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CF-Andrew Park 1-CS 820628June 28, 1982 Federal Emergency Management Agency Denton Federal Center Loop 288 Denton, Texas 76201 Attention: Mr. Dell Greer Reference: Floodway Revision Community Number 480170 City of Coppell, Texas Dear Mr. Greer: The City of Coppeli respectfully requests that you review the enclosed Floodway Revision material. We submit this data as a refinement based on more accurate information than was available during the original study, i.e., 2 foot contour mapping rather than l0 foot contours and additional surveyed floodplain cross sections. Should you find the material satisfactory we would further ask that FEMA approve the Flood- way Revision and subsequently issue a Letter of Map Amendment (LOMA) for this segment of floodway located in the City of CoDpell. The affected Floodway Map is Community Panel Number 480170 0005 B, effective date August 1, 1980. Your assistance in this matter will be greatly appreciated. If you need any further informatiop to aid in your review process, please call ///James R.._E!ium, III ~ JUL%%lgB2° ~/' City Administrator August 2, 1982 FEDERAL EMERGENCY MANAGE[ViENT AGENCY Washington D.C. 20472 IN REPLY REFER . SL-NT-NH (123) MEMORANDUM TO: Mr. R. Dell Greer, Chief Natural and Technological Hazards Division Federal Emergency Management Agency FROM: Richard W. Krimm Acting Administrator, I SUBJECT: Notification of the receipt of an appeal of flood boundaries along Denton Creek and Cottonwood Branch on the Flood Insurance Rate Map for Coppell, Texas A request to revise flood boundaries on the Flood Insurance Rate Map has been received. The request will be evaluated and you will be informed of the outcome. A copy of the request is attached for your information. Attachment Acting Assistant Associate Director Office of Natural and Technological Hazards FLOODWAY REVISION REQUEST CITY OF COPPELL Community Number 480170 MATERIALS 1. HEC-2 Printouts a. Denton Creek Natural Conditions 10, 50, 100, 500 yr b. Denton Creek Encroachment Run c. Cottonwood Branch Natural Conditions 10, 50, 100, 500 yr d. Cottonwood Branch Encroachment Run 2. Topographic Map - Exhibit A Containing: a. 2 foot contour intervals b. Scale 1" = 200' c. Cross section locations d. Delineation of proposed floodway 3. Flood Profiles a. Sheet llP- Denton Creek b. Sheet 14P - Cottonwood Branch CARTER BURGESS. INC. ENGINEERS · PLANNERS !1OOMACON ST./ P. O. BOX 2973/ FT. WORTH. TX. 76113 / ¢817)335-26!1 June 25, 1982 Mr. James R. Elium City Administrator City of Coppell P.O. Box 478 Coppell, Texas 75019 Reference: Floodway Revision City of Coppell Dear Mr. Elium: Carter and Burgess, Inc., on behalf of Univest Corporation, respectfully requests that you review and then submit to the Federal Emergency Management Agency (FEMA) the enclosed floodway revision materials. A floodway revision is being proposed for a 2.16 mile stream segment of Denton Creek extending from DeForest Road to approximately one half mile upstream of Denton Tap Road. The floodway revision is being requested more as a refinement rather than a major change. Additional topographic data and additional field surveyed cross sections have been obtained which provide more accurate information than was available during the original flood study performed for FEMA. The regional office of FEMA was contacted at the onset of this study and they provided input as to the proper steps to be performed in a floodway revision. They also said that the Fort Worth District of the Corps of Engineers had performed the detailed study for FEMA. The source data that was used in the FEMA study was requested and received from the Fort Worth District Corps of Engineers, Flood Plain Management Branch. This data was formatted into a HEC-2 computer model. The above data was supplemented by nine new field surveyed cross sections. The new cross sections were spaced approximately lO00' apart and were approximately 7000' in length. They accurately covered the floodplain on both sides of Denton Creek. The input from these cross sections was then added to the HEC-2 computer model of Denton Creek. The new flood study was performed in accordance with standard FEMA procedures using the HEC-2 computer model. The floodplain was delineated on 2' contour interval mapping. The original mapping was performed for FEMA on 10' contour intervals from U.S.G.S. maps. C&B No. 8135701 Floodway Revision City of Coppell June 25, 1982 Page two A new floodway configuration was determined in accordance with FEMA procedures. Equal reductions in conveyances were made on both sides of the creek. The more accurate data and flatness of the floodplain proved by engineering analysis that the floodway line can be moved from the line marked 'A' to line 'B' on the attached Exhibit. Additional enclosures include flood profiles and HEC-2 computer printouts. We trust this information is sufficient for submission to FEMA. Should you need additional information for your submittal, please contact me. Please confirm that the City of Coppell desires to make the Floodway Revisions requested by signing in the space provided below. Sincerely yours, CARTER & BURGESS, INC. M. Leslie Boyd, P.E. James R. Elium, City Administrator MLB:ddb Enclosures cc: Mike Allen, Univest CARTER BURGESS. INC. ENGINEERS · PLANNERS 1100 MACON ST. / P. O. BOX 2973 / FT. WOI~TH. TX. ?S113 / ¢817lS35-2611 AprU 1~, 1982 Mr. 3ames R. ELium City Manager City of Coppell P. O. Box Coppell, Texas 75019 Reference: Floodway Revision Denton Creek Dear Mr. Elium: Carter & Burgess, Inc. was engaged by UNIVEST Corp. to prepare a detailed flood study investigation on a stream segment of Denton Creek. The stream segment is located in Coppell and is approxi- mately 2.16 miles in length and extends from DeForest Road to approximately one-half mile upstream of Denton Tap Road. The flood investigation has been completed and floodway Limits were developed. We respectfully ask that you review all data submitted herewith and consider whether th~s information is sufficient to satisfy your needs. A brief explanation of the investigation will be presented below so that you will better undeKstand the various processes and logic of the new floodway configuration. The last Flood Insurance Study for Coppell was published by FEMA in 1980.'