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CF-Andrew Park 1-CS 870921 ! I ! ! ! ! PAWA'Winkelmann & Associates, Inc. September 21, 1987 Ms. Maryetta Cunningham Natural Hazards Program Specialist Federal Emergency Management Agency - Region ¥I Federal Center 800 North Loop 288 Denton, Texas 76201-3698 re: Improvements to Andrew Brown Community Park Denton Creek Floodplain - City of Coppell, Texas Dear Ms. Cunningham: The City of Coppell desires to make improvements to its existing Andrew Brown Community Park, which is located in the common floodplain of Denton Creek and Cottonwood Branch, just west of Denton Tap Road. Improvements will involve minor cut and fill operations (within 3 ft above or below existing grade) for proposed playing fields. Concession buildings may also be placed, which will be floodproofed or considered as enclosures. The proposed actions are partially within the designated regulatory floodway of Denton Creek. However, the impacts of the proposed project have been analyzed and found to have no effect upon 100-yr water surface elevations or floodway widths. The following sections briefly describe the proposed project and its ramifications. Maps of the vicinity and the specific site details are also provided for clarification. BACKGROUND & STUDY APPROACH The existing conditions HEC-2 encroachment model for Denton Creek is based on the currently effective Flood Insurance Study (FIS) computer model. Field surveyed cross sections have been added, based on data supplied by Ginn, Inc. Section extensions were supplemented by aerial topographic information with an original scale of 1 in = 200 ft, and are plotted in Appendix 3. The proposed park improvements are located in the right overbank area of Denton Creek, from stream station 327+25 to stream station 342+60. In the existing conditions scenario, the Manning's "n" value assigned for the right overbank through the project reach length was 0.055. 12660 COlT ROAD, SUITE 200. DALLAS, TEXAS 75251 - 2141661-9406 Ms. Cunningham, 9/21/87, Page 2 of 3 The existing conditions HEC-2 encroachment model of Appendix 1 is based on the principle of equal conveyance reduction in both overbanks, based upon the Method 1 and Method 4 options of the HEC-2 model. The results indicate floodway widths through the project site ranging from 110 ft at stream station 327+25 to 968 ft at stream station 342+60. It should be noted from this computer model that the 100-yr discharges vary between the natural profile and the encroached profile. This is due to the common floodplain with Cottonwood Branch that occurs in the natural profile for Denton Creek. In the encroached profile representation, the floodway determination procedure of the HEC-2 model is based on the placement of vertical walls on each side of the channel such as to optimize the target rise in water surface elevation. Thus, the channel sections of the two streams must be separated, and the discharge conveyed by each determined. This has been accomplished by way of a split flow analysis such that the energy grade line elevations of each stream are approximately equal, with their combined flow being equal to the total base flood discharge. EVALUATION The proposed conditions model for Denton Creek is included as Appendix 2 of this report. The geometry of the sections has been modified to account for the proposed park improvements in the right overbank of Denton Creek. These modifications are shown on the cross sections plotted in Appendix 3. The Manning's "n' value has been lowered from 0.055 to 0.050 to account for the more manicured condition of the playing fields in the right overbank. As with the existing conditions model, the discharges vary between the natural and encroached profiles due to the functioning of the split flow action between Denton Creek and Cottonwood Branch. Comparison of the floodway widths and encroached water surface elevations presented in Appendices 1 and 2 indicate no adverse impacts of the proposed park improvements. Floodway widths in the proposed conditions model are less than or equal to those of the existing conditions model. The encroached base flood water surface elevations also compare in a similar fashion. Ms. Cunningham, 9/21/87, Page 3 of 3 II II I! II IJ I 1 1 CONCLUSIONS & RECOMMENDATIONS The proposed improvements to the Andrew Brown Community Park within the City of Coppell, and partially within the regulatory floodway of Denton Creek, present no adverse impacts upon the floodway conditions. Regulations of the 44 CFR Parts 59, et.seq, as administered by the FEMA state that, "Once the floodway has been adopted, encroachments, including fill, new construction, substantial improvements, and other development within the adopted floodway that would result in any increase in flood levels within the community during the occurrence of the base flood discharge is prohibited." Conditions and Criteria [or Floodway Revisions, (FEMA, August, 1984). Documentation is hereby supplied illustrating that no increase in flood levels during the occurrence of the base flood discharges will result from implementation of the proposed improvements. To the best of our knowledge, the attached data should suffice for FEMA review of the proposed project. If additional data are required, or if you have any questions, please advise. Sincerely, PAWA-Winkelmann & Associates, Inc. Carl V. Anderson, PE Vice President attachments: Appendix 1 - extended FIS HEC-2 encroachment model Appendix 2 - proposed conditions encroachment model Appendix 3 - cross sections Appendix 4 - maps of project area enclosure: grading plan cc: John Karlsruher, PE - City Engineer with Ginn, Inc. Shohre Daneshmand - City of Coppell L E G E N D Existing Proposed 0 I000 2000 ~000 Scole in Feet FLOODPLAIN MAN/~EMENT~ Dote: June 1987' Approved by: ~ ANDERSON ENGINEERS, INC. Engineering Hydrologists ':' i:. b NTH Federal Emergency Management Agency Region VI, Federal Center, 800 North Loop 288 Denton, Texas 76201-3698 October 6, 1987 Mr. Wayne Ginn, P. E. City Engineer City of Coppell Post Office Box 478 Coppell, Texas 75019 FILE COPY Dear Mr. Ginn: This letter relates to a report submitted on September 21, 1987, from PAWA-Winkelmann and Associates, Inc., to support a proposed improvement project to the City of Coppell's Andrew Brown Community Park, located within Denton Creek and Cottonwood Branch. In reviewing the report Maryetta Cunningham, Natural Hazards Program Specialist, has identified several important concerns relating to the project as submitted; these are: 11 When an adequate engineering analysis of hydrologic and hydraulic data establishes the NO IMPACT on the base flood elevations, floodway boundaries or carrying capacity would be caused by the placement of the project, a submittal to the Federal Emergency Management Agency is not required; the final decision to accept the report lies with the Community. However, if the project will result in any revision to the identified flood hazard characteristics, it is necessary that a FEf.~ approval be obtained. 2) The report, while it states that the technical analysis establishes no effect upon "lO0-year water surface elevations or floodway widths," fails to provide any evaluation or information relating to the "Concession buildings which may also be placed, which will be floodproofed or considered as enclