of Map Amendment." A Conditional Letter of Map Revision, or belief letter, issued under Part 65 of the NFIP regulations is FEMA's comment on the effectiveness or impacts of a proposed flood control project or flood plain modification. It is based on FEMA's review of the proposed project and states that were the proposed project built as designed, it would be cause for a map revision. A Letter of Map Revision (LOMR) can be issued under Part 65 of the NFIP regulations as an expeditious means of revising a FIRM or FBFM. The LOMR gives a detailed description of the Base Flood Elevation (BFE) and graphic changes ~hat will be made to the SFHA currently delineated on the effec- tive FIRM and/or FBFM. LOMRs involving BFE changes are always succeeded by a physical map revision. A physical map revision under Part 65 of the NFIP regulations involves changing and republishing the existing FIRM and/or FBFM to reflect any updated condition. Valid LOMAs and LOMRs are incorporated into map revisions. C. Reasons for Revision FEMA will revise an effective FIS to reflect new information which shows the original FIS to be incorrect and to reflect physical changes which invalidates the original FIS analyses or presentation of data. Updated or corrected topographic mapping, hydrologic data, or hydraulic data FIA/OP~A/RSD 7/27/S4 constitute new information -which may warrant a revision. Flood protection projects and any form of topographic alterations (cut and fill) constitute physical changes which may also warrant a map revision. D. General Revision Policy Within the statutory criteria established for appeals, revision requests will be evaluated based on t~he possession of knowledge or information indicating that the effective elevations are scientifically or technically incorrect. In certain situations ,where new information shows mathematical or measurement errors in the original FIS, F~A will accept the respon- sibility for revising the FIS provided the requestor submits the basic data necessary to correct the problem. Where revision requests are based on the application of alternative methods, appellants are required to demonstrate that the new analyses result in more correct estimates of base flood elevations, thus demonstrating that FEMA' s estimates are incorrect. In these situations, where a revision request is being made based on improved data, methods, or applications, the community (or requestor through the community) must accept responsibility for providing all data and analyses necessary to update the FIS. If a physical change resulting from development warrants a revision to the existing analyses, FF~MA will not undertake this new technical analysis. The requestor, through the community, must accept responsibility for providing all data and analysis necessary for u?dating the FIS. E. Community Map Revision Requests The map revision process cannot be initiated without the community's endorsement since it is the community that adopts the effective FIS. Therefore, any individuals requesting a change to the FIS, must do so through the community. The community, in turn, may support the request and forward the information to FEMA for evaluation. FEMA will review the request to ascertain that the following conditions are met before eval- uating the revision request. Condition Number 1 The community must endorse the revision request and demonstrate prepared- ness to accept responsibility for the operation and/or maintenance of any structural measures (e.g. channel improvements or levees) involved. The request submittal should include the following: I .1 A statement that the community endorses the revision request. 