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Copperstone FS-CS 890824Jerry Parchd Consulting Engineers August 24, 1989 Mr. Ron Morrison, P.E. Kimley-Horn and Associates, Inc. 12660 Colt Road, Suite 200 Dallas, Texas 75251 214/386-7007 HAND DELIVERED RE: JP No. 88009; Request for a Conditional Letter of MaD Revision; Parks of Coppell; Coppell, Texas Dear Ron: The enclosed letter from the Federal Emergency Management Agency was received by our firm on July 6, 1989. In this letter FEMA cites differences between a study submitted by our office to the City on February 21, 1989, entitled Flood Plain Reclamation Study on Cottonwood Branch and Denton Creek for Flood Plain Reclamation and Construction of Parkway Boulevard Bridge and a study submitted by Kimley-Horn and Associates, Inc. to FEMA to revise and update the City of Coppell Flood Insurance Maps. FEMA requests in their letter that these differences be resolved so that a single HEC-II model for existing conditions in Denton Creek and Cottonwood Branch results. The referenced letter from FEMA states that a resolution of this matter must be received within 90 days of the date of the FEMA letter, which is July 3, 1989. Therefore, our response must be provided to FEMA by September 30, 1989. In response to FEMA's request, you and I have met on June 21, July 26, August 2 and August 10, 1989 in order to resolve differences in our hydraulic models. Enclosed are copies of the four hydraulic models of Denton Creek and Cottonwood Branch, which we originally submitted to the City in February, that are revised according to our recent discussions so that our models of existing conditions in Denton Creek and Cottonwood Branch are now similar. I trust that the revisions included in the hydraulic models attached hereto and Mr. Russell Doyle's confirmation of June 20, 1989 that this study is exempted from provisions of the October 1987 City Flood Plain Ordinance and should be reviewed in accordance with City Ordinance Number 219 will permit you to complete your review of our study. If approved, I request on behalf of ABQ Development that the City of Coppell notify FEMA of 320 V~'estway Place, Suite 501 · Arlington, Texas 76018 · Metro (817) 467-2418 · (817) 465-0259 Mr. Ron Morrison, P.E. JP No. 88009 August 23, 1989 Page 2 of 2 the CitY's endorsement of the proposed project and that a subsequent submittal from the City will contain hydraulic models of existing conditions in Denton Creek and Cottonwood Branch similar to the models attached hereto. This documentation should permit FEMA to continue their consideration of our project for a Conditional Letter of Map Revision. Thank you for the assistance you have provided in resolving this matter. If you have any questions or if I can provide any further assistance, please call. Sincerely_ Jerry Parche, P.E. JP:ch Enclosures: HEC II Computer Outputs: Model DCEXKH.DAT-Existing Conditions Denton Creek Model CEX3KH.DAT-Existing Conditions Cottonwood Br. Model DCPRKH.DAT-Proposed Conditions Denton Creek Model CPR4KH.DAT-Proposed Conditions Cottonwood Br. Letter from FEMA dated July 3, 1989 cc: Mr. Jay Chester, P.E./ABQ Development Corporation Mr. Russell Doyle, P.E./City of Coppell Federal Emergency Management Agency Washington, D.C. 20472 Mr. Jerry' Parch~ Jerry Parch~ Consulting Engineers 320 Westway Place, Suite 501 Arlington, Texas 76018 Dear Mr. Parch~: Case No.: 89-06-31R Community: City of Coppell, Dallas and Denton Counties, Texas Community No.: 480170 This is in reference to a February 24, 1989 letter from Mr. Russell R. Doyle, P.E., City Engineer and Floodplain Administrator for the City of Coppell, in which he requested, on your behalf, that the Federal Emergency Management Agency (FEMA) issue a conditional Letter of Map Revision (LOMR) for the proposed Parkway Boulevard bridge located near the confluence of Cottonwood Branch and Denton Creek. With his letter, which was forwarded to us by our Region VI office, Mr. Doyle submitted a technical report prepared by your firm, entitled Floodplain Reclamation Study on Cottonwood Branch and Denton Creek for Floodplain Reclamation and Construction of Parkway Boulevard Bridge, dated February 1989. This report contained the following: HEC-2 hydraulic models representing existing and proposed lO0-year flood and floodway condi- tions for Denton Creek and Cottonwood Branch; topographic mapping delineating the proposed lO0-year floodplain boundary and the proposed floodway configura- tion; and plotted cross sections for Denton Creek and Cottonwood Branch. Please note that we have also received a January 12, 1989 letter from Mr. Doyle, tn which he requested that FEMA revise the September 22, 1988 Flood Insurance Study and Flood Insurance Rate Map for the City of Coppell to include updated corporate limits information and the results of a technical report prepared by