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Park West CC(1)-CS 891213 (2) The City With A Beautifu'PF~ro December 13, 19890 ~~' Mr. Don Davis 5506 Cockrell Hill Ro6 Dallas, Texas 75236 Initial Gheck TO & C[mulate X SGG X RRD LD X SD aDC BCB FILE ~vine Cree--~D~cent~ West Commerc~~ reliminary Dear Mr. Davis: P.O. Box 478 Coppell, Texas 75019 214-462-0022 Plat Submittal As we have discussed, I have received your letter, dated December 11, 1989 notifying me of your client's intention to dedicate Grapevine Creek area to the City of Coppell. I do appreciate the information, but that matter will be handled through normal channels (i.e. the Development Review Committee -DRC- Process). Per the latest addition of your preliminary plat dated received by the City of Coppell on November 21, 1989, you have not so indicated the proposed disposition of the Grapevine Creek area. As I understand, the DRC requested of you, your clients direction in this matter (and assumingly your December I1, 1989 letter to me is in response to that inquiry). You need to place these intentions on the preliminary plat and have them addressed in that prescribed process. The current final plat for this tract (i.e. Gateway Park) did not dedicate Grapevine Creek as such and current staff advises me that the City then had no intentions of accepting that creek area for operation and maintenance. The City may still be of the same disposition. In any case, the Public Works Department's current recommendation is to not accept the creek for the City. By copy of this letter, I am providing Gary sieb, the Planning Director with a copy of your letter, so that your clients intentions can be included into your platting file for the record. Additionally, enclosed is a copy of an August 18, 1987 FEMA letter (furnished you last Thursday by Shohre Daneshmand) dealing with Grapevine Creek in the area and indicating that a request for a conditional letter of map revision was approved subject to a number of items being furnished to FEMA, including the provision for reduction of "excessive velocities". To the best of my knowledge, there has not been a response to this letter furnished through the City of Coppell and approved by both FEMA and the City of Coppell. Please let me know if you concur in this understanding or not (If not, please furnish any related documentations). While this is a final plat item, we want to continue to make sure that this matter is resolved, so that it need not delay any final plat once one is filed, etc. City staff desires to continue to work with you to assure that any and all issues are identified, addressed and resolved in as an efficient, effective and expeditious way as possible, so as not to delay unnecessarily any processing of this development's applications, etc. Please let me know if we can be of further assistance in this matter. O~ity Engineer RRD / bb xc: Steve Goram, Director of Public Works Gary Sieb, Director of Planning and Community Services Shohre Daneshmand, Civil Engineer Mike Tucker, Prentiss-Copley Attachments DONDAVIS.RRD illJJl , -? I] The Honorable Leu Duggan Mayor of the City of Coppell Federal Emergency Mana§ement A§¢ncy Was}fin§ton, D.C. 20472 i~N Case #86-06-47R P.O. BOX 478 ."'.'.. "" Coppell, Texas Dear Mayor Duggan: This is in reference to a letter dated M~ay..-29,~986, and technical data submitted by Mr. Ed Powell, P.E., former City Engineer for the City of Coppell, Texas. In his letter, ~%ich was forwarded to us by our Region VI office, Mr. Powell requested 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 Creek between Coppell Road and Interstate Highway 635. This proposed project involwes realignment of the Grapevine Creek channel including construction of a grass-lined channel with the base width varying from 70 to 200 feet, construction of a concrete drop structure downstream of Interstate Highway 635, and construction of a bridge at Freeport Parkway. Technical data submitted in sup~rt of this request included a report entitled "Conditional Letter of Map Revision Request for Grapevine Creek in Coppell, Texas," prepared by A1D=rt H. Halff Associates, Inc. ~iis report contained a description of methodologies used and hydraulic backwater models for existing and proposed conditions. Additional technical data were submitted on several occasions at our request; all required data to process this request were received by March 18, 4987. During the course of review, we were informed that the channel modification between Coppeli Road and Freeport Parkway and the construction of the Freeport Parkway bridge were completed. Mr. B. 3dlatole Falagan of Albert H. Halff Associates, Inc., submitted hydraulic analyses, ~{nich reflected the completed portion of ~le project and requested that FEMA review these analyses and issue Letter of Map Revision. In reviewing the analyses, we noted that excessive elocities exist in the earthen channel between the downstream end of the ompleted channel modificationo~ and the Freeport Parkway bridge, which could esult in severe erosion and undercutting of both the bridge structure and the edified channel during flooding events. FEMA will not perform a revision at his time to the effective Flood Insurance Study (FIS), Flood Boundary and loodway Map (FBFM), and Flood Insurance Rate Map (FIRM) for the City of Coppell to reflect the completed portion of the channel modification and bridge construction project due to the unstable conditions created by the partially completed project. Our regional office has been notified of the stability problem with the completed portion of the project, and will be contacting your community in an effort to resolve this problem. We have reviewed the data submitted for the entire channel modification and bridge construction project, and have determined that the project as a whole ~eets the minimum floodplain management criteria of the National Flood In- ~/surance Program. The drop structure that is proposed between the completed ~-channel modification and the Freeport Parkway bridge would appear to create stable channel conditions during flood events. If the entire project is completed as proposed, a revision to the effective FIS, PBFM, and FIRM for the 2 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 PIS 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- agement criteria established under the National Flood Insurance Program. The 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 c~npletion 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: "As-built" plans of the channel modification and bridge construction 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 FII~4. ~ ~.~ !~_...~ Revised water-surface profiles of the 10-, 50-, 100-, and 500-year floods reflecting "as-built" conditions, including a zone determi- nation. 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 4e 5J 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 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. 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 basellne model and will be used to accurately measure the effects of the completed project. 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. 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. 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'~loO~ay elevations may not exceed revised base flood elevations by more than 1.0 foot. Delineation of the ~00- 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. This information should be show~ on a map of suitable scale and topographic definition to provide reasonable accuracy. Ail items should be labeled for easy cross-referencing to hydraulic model and summary data, 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 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) 898-9127 or members of our Headquarters staff in Washington, D.C., at (202) 646-2754. Sincerely, ~ ~L.~icks F 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