1 .2 If the basis for the revision request is a channel modification, the completion of a dam, levee, or any other structural measure, evidence is required to demonstrate the design is adequate and that mainte- nance and operation provisions, where applicable, have been made. The submittal must indicate what entity has operation and maintenance responsibility and how they will be accomplished. If maintenance is to be accomplished by an agent other than the community, a legal 2 provision (ordinance) for community monitoring and backup maintenance is required. 1 .3 The submittal must indicate that, where necessary, state approval of the revision has been obtained. Condition Number 2 If the revision request is based on information showing a mathematical or measurement error in the original analysis, the submittal must include the following: 2.1 Documentation that identifies the specific source of the error. 2.2 A written description of the new information and how it differs from the existing FIS information. 2.3 Basic data supporting the revision request to enable F~A to revise the FIS. In most cases, this data must be certified by a registered professional engineer or licensed land surveyor. Updated topographic maps, revised community boundary maps including annexation ordi- nances, and channel, dam, or bridge structure plans which illustrate the correct dimensions are examples of the basic data required. Condition Number 3 If the revision request is based on improved application of hydrologic, hydraulic, or other methods, or use of better data in applying such methods, the requestor must submit the following: 3.1 Documentation that identifies the error in the application or in the inferior data in the original analysis and supports why the appli- cation is incorrect or the data is inferior. 3.2 Engineering analyses applying the same basic methods utilized by FEMA but with the changes itemized. 3.3 Alternative engineering analyses utilizing the methods or assumptions determined to be correct. 3.4 Background technical information which supports the requestor's changes as being more correct. 3.5 Certification by a registered professional engineer or licensed land surveyor of the correctness of any alternate data utilized or mea- surements made. 3.6 Documentation of all locations where the requestor's base flood elevations are different from FEMA's. 3 Ail engineering analyses performed must substantiate that the revised hydrologic or hydraulic analyses meet F~A requirements as well as any state or community requirements. F. Genera]. Technical Guidance Flood plain revisions should be based on the hydraulic model used ~o develop the flood profile and flood boundaries currently in effect. The community should request, through the FEMA Regional Office, a copy of the input data used in ~he computer model for its effective FIS. %~ere the input data representing the original hydraulic model is unavailable, or where a t~chnically superior model can be used, an approximation should be developed. A new model should be established using the original cross- section topographic information, where possible, and the discharges contained in the FIS which established ~he original BFEs. The model must use the same effective flow areas as established in the original analysis and must be calibrated to reproduce the original base flood elevations within 0.1 foot. Any differences between the new analysis and the orig- inal profile must be justified on an engineering basis. After the model has been checked and matches the original base flood elevations, the model can be modified for new flood plain runs. The analysis will be accomplished as specified for the following conditions: Discharge Dec~eases. Hydraulic analyses may be revised .when a base flood discharge decreases as a result of structur&l improvements, such as the construction of a flood control dam or other significant retention facil- ities. Hydraulic analyses should not be revised based on a computed discharge reduction which results from changed methodology or longer stream gage records, unless the change is statistically significant. The statistical significance criteria are