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Riverchase-2/FP-CS 850516CERTIFIED MAIL RETURN RECEIPT REQUESTED Federal Emergency Management Agency Washington, D.C. 20472 Honorable Andrew Brown Mayor of the City of Coppell P.O. Box 478 Coppell, Texas 75019 Dear Mayor Brown: This is in reference to a letter of transmittal, dated April 4, 1985, sub- mitted bY Mr. Err A. Meyer, P.E., of Nathan D. Maier Consulting Engineers, Incorporated. In his letter, Mr. Meyer requested that the Federal Emergency Management Agency (FEMA) review plans for the proposed fill of a portion of the flood plain of the Elm Fork of the Trinity River known as the "Lesley Tract," on the north side of Beltline Road. The project also includes the construction of a swale paralleling the main channel of the Elm Fork of the Trinity River. With his letter, Mr. Meyer submitted technical data prepared by his firm, including a report entitled En~ineerin~ Re~ort to the Dallas County Commissioners Court for the Proposed Creation of Dallas County Levee ImRrovement District Number 18, dated October 1984. Based on our review of the submitted data, we believe that the proposed project will be sufficient to reduce the Special Flood Hazard Area within the flood plain of the Elm Fork of the Trinity River, provided that the project is completed as proposed. In addition, we believe that the proposed fill will not increase the lO0-year flood elevations upstream of the project area. This determination was made using the lO0-year discharge as computed in the ef- fective Flood Insurance Study (FIS) for the City of Coppell, Texas. The potential hydrologic impacts associated with the development of this area or upstream areas have not been considered in this determination. This con- ditional Letter of Map Revision is issued with respect to minimum standards of the National Flood Insurance Program (NFIP). Because your community may .' enforce more restrictive flood plain management criteria, final approval of the project rests with your community. The basis of this conditional Letter of Map Revision (LOMR) is a proposed swale which will carry a significant portion of the 100-year flood discharge. It should be noted that 'National Flood Insurance Program (NFIP) regulation 44 CFR 60.3(b)(7) requires that communities "assure that the flood carrying capacity within the altered or relocated Portion of any watercourse is main- tained." This provision is incorporated into your community's existing flood plain management regulations. Consequently, upon completion of this proposed project, the ultimate responsibility for maintenance of the swale as well as the main channel will rest with your community. Upon completion of the proposed modifications, your community may request a revision to the effective FIS and Flood Insurance Rate Map (FIRM). The revision request must include the data listed below: "As-built" plans of the project, certified by a registered engineer. A written description of the methodology used to determine hydrologic and/or hydraulic parameters, if different from the effective FIS and FIRMs, ' Revised water-surface profiles of the ]0-, 50-, 100-, and 500-year floods reflecting "as-built" conditions, including a zone determina- tion. bo 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.) Since only a portion of the existing profiles is revised, the upstream and downstream portions of the revised profiles should coincide 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 PIS. Two floodway hydraulic backwater models should be submitted. 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 proposed fill and include any other channel modifications or encroachment that may have occurred in the flood plain 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 i~s effects on flood hazards, an intermediate model should also be submitted. This model must reflect flood plain 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 m~y 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 proposed fill. 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 s~ecified in item 3a ~ave been approved by FEMA. b. Since only a portion of the floodway is being revised, At must tie into the effective FIS floodway by duplicating the results of the original baseline model at cross sections upstream and downstream of the proposed fill. ' c. . The revised floodway must carry the waters of the proposed base flood (100-year) without increasing the water-surface elevations of that flood by more than 1.O 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. 5. Delineation of the lO0- and 500-year flood boundaries, the 100-year floodway boundary, and the location 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. Ail items should be labeled for easy cross-referencing to hy- draulic model and summary data. 6. SourCe data and engineering documentation for the previously men- tioned items, as well as a bibliographic list of other sources of information used. Upon receipt and review by FEMA of items 1-6 above, and verification that the fill project conforms to sound engineering practice and meets all applicable NFIP standards, FEMA will take action to incorporate the effects of the completed project into the effective FIS for your community. We have enclosed documents, entitled Conditio_~ns.and Criteria for M~ Fisions and Conditions and Criteria f~r~oodw~~ describe , ~h~c~ furthe~ the nature and extent of the material needed to support a request to revise an effective FIS. Compliance with the criteria outlined in these documents will expedite FEMA's review process, thus allowing the effective FIS and FIRM for your community to be revised as appropriate,' in a timely manner. 4 Should you have any questions regarding this matter, please do not hesitate to contact the Chief, Natural and Technological Hazards Division of ~he Federal Emergency Management Agency in Denton, Texas, at (8]?) 387-58]] or members of our Headquarters staff in Washington, D.C., at (202) 646-2?54. Sincerely, Federal Insurance Administration Enclosures cc: Mr. Err A. Meyer, P.E., Nathan D. Maier Consulting Engineers, Incorporated, w/enclosures Mr. H. Wayne Ginn, P.E., City Engineer, City of Cop~ell, w/enclosures Mr. Milburn Ssith, Fort Worth District, U.S. Army Corps of Engineers, w/o enclosures