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Copperstone FS-CS 890902Jerry Parcl & Consulting Eng,neer 10 Oy t. September 2, 1989 Mr. David Lushbaugh Management Engineering and Technical Services Division Dewberry & Davis 8401 Arlington Boulevard Fairfax, Virginia 22031 RE: JP No. 88009; Flood Plain Reclamation; Denton Creek and Cottonwood Branch; Coppell, Texas Dear David: On February 21, 1989 we submitted a study to the City of Coppell entitled Flood Plain Reclamation Study on Cottonwood Branch and Denton Creek for Flood Plain Reclamation and Construction o: Parkway Boulevard Bridge. This study was prepared for submittal to FEMA in support of our request for a Conditional Letter of Map Revision for the Parks of Coppell project. The proposed project is located in the City of Coppell, Dallas County, Texas, on the west side of Denton Tap Road. Our previous submittal contained the following: Four HEC II hydraulic models of the 100-year flood in Denton Creek and Cottonwood Branch as follows: Model DCEX - Existing Conditions on Denton Creek from Section 32150 to Section 39440. Model COEX - Existing Conditions on Cottonwood Branch from Denton Creek Section 32150 to Cottonwood Branch Section 8000. Model DCPR - Conditions on_ Denton Creek with proposed fill in the Parks of Coppell project. The reach analyzed in this model extended from Denton Creek Section 32150 to Section 39440. Model CPR4 - Conditions on Cottonwood Branch with proposed fill in the Parks of Coppell and a proposed Parkway Boulevard bridge over Cottonwood Branch. The reach analyzed in this model extended from Denton Creek Section 32150 to Cottonwood Branch Section 8000. B. Exhibit "A" showing Existing Conditions Prior to Construction. C. Exhibit "B" showing Conditions with Proposed Reclamation and Bridge. 320 L~'estway Place, Suite 501 * Arlington, Texas 76018 * Metro (817) 467-2418 · (817) 465-0259 Mr. David Lushbaugh JP No. 88009 September 2, 1989 Page 2 of 5 Cross sections of Denton Creek and Cottonwood Branch as included in the hydraulic models presented in the February, 1989 report. These were submitted separate from the bound report. The information presented herein provides an update of the aforementioned February, 1989 submittal in response to a letter received from Mr. John L. Matticks, Chief, Risk Studies Division, Federal Insurance Administration, Dated July 3, 1989. A copy of Mr. Matticks letter is enclosed. In his letter, Mr. Matticks cites differences in the technical report we presented in February, 1989, and a report submitted by Kimley-Horn and Associates, Inc. in January, 1989 on behalf of the City of Coppell to revise the City's Flood Plain maps. Mr. Matticks letter states that these differences should be resolved so that a single HEC II model of existing conditions in Denton Creek and Cottonwood Branch results. In response to this request, the city's flood plain consultant, Mr. Ron Morrison of Kimley-Horn and Associates, and I have met on several occasions and have resolved all differences between our respective' existing condition models in the project area. A letter regarding this is being provided to FEMA by the City. The City of Coppell F.I.S. containing these revised models of Denton Creek and Cottonwood Branch will be submitted to FEMA in early September, 1989. Ail hydraulic models presented in our February, 1989 submittal have been rerun and are included herein. The downstream limit of these hydraulic models is Section 24700 on Denton Creek. Hydraulic models of the 100-year flood with existing conditions in Denton Creek and Cottonwood Branch are included herein in Appendix A. Hydraulic models of proposed reclamation and the proposed Parkway Boulevard bridge over Cottonwood Branch are included herein in Appendix B. These hydraulic models are identified below. Model DCEXKH - Existing Conditions in Denton Creek from Section 24700 to Section 39440. Model CEX3KH - Existing Conditions in Cottonwood Branch from Denton Creek Section 24700 to Cottonwood Branch Section 8000 in the Villages of Cottonwood Creek project which is upstream of the Parks of Coppell. Mr. David Lushbaugh J? No. 88009 September '2, 1989 Page 3 of 5 Model DCPRKH - Proposed Conditions in Denton Creek. From section 24700 to section 32230 on the upstream side of Denton Tap Road this model is similar to model DCEXKH since this reach is outside of the project area and no reclamation is proposed. Sections 32250, 32305 and 32725 are in the combined Denton Creek-Cottonwood Branch flood plain area and contain proposed reclamation on the south side of Cottonwood Branch authorized by a Conditional Letter of Map Revision received from FEMA dated January 26, 1989 (Case No. 89-06-06R). Sections 33145 through 34950 include proposed reclamation on the south side of Denton Creek in the Parks of Coppell, if any. Sections 36970 through 39440 are upstream of the project and are similar to existing conditions model DCEXKH. Model CPR4KH - Proposed Conditions in Cottonwood Branch. From sections 24700 to section 32725 this model is similar to model DCPRKH as described above since this reach is in the Denton Creek flood plain or combined Denton Creek- Cottonwood Branch flood plain area. Sections 32816 through 33116 model the proposed Parkway Boulevard bridge and channel improvements. Sections 33560 through 34472 model proposed reclamation in the Parks of Coppell. Sections 6710 and 8000 are existing cross sections located upstream in the Villages of Cottonwood Creek project. In his letter of July 3, 1989, Mr. Matticks cites two specific differences between the models we submitted to the City in February, 1989 and the models submitted to FEMA by Kimley-Horn on behalf of the City of Coppell in January, 1989. These differences and the resolutions arrived at by Kimley-Horn and our firm are described below. Differences in the location of the upstream limits of the common Denton Creek-Cottonwood Branch Flood Plain. After reviewing the detailed topography available for the Parks of Coppell project and comparing bank elevations on the south side of Denton Creek with 100- year flood levels, our firm and Kimley-Horn have agreed that the upstream limits of the common flood plain are approximately at Denton Creek section 32725. Both the revised city models to be submitted to FEMA and revised Mr. David Lushbaugh JP No. 88009 September 2, 1989 Page 4 of 5 models contained herein for the Parks of Coppell project evidence this by a change in discharge from the combined flood plain discharge at section 32725 to the 100-year flood discharge in each creek at the next section upstream. Differences is the 100-year Flood Discharges used in the City's models and the Parks of Coppell models. Discharges used in the City of Coppell study prepared by Kimley-Horn & Associates that will be submitted to FEMA and our models of Denton Creek and Cottonwood Branch prepared for the Parks of Coppell project as revised and presented herein are now similar. Current hydraulic models use the following 100-year flood discharges at the sections listed in Mr. Matticks letter. CROSS SECTION 100-YEAR ULOOD DISCHARGE (c.f.s.) Kimley-Horn & Jerry Parche' Associates, Inc. Consultinq Enqineers 28870 (below Cottonwood Branch) 21,300 21,300 32150 21,300 21,300 *32725 21,300 21,300 33145 14,900 14,900 36970 15,600 15,600 39440 15,600 15,600 39540 15,400 * This section was incorrectly identified in Mr. Matticks letter as 32780. The study description included with our initial submittal to the City of Coppell in February, 1989 is included in Appendix C of this report. Much of the information regarding the Parks of Coppell project and the Denton.Creek and Cottonwood Branch flood plains presented in that report is still current. Flood plain and floodway elevations and flood discharges presented in our February, 1989 report, however, are superceded by information presented in this letter and in Appendices A and B contained herein. Mr. David Lushbaugh JP No. 88009 September 2, 1989 Page 5 of 5 Exhibits "A" and "B" showing existing and proposed flood plain conditions respectively in the Parks of Coppell are included in the map pocket in the back of this report. Cross sections of Denton Creek and Cottonwood Branch modelled for this study, as revised herein, are presented separate with this report. I trust that the information presented herein will permit Dewberry & Davis to proceed with the review and issuance of a Conditional Letter of Map Revision for this project. The City of Coppell has informed me that they will provide a letter to FEMA endorsing the proposed reclamation and Parkway Boulevard bridge concurrent with this submittal to you. If you need any other information or have any questions, please call. Since time is now extremely important, I appreciate your prompt attention to this matter. Sincerel~ p/P.E. JP:ch Enclosures: .July 3, 1989 Letter from Mr. John Matticks Appendix A - Models DCEXKH and CEX3KH Appendix B - Models DCPRKH and CPR4KH Appendix C - Report included with February, 1989 submittal of this study Map Pocket - Exhibit A - Existing Conditions Exhibit B - Proposed Conditions Separate - Creek Cross Sections - seven sheets cc: Mr. Jay Chester, e~--/A opment Corporation w/enc. Mr. Russell Doyle, P.E./City of Coppell w/enc. Mr. Ron Morris,, p.E./Kimley-Horn & Associates, Inc. w/enc.