- The Flood Insurance Study engineering analysis was performed by the Flood Plain Management Branch of the Fort Worth District Corps of Engineers. A copy of the Denton Creek computer model card deck was requested from the Corps of En§ineers. A source deck was not available since the analysis had been performed using the Thomas program. The Thomas program wa~ an in-house program used by the Corps of Engineers. The current program most commonly used in flood study investigations is HEC-2. HEC-2 is a water surface profile program developed by the U.S. Army Corps of Engineers Hydrologic Engineering Center (HEC) in Davis, California. The input data listing received frOm the Fort Worth Corps of Engineers was trans- formed in format to that of HEC-2 for use in our investigation of Denton Creek. There were only two cross sections, excepting the bridge sections at Denton Tap, within the ;tream segment of the UNIVEST property. In GSdB No. 8135701 Mr. 3ames R :lium April 14, 1992 Page 2 order to develop a more accurate stream model for the flood Investi- gation, nine additional cross sections were surveyed in September and October of 1981. The locations of the new cross sections are shown on Exhibit A. The 'data from these cross sections was coded into the HEC-2 computer model. The construction of the computer model was completed In October 1981. The initial computer results matched those shown for the 100-year flood profile In l~e 1980 FIS within 0.3 foot. This was considered to be a good check and verifi- cation of the beginning computer model was assumed at this point. Following the above verification of the basic model, the model was updated to reflect existing hydraulic conditions. This consisted of removing certain non-conveying pits located In a few of the cross sections. The areas were not conveying since they were localized depressions without continuity in the direction of flow. Once these changes were made, the base flood elevations changed slightly. It is this revised base flood profile which was used in later floodway dele r minations. Floodway determinations were accomplished in the following sequence: Optimize the maximum encroachment to achieve close to one foot rise above base flood at each cross section. This results in an irregular shaped floodway which is generally not acceptable to FEMA. · Modify the floodway configuration from the above step into a smooth shaped floodway. Make additional computer runs and trial changes until floodway is smooth and all increases above 100-year base flood elevations are within acceptable limits. The final floodway configuration as deter mined from the above methods is shown on Exhibit A. It is this floodway which you are respectfully requested to review. HEC-2 computer results are being transmitted herewith to substan- tiate all floodway changes. We trust the information contained in this letter and attachments is sufficient for your review process. However, if you have any questions or need anything more, please feel free to call.' Please contact us after you have reviewed this material. Sincerely yours, MLB/cb Enci. cc: Mike AlLen CARTER & BURGESS, INC. M' LeSlie Boyd, P.E. ~. FILE COP:Y U.I. OIII)ARTI~MT OIl MOUIINO ANO URBAN OIV~L. OltIENT NAME. TITI. I ANO ORGANIZATION Opt CAI. I. ER (Si/ VISITOIll (I) SUBJECT PROJECT NUMBER ACTION REQUIRED ACTION TAKEN COMMENTS: Co.dinated with:~. Infraction Copi/s't'o: Confirmti~ Co~ to: TIT~ ANO ORGANIZATION SIGNATURE FIUD-734 lo-61! P~EvIOUS EDITION MAY I! USED ~.S. GC//EllJS~NT /'ltXIrgXIC OITZCK: 19751671-1~Ill2sS U.I. I)I[IIAI~TIdI, iiI,, O1' ~UItH ANO URBAN OEYEI.,OIDMIINT OAT"' MIM0k.MINM FOil FILE '",,.,,'NO,, CA,.,. NAIdI'. "r I?l.E AND ONQANIZA?ION OF CAI. L. EIR II)/ VIIITON SUlJEC? PNOJI[CT NUMe,N ACTION I~EqI. IINEO ACTION TAKEN COMMON?S: 5~F~i-P~'~°TI~ '~'Or~T~O,'~ ~F T-HI .'~ I0£,-'-r'c-~l~ ~LOO.-Ol~c_~t,a, ~ ,~/~_~ A e--o/-~.~ o~' -r-H~)-r- Po~--r~o,x.) ~F ~-~ Co~dinated wit~ In~ti~ Co~~ Confirmti~ Copy to: USI,o MIMM~U41NM F011 FILl NAME. ?I?~.E ANO ONQANIZATION OF CALKER III/ VIIITON III ACTION NEQUII~EO ACTION TAKEN CQMMENTS: Co~dinated with: ~ ~ / Confirmti~ Co~ to: TIT~E ANO ORGANIZATION IIGNATURE IIIMOllANO4JM FOIl FILl NAME, TirOl 'l~lJ[C~ PMOJICT NWIIM ACTION M[qUIMiD ACTION TAKIN COMMBNTSt. I! UIID CARTER O BURGESS, INC. ENGINEERS. PLANNERS File Les Boyd Univest Floodplain STATUS'- REPORT Subsequent tO_ the receipt of the_~__ield .survey. data we have _performed the following items of work: A. Plotted the cross-sections at scale of 1"=100' horizontal and 1"=5' ver- ticnJ. Reduced the cross-sections to coding for HEC-2 computer model of Denton Creek. C. Keypunched and established data files for the computer model. D. Made computer runs using the 100-year discharge to verify reasonable con- formance with previously published flood elevations. - Modified the computer-model to determine floodway limits. Refined the floodway determinations to maximize the amount of land to be recovered. G. Checked the encroachment model for sensitivity at certain problem sections. H. Reviewed the results and conferenced with Bill Bell to check for general reliability. I. Made minor changes to model on the west side of Denton Tap. C&B No. 813J57-05 FILE COPY