discussed in Section 2-6 "Hydrologic Analyses" of FEMA's Guidelines and Specifications for Study Contractors (dated September 1982). If it is evident that the change is statistically significant, the original hydraulic model should be rerun, changing only the discharges. In cases where the statistical significance test does not apply, a determination by a state or Federal agency that the change is significant may be accepted as a basis for a map revision. Discharge Increases. If watershed conditions have resulted in a signifi- cant (defined as above) increase in discharge, the original hydraulic model must be run with the increased discharges. Channel Modification. The original hydraulic model will be modified to include the channel modification, and any other encroachment occurring subsequent to the original flood plain delineation. In all situations, the revised hydraulic analysis should be modified to include current conditions as well as any encroachment to the flood plain that may have occurred since the original model was developed. The revised hydraulic analysis should also include a zone determination, and if only a portion of the existing hydraulic analysis is revised, the upstream and downstream portions of the revised analysis should coincide 4 with the effective FIS profiles, i.e., hydraulic calculations should be continued for a great enough distance upstream and downstream of the revised area until water-surface elevations coincide with those in ~he effective FIS. The submittal must include the technical data that enables FF~A to deter- mine whether the flood plain revision meets FF~A's requirements. The sub- mittal must include the following: o A copy of the printout for the original hydraulic computer model representing the 100-year flood profile run for conditions existing at the time the currently affective hydraulic analysis 'was developed. The printout must include full input and output listings. o A copy of the printout from the hydraulic computer model reore senting the new 10-, 50-, 100-, and 500-year profiles. The model should be the same as that used in the preceding item, but modified to include any channel modification, fill or other encroachment that may have occurred in the flood plain since the original flood plain was delineated. o Delineation of the 100- and 500-year flood boundaries and the location and alignment of cross sections and flow line used in the hydraulic model. This information should be shown on a map of suitable scale and topographic definition to provide reason- able accuracy. o A copy of the currently effective flood profiles showing the existing and the revised flood elevations. o Certification from a registered professional engineer that the physical parameters used in the proposed flood boundary delin- eation represent actual conditions and that the standards contained in these "Conditions and Criteria" are met. When a map is revised as a result of a new hydraulic analysis, the effect of the revised hydraulics on the floodway must also be considered, if a revised floodway analysis is necessary, the community should refer to the "Conditions and Criteria for Floodway Revisions" for additional informa- tion that will be required. G. FEMA Response to Map Revision Request FEMA will evaluate a request from the community for a revision and respond by one of the following means: (1) A preliminary copy of the revised FIS will be sent to the community for their review. If the revision involves BFE changes or the estab- lishment of new BFEs, FEMA 'will initiate a 90-day Appeals Period. Upon review and resolution of any comments and/or appeals received, FEMA will issue a new effective FIS which will be sent to all pre- vious recipients of the maps. 5 (2) Send a Letter of Map Revision (LOMR) to the community stating that the review of the submitted material has resulted in an official map revision but that a physical map