Kimley-Horn and Associates, Inc., consulting engineers for the City of Coppell. A comparison of the hydraulic models for Cottonwood Branch and Denton Creek from your submittal and those from the Kimley-Horn and Associates, Inc., submittal indicates differences in the technical analyses that yield different results. These differences are listed below. Kimley-Horn and Associates, Inc., has modeled the two streams with an assumed common floodplain from their confluence upstream to a point approximately 1,220 feet upstream of Denton Tap Road. Your submittal models the two streams as having an assumed common floodplain from their confluence upstream to a point approximately 475 feet upstream of Denton Tap Road. The lO0-year flood discharges utilized in the submitted HEC-2 hydraulic models for Denton Creek are outlined below. JUL 6 1969 lerrt' PiTche Co~sultlrll En~. Cross Section lO0-year Flood Discharges Kimley-Horn and Jerry Parch~ Associates, Inc, Consulting Engineers 28870 (below Cottonwood 21300 Branch) 32150 21300 32780 (above Cottonwood 21300 Branch) 33145 21300 36970 15600 39440 (at Interstate Route 15600 i2i) 39540 15400 21300 21300 14900 14900 15600 Please note that the flood discharges listed above are for the lO0-year frequency storm only and that there may be similar discrepancies in the 10-, 50-, and 500-year frequency storms. Also note that the resolution to these differences should be coordinated with Kimley-Horn and Associates, Inc., and the City of Coppell so that a single HEC-2 hydraulic model for Denton Creek and for Cottonwood Branch will result. Please forward your response along with any supportive technical data to our technical evaluation contractor, Dewberry & Davis, 8401 Arlington Boulevard, Fairfax, Virginia 22031, to the attention of Mr. Dave Lushbaugh, Management · Engineering and Technical Services Division. Our processing of this request will be considered inactive until the aforementioned differences are resolved. Please note that if a complete resolution is not received within 90 days of the date of this letter, this case will be treated as an original submittal and will b~ subject to all submittal/payment procedures including the initial fee. 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 Matthew B. Miller of our Headquarters staff in Washington, D.C., at (202) 646-3461. Sincerely, Federal Insurance Administration cc: Mr. Russell R. Doyle, P.E. Mr. Ronald Morrison, P.E., Kimley-Horn and Associates, Inc. Following is documentation of input data to the Jerry Parche' Consulting Engineers hydraulic computer models of Denton Creek and Cottonwood Branch prepared in August, 1989 so that models prepared by Kimley-Horn & Associates, Inc. for the City of Coppell restudy and Jerry Parche' Consulting Engineers for the Parks of Coppell Project are similar. Previous revisions made by Ron Morrison to the Denton Creek model downstream of Denton Tap Road are not discussed herein. Denton Creek downstream sections 24700 through 31230 have been added from the Kimley-Horn model. ET cards were modified for using appropriate fields for Method 1 Encroachment or Effective flow limits. The ET Encroachment Run Stations at section 24700 from the January, 1989 Kimley-Horn model are input into the Jerry Parche' Consulting Engineers' models of Denton Creek. Encroachment Stations input at sections 25530 through 32150 are taken from the revised Kimley-Horn model received from Ron Morrison on August 10, 1989. The encroachment stations at section 32150 are similar to encroachment stations used at this section in the February, 1989 JPCE model. Water surface elevations for the lO0-year flood plain and encroachment models at Denton Creek section 24700 are taken from the January 1989 Kimley-Horn model. Sections 32150, 32200, and 32230 February, 1989 JPCE model with January, 1989 Kimley-Horn model. were these replaced in the sections from the Section 32250 is from the February, 1989 Jerry Parche' Consulting Engineers model. It is similar to the January, 1989 Kimley-Horn model. Distances upstream to this section are revised to be in accordance with the Kimley-Horn model. Section 32305 to section 34950 which partially lie within the Parks of Coppell are from the February, 1989 JPCE model. Section 35170 from the February, 1989 JPCE model was deleted since this section is not in the Kimley-Horn model. Sections 38200 and 39290 on Denton Creek from the Kimley- Horn model were added to the JPCE model. Upstream sections 36970, 38200, 39290 and 39440, which are upstream of the Parks of Coppell, are exactly the same as these sections in the Kimley-Horn model. Values on NC and NH cards were adjusted to agree with values in the Kimley-Horn model. 10. All sections in the current Cottonwood Branch models upstream of the combined Denton Creek/Cottonwood Branch Flood Plain area were taken from the February, 1989 JPCE model.