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Park West CC(1)-CS 861211Federal Emergency Management Agency The Honorable Lou Duggan Mayor of the City of Coppell P.O. Box 478 Coppell, Texas 75019 Washington, D.C. 20472 December 11, 1986 Dear Mayor Duggan: This is in reference to a letter, dated February 25, 1986, and technical data submitted by Mr. Ed Powell, P.E., City Engineer for the City of Coppell. In his letter, which was forwarded to us by our Region VI office, Mr. Powell requested that the Federal Emergency Management Agency (FEMA) issue a con- ditional Letter of Map Revision (LOMR) for a proposed swale construction and floodplain fill project along Denton Creek immediately upstream of Denton Tap Road in Coppell, Texas. The technical data submitted with Mr. Powell's letter were prepared by Jack R. Davis & Associates, Inc., and included HEC-2 hydraulic backwater computer models of Denton Creek for both existing and proposed conditions, location and topographic maps that include cross-section locations and base (100-year) flood and floodway boundaries, and plotted cross sections. Additional data were submitted on July 9, 1986 by Mr. Mark W. Roberts, P.E., of Brockette- Davis-Drake, Inc., (formerly Jack R. Davis & Associates, Inc.), which included revised HEC-2 hydraulic backwater computer models and a revised topographic map delineating the base (100-year) flood and floodway boundaries. The revised models and revised map tie the proposed swale and fill project into another proposed project, currently under construction, located immediately downstream in an area known as the Gateway Development in the City of Lewisville, Texas. A conditional LOMR was issued to the City of Lewisville On September 3, 4985 for the Gateway Development project. We reviewed the data submitted by Mr. powell and Mr. Roberts and have de- termined that the proposed swale construction an~ fill project meets the minimum floodplain management criteria set forth by the National Flood In- surance Program (NFIP) regulations. If the project ware cempletnd as proposed, a revision to the effective Flood Insurance Study (FIS), Flood Boundary and Floodway Map (FBFM),.and Flood Insurance Rate Map (FIRM), for your community would be warranted. This revision would show the revised flood and floodway boundaries and Base (tOO-year) Flood Elevations (BFEs) as computed in the revised H~C-2 models submitted on July 9, 1986, by Mr. Roberts. Please note that future revisions to the FIS, FBFM, and FIRM for the City of Coppell Or restudies of the flood hazards in this area could modify this determination. Also note that this detezmination is made with the assumption that the down- s~ream Gateway Development project is constructed as proposed. In addition, wa have a concern with the proposed stream velocity at cross section 32305. Due to the high velocity in this portion of Denton Creek, channel protection measures must be made prior to FEMA issuing a revision to the effective FIS, FBFM, and FIRM. You should require that this concern be resolved prior to approving any construction within the currently effective floodway or place- ment of structures in the floodway fringe at an elevation that is below the effective BFE. 2 The determination made in this letter is based on the lO0-year flood dis- charges computed in the effective FIS for your community, and does not con- sider subsequent changes that would tend to increase flood discharges. The development of this project and other projects upstream could result in increased flood discharges, which, in turn, could result in increased 100-year flood elevations. Future restudies of the flood hazards in this area, which would take into account the cumulative effects of development on flood dis- charges, could establish higher 100-year flood elevations in this area. This conditional LOMR is based on minimum floodplain management criteria established under the NFIP. Your community is responsible for approving all proposed floodplain developments, including this request, and for assuring that necessary permits required 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 NFIP requirements. NFIP regulation 44 CFR 60.3(b)(7) requires communities participating in the NFIP to "assure that the flood carrying capacity within the altered or re- located portion of any watercourse is maintained." Without proper mainte- nance, such as the regular clearing of a channelized stream, channel modifi- cation projects will, in time, fail to function as designed, thereby recreating the flood hazard that they were intended to mitigate. Therefore, before FEMA will revise the FIS, FBFM, and FIRM to reflect the effects of the completed project, we require a written statement that your community will uphold its responsibility for assuring that the channel is maintained in order to preserve its design function. Upon completion of the proposed swale construction and fill project, your community may request a revision to the effective FIS, FBFM, and FIRM. The revision request must include the data listed below. Several of these items have already been submitted by Mr. Roberts, but may need to be revised and re- submitted to incorporate any changes resulting from the aforementioned concern and any design changes that may take place during construction. Written endorsement of the revision request; a written statement concerning maintenance of the project, as described above; and documentation demonstrating that the above-mentioned concern has been resolved· "As-built" plans of the swale construction and fill project, certified by a registered engineer. HEC-2 hydraulic models of the 10-, 50-, 100-, and 500-year floods and the floodway model representing "as-built" conditions with the aforementioned concern resolved. 4e Delineation of the 100- and 500-year flood boundaries, the floodway boundary, and the locations and alignment of cross sections and flowline used in the hydraulic model representing "as-built" con- ditions. Revised water-surface profiles of the 10-, 50-, 100-, and 500-year floods reflecting "as-built" conditions, including a zone determi- nation. 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 FEMA's review process, thus allowing the effective FIS, FBFM, and FIRM 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) 898-5127 or members of OUr Headquarters staff in Washington, D.C., at (202) 646-2754. Sincerely, Acting Chief, Risk Studies Division Federal Insurance ~m~nistration cc: Mr. Ed Powell, P.E. Mr. Mark W. Roberts, P.E.