revision is not warranted at this time. The LOMR will give a detailed description of the BFE and graphic changes that have been made to the SFHA currently delineated on the effective FIS. The material, including a copy of the maps showing the revised information, will be filed for incorporation at a later time into a physical map revision. The approved map revision copies will be dated. Since it is possible that FEMA has responded to a map revision request in this manner, all FIS users are encouraged to contact the community before proceeding with plans for development within the flood plain areas. LOMRs that result in an increase in BFE will also be preceded by a 90-day appeals period. An appeals period for a LOMR which only decreases BFEs will be provided subse- quent to the issuance of the LOMR. (3) Send a letter to the community with a copy to any other requestor stating that the submitted material is not adequate to make an evaluation and offer them the option of submitting additional infor- mation. (4) Send a letter to the community with a copy to any other requestor stating that the submitted material appears to justify a map re- vision, but because not enough information was submitted the com- munity will be placed on a list for restudy consideration, as funds per~it, unless the community can submit additional information. (5) Send a letter to the community with a copy to any other requestor stating that the submitted material does not substantiate their request for a map revision and that the file will be closed unless the community can submit additional information to substantiate their request. H. Submittal of Material Submit material to the FEMA Regional Office. The Regional Office will review material for completeness, concur with the proposed changes, approve any maintenance ordinances, and forward the necessary material to FEMA Headquarters. FEMA Headquarters will review the hydrologic/hydraulic backup material, engineering design, and any plans of operation and maintenance (where ap- propriate), request additional information as necessary, then respond to the community by means of the appropriate action defined under Section F abov e. To obtain further information on the conditions and criteria for map revisions, communities are encouraged to contact the appropriate FEMA Regional Office or FEMA Headquarters in Washington, D.C., at (202) 287- 0230, prior to requesting a map revision. "'- AUGUST, 1984 AGENCY FEDERAL EMERGENCY MANAGEM=I~T Conditions and Criteria for FLOODWAY REVISIONS CONDITIONS ~ND C!~ITERIA FOR FLOODWAY .REVISIONS FOREWORD Th~.National Flood Insurance Program makes flood insurance available to property owners in communities that adopt and enforce flood plain management measures to reduce future flood losses. The Program provides' flood hazard maps and risk information on which local flood plain management measures are based. One aspect of a sound flood plain management program is the maintenance of a floodway area to assure that the elevations of future floods will not be increased significantly. The adoption of a floodway by a community preserves the necessary conveyance area for passage of the flood waters by restricting actions within the floodway which will result in any increase in flood elevation. After a floodway is adopted, a community may encounter a compelling need to change the configuration of their floodway and therefore request that the floodway map prepared by the Federal Emergency Management Agency (FEMA) be revised. The purpose of these conditions and criteria is to set forth the nature and extent of the material needed to support such a request. Compliance with the criteria described herein will allow FEMA to review the material and revise the floodway maps as appropriate in a timely manner. e~frey S. Bragg ministrator Federal Insurance Administration C~DZ'Z'ZCi~S ~ C~Z~ ~ ~ This docu~en~ aet~ £orth FEHA poLic~ concern£nq ~equests fo~ zev~s~ng ~e co~un~7 ~ ~egulate ~l~a~s ~n accordance ~i~ Title 44, ~e ~e of F~eral ~gu~a~ons. A. ~ ~le ~ Establ~sh~nq F~wa~s The Nationa~ Fi~ lnau~ce Act o~ ~968, S~t~on 136~ au~o~lzed F~ to ca=~ out ~L~ atudSes =eSat~g ~ enc=oac~en~ a~ ~st=uct$ons on a~eam ~aflflels ~d f~ways. ~e ~r~se of ~e st~es ~s ~ support (1] ~equlate ~e deve~o~ent o~ ~d ~he~e app~opr ~ate (2) 9u~de p~o~a~ ~s~uct~on awa~ from l~t~s ~ch a~e ~eatened b~ fl~ hazard, (3) assist ~ ~educ~n9 ~qes (4~ o~e~w~se ~prove ~e ~ng-r~qe ~d ~aqement ~d ~se o~ ~e section ~a e~l~c~t ~ ~t ~ aha~ ~k closely ~, ~ provide necessa~ techn~ ass~stan~ gove~en~ agencies ~ ~e~ ef~oc~s ~acd ~u~ ~ p~a~n ~ageMn~. ~e ~ cole c~a~sO of establishing m~n~m a~da~ds ~ provid~ng info~M~ ~ s~Ce ~d ~ reguh~ng agencies. Established cr~ec~a ace con~n~ ~ PR~ 60.3 (d) o~ ~p~ec ~, Ti~e 44 of ~e ~e o~ F~e~a~ bgu~a~i~s. ~e regu~a~im s~es ~ ~en F~ has principle ~ ~e area ~os~ ~o~ ~e cegu~a~ ~L~va~ ~s~ cacc~ ~e va~e~s of ~e ~se f~ (~00-~ea~) v~ouC ~c~eaa~ng ~e ~e~ so,face e~eva~im o~ ~ fL~ ~ce ~ ~.0 ~ a~ ~ ~n~. Howve~, ~e ~ is less ~ ~.0 ~. s~d~in~,' ~ ~rdi~os wi~ ~un{~{es during ~e ~ genera~ a~ge ~ prepares c~t~nues M au~=t ~ ~n~ty ~ iU e~o=u ~ en~oc~ ~e ~X~way ~ provLdes ~/es of ~ Mpa ~ users. Definition and Purpose of Floodway The floodway Ls defined as the ch·ntis1 of a stream and any adjacent flood plain areas that must be reserved in order ~o discharge the 100-year flood without increasing flood heights by a specified amount. The purpose for establishing a £1oodway is to provide for the balancing of the competing uses of development against the resulting increase in flood hazards. FEMA ~s set a m~n~ s~dard, l~m~ing ~e ~l~way ~id~ to ~t ~ch w~L~ ~t result ~ ~ ~ease of ~e 100-year fl~ ~ ~re ~an 1.0 f~t. Several s~tes have adopt~ ~ui~e~n~ whi~ limit ~e ~ncreases ~ less ~n ~e ~ m~n~ standard. In ~ses ~ere ~e · ~e ~s adopt~ ~re s~ngen~ stud·fda b~ ~egaLl~ enforceable s~tu~e or ~egu~at~, ~ co~u2es f~wa~s bas~ ~n ~ese standards. Once ~e fL~a~ hs ~ adop~ed, encroac~n~s, inc~ud~ng ~i~, new c~s~uc~on, subst~tiaL ~proveMnts, ~d othe~ deve~o~en~ wi~n ~e ~op~ed ~l~va~ ~t ~ld ~esul~ ~ ~y ~nccease ~n ~1~ levels.v~h~n ~e ~un~C~ du~ing ~e ~urrence of ~e ~se fl~ d~scha~e pcohibit~. C. Floodway Revision - General Policies #if. bin the criteria established by FZ~A, ~any flc~dwsy c~nfigurat£ons may be generated. However, once adopted by a community, a par2icular floodway configuration ~comes ~ministra~vely es~a~l~shed, much ~e sm as o~ef country ~egulat~ons. ~ adopt~ flyway ~ep~esen~s a · eas~able all~i~ of ~ ~ea fo= ~e passage of fl~ wa~e=s ~d d~s not necessarily rep=·sent ~e n~n~m~ area E~=~ ~ Me= ~e ~ mln~n~ s~andard. ~ce ~e ~l~way ~s ~op~, ~e flyway ~u~a=~es a~e An~end~ ~ reM~ s~ic. During a restudy, the existing floodway co~figuration should be checked ~o assure that it mee~.s acceptable surcharge lhnits under pr·sen= conditio~s. Mb·re surcharge lhnits are exceeded, the floodway should be expanded o A floodway may be reduced ~n size it the discharges have been reduced as a result of a physical change, such as a diversion of the flov, or the const~uctic~ of · dmn or other na~or retenticm measures. A floodway may Ltso be reduced as a result of stream nodifications. Flc~dway changes should not be baaed on physical changes, unless it has been established that adequate maintenance has been provided to assure con~inua~ion of the respond to · social o~ econc~tc need fo~ developuen~ within a floodway. %'hAs need may be met by shifting the floodway boundaries, using appropr~u~e b~draul~c analysis, whib mainta~n£ng the flc~d carrying capac~j'of the floodway. A~ter a floodway is es~ublished, development may occur in the £~lnge &rea (the area within the f~ood plain hut outside the £1oodwey), but should not occur in the floodway unless it is ahotm tha~ the development viler not reduce tbs conveyance o~ the floodway. It it bec~-s necessary to 2 revise the £~oodway, the ocigina~ hydraulic B~de~ must ~e used es~ab~ish the base profile £or ~e new enc~oac~en~ analysis. The ~e~ ~u~d ~en ~ ~fi~ ~ ~nc~ude ~e ~-p~ace ~croac~en~ for ~e revised ~vay ~unity FlYway ~p Revision ~quest Th {l~way ~sig~tim Is ~t' 'a~alable' to Y~ by individuals, ll~e it is ~ ~unity ~t alec~ ~d gopts ~e regularly flyway. ~us, n ~dividu~ should I~e~ ~e flyway desigu~ion ~ mk mn~en~ f~om, ~e ~uniU. ~e ~muni~y, iff tu=n, 8up~t ~ ~en~en~ ~ =~es~ ~ ~ revue ~e original, p~uc~ Fi~ ~u~a=y ~d Fl~way ~p (~FM). ~ will review z~ues~ ~ Iscertain ~t ~e foll~in9 conditions a=e ~ belo=e e~ending effo~ m ~e revisits Condition N~er 1 ~e ~unitM Bust demonstrate ~t it is p~epa~ed ~ adop~ ~e ~ified flyway a~ ~ a~ legal r~virenen~ will have been met before ~e fl~wa2 As revise, bfore ~aflges ~e male ~ ~muni~M regulated -. a~eas, co~unities a~e r~ui~ ~ ~ sure ~ foll~ certain a~inis~ative p~ures. ~he ~eques~ ~ F~ needs ~ include evidence ~t appropriate p~edu~es have ~en ~ollo~d. ~lc~ infor~ion ~ su~it~ a~e as ~o~s: l.l ~ of a ~bllc ~tice ot ~e intent ~ ~iIy ~e f~, ~y adversely ~acC~ pr~et~ ~ets ~ ~jacent 3urisdic~i~s. 1.3 In lieu o~ ~e ~ve, a s~te~t ~t ~e ~ge viii cesult ~ adverse ~ndition N,,-~r 2 ~ ~gineering ~alysis ~s ~n ~o~ ~ subs~ia~ ~ ~e revis~ ~l~way ~e~ ~ min~ st~dafds as ~ as ~y 8~ o~ FlYway rev/a/~o ma~ b ~ ~ ~ ~drau~Lc ~el us~ M devel~ repres~t~g ~ ~ig/~l ~aul/c ~el b Gvailable, using ~ ~ig~l cross ~ti~ ~Eaph~c ln~ocmti~, where ~ ~ dis~i:ges ~in~ ~ ~e Fi~ lnlu~e St~ repcoduce the ociginal base flood elevations within O,1 foot. Where reproduction of the oc~gina~ base ~ e~eva~ons within 0.1 ~ is no~ ~ssible, o~ ~esul~ ~ un~und ~ginee~inq pzactices, ~e co~uni~y ~ould con~act ~e a~opriate ~ ~9io~1 Office for dir~tion. After the model has ~een c~ecked and latches the original base flood elevations, the ~odel c~ ~ ~f~ fo~ new flyway runs. The ~alysis will ~ ~c~l~shed as ~ec~f~ fo= ~e foll~ing basis ~evis~on= Discharge Decreases Floodways may' be revised w~en a base £1oc~ discharge dec:eases as a result of s~uctural hnp:ovenents, such as the construction of a flood cont=ol dan or other significant retention facilities. Floodways should not ~e revised based on a computed discharge reduction which results from changed aethedoloTy or longer stream gage records, unless the change is a~at£s~icslly significant. The statistical significance criterio are discussed in Section 2-6 'H~drolo~jic Analyses' of FEe's Guidelines and Specifications fo~ Study Contractors (dated Septenber 1982). l~' cases where the statistical significance test does nok al~ly, a dete~minat$on b~ a state or Federal agenc~ that the change is significant nay be ' accepted as a basis for a floodway ~evision. If evident that the change ' is statistically significant, t~e original hydraulic model abound be re~un, cAanging only the discharges. This will serve as the base run for the subsequent floodway encroac~nent runs. The floodway model should then be modified ~o include current conditions, as wll as any encroaclment that may have occurred since the origina~ model was p~oduced. The resulting floodway elevations mus~ not exceed the base run elevations b~ mo~e than 1.0 foot, or as specified b~ the a~ate connunit~. Discharge Increases If watershed conditions have resulted in a significant increase in discharge, the original hyhraulic model needs to be run with the tmc=eased discharges. The resul~ing base flood profile will serve as the base for the £1c~dway runs. The floodway runs musk include any encroachment that maF have occurred since the delineation of the original floodway. Channel Mo~ification The o~iginal ~odel will be ~odified ~o include the channel ~odification, and any other encroachment ~cu==in~ a~s~uent ~ ~e origin~ flyway delineati~. ~e Eesult~g ~se fl~ profile will ~e fI~way c~s. ~e fl~y c~s ~8t include my ~ve =u=c~ sin~ ~e del~at~ of ~e oc~gl~l ~l~vay. The reault~g fl~y eleva~l~s ~aC not ~ce~ elevators ~ ~e ~igi~l base ~ elevators by ~ce ~ ~.0 ~, Social ot ~-cono=ic Where it is desired to shift the £1oodway for socia~ os economic reasons, ~e ~se f~ e~eva~s ~t~ ~e or~na~ ~e~ ~u~d serve as ~e ~ase prof~e fo; ~e ~ f~a~ tun. ~he ~eL ~u~d ~en ~ ~ed ~e ~v f~way ~i~s a~e ~ ~ ~e~ in a ~ne~ which vi~ ~ tesu~ in ~ ~nc:ease ~ ~e ozig~na~ ~se {~ eZeva~ In excess Condit~on N~er 3' S~b=~taL includes tKhnica~ da~ ~at ~ables ~ ~ de~et=~ne whether ~nc~ude ~e X c~y of ~e pf~ntout tot ~e otiqi~l h~draulic cobuyer ~el teptesent~1 ~e ~00-yeat fL~ profile run tot conditions at ~e t~e ~e cuttentl~ effective f~va~ vas developS. The pr~n~u~ =usk ~nc~ude ~uZ1 input ~ output 3.2 X cop~ of ~e pt~nto~ tot ~e original h~drauL~c co~u~e~ ~eZ representing ~e fZ~va~ run for conditions under which ~e cuttent~ ef[ec~ve fl~va~ ~s developS. The pt~n~ou~ ~sc include ~u~ ~npu~ a~ ou~pu~ 3.3 X co~ o~ ~e pt ~ntout ~o= ~e h~dtau~c ~u~et ~e~ representing ~e ney L00-ye~ profile. The ~e~ ihou~ I~e as ~t us~ ~ ite~ 3.~, but ~f~ ~ ~nc~ude ~y ~anne2 ~if~cat~on, ~ or other enctoac~en~ ~ ~ have ~cutt~ ~n ~e fL~ p~a~n I~nce ~e oclg~nal fl~va~ vas delineate. X c~ o~ ~ printout tto= ~e h~d;auL~c co~uCe~ ~el representing ~e fZ~va~ r~ f~ ~e pt~s~ ~Z~va~, ~nclud~ng ~anne~ ~f~cat~s ~d enctoac~ents I~nce ~e origami ~l~va~ vas eitabl~sh~. ~e ~ua~ conveyance teduct~ ~ should ul~ ~ ~ute ~e revised fl~va~ ~ un~ess agree=en~s have ~. The ptin~out ~st Lnclude ~u~ ~npu~ ~d output l~sC~nqs vi~ a~ ln~t ch~ges fr~ ~e original ~e~ highlighted. The net effec~ of ~e ~a~e~ ~f~cat~ons, ~e ~ctoac~ents ~de I~l~uent ~ ~e ei~l~s~en~ ot ~e or~g~n~ fl~y, and ~ ~ ~it ~t' ~d ~.0 ~t ~ve ~e ~v L00-~eat e~eva~ons ~t~ ~ lt~ 3.3. 3.5 A ~ o~ ~e revil~ Fl~va~ ~ T~b~e representing ~ fo~ ~e pr~s~ ~u~ conf~gura~. 3.6 ~e ~ of ~ ~E~en~~ effective ~ ~ng ~e existing flyway ~ o~ ~ ~ing ~e pr~8~ f~wa~ configurations. 3.? Cert£fication f:om a registe:ed p~o£ess~onal eng£nee= that ~he physical parameters used ~ ~e p=o~s~ flyway delineation ~epresen~ ~ual c~di~ions ~d ~ ~e s~dards ~n~ained ~n ~ese "Condit~s ~d ~te=~a' are ~. 3.8 If ~e basis fo= ~e p=o~s~ flyway =evisi~ ~ a ~annel ~ficat$on, ~e ~iet~ of a d~, o= ~y o~e= .~uc=u:al ~asu=e, evSdence $~ required ~ ad~2e, a~ ~2 ~nten~ ~ ~e=at~ p[ovis~ons, where a~l~cable, ~ve ~n Mdc. The en~ hs ~nten~ce res~nsib~l~ acco~l~shed. If M~n~enance ~ ~ ~ ~co~l~shed ~ agencies o~e= ~ ~ co~nity, a legal p=ovis~ ~o~ ~n~=F ~=~ng ~d ~c~-up ass~s~ce Is =~u~red. 3.9 ~ntat~m of a~roval o~ ~e p=o~s~ flyway revis~ from ~e app=op=ia~ s~te agen~ loc co~nlt~es ~e=e ~e ' regulates ~e flyway. 3.10 ~uMn~tim of ~y va=iati~ from ~,dit~on 2 ~d/or ~nd~=~on-3. Th~s includes app=ov~ rrm ~e e~lanat~ of ~e rea~n for va=~a~ion. ~ ~nse ~ Fl~wa~ Revision ~ues~ ~ wi~ revi~ a ~n~ts r~ues~ f= a lflfu~mce Study users o~e~ ~m ~e co~ni~y my ~Wues~ ~pies of ~e flyway ~l~nea=~s. The=efo=e, In sups== of ~e ~un~ ~d o~e= users, ~ ~flCa~s a sysc~ fo~ d~s~bu~ng revising ~l~a~s as ~ Mad ~ses. ~en ~e dis~r~bu~e ~en ~ rec~niz~. ~er8. Signi~ic~t ~i~vaF revis~s ~e ~den~ bes~ ~ ~e size of area, as w~ as ~e n.--~e~ o~ ln~e~es~ ~ wi~ re,fid ~ a request ~ron ~e ~unl~ ~or a ~l~waF revision ~ o~ o~ ~e ~o~ing ~s. T~ ~nels will ~ a~~ ~ a revis~ in~ex panel ~es of ~e revis~ ~l~uy p~els w~ll ~ sen~ ~ previous r~ipienu of ~e ~l~waM ~pa. A revis~ T~lea. (2) ~ ~e ~ ol ~ ~ desig~ting ~ a~c~ ll~vaM cevisi~ ~ ~ ~i~ ~ a letter ot fl~y revisi~ a~ing ~ ~ Eerier ot ~ su~it~ed ~e de~er~i~ ~ ~ ~n~ c~i~e~n~ ~ve m~te~iaX, ~cLud~ng the ~ o£ the lPBFM showing the revised ~. ~lflce 1~ Ls ~sl/b~e ~a~ ~ ~s res~nd~ ~ a ~l~vay rev~si~ :~ues~ Ln ~Ls ~ne~, I~ Fi~ lnsu~ce S~udy use:s ere encou~ag~ ~ ~ntact ~e plus fo~ ~velo~n~ (3) Send a Le~e~ ~ ~e a~ove ~y ~ntenance oEd~nces, ~e ~ Cefl~ office. ~ ~fl~:a~ wL~ review ~e b~dro~ic~d:au~Lc hck-up ~g~neering design, ~ ~ pL~s o~ ~eca~on ~ uifl~enince (vhece bg~ona~ ~ce o~ ~ Central ~ ~esh~ng~n, D.C. ~ (202) 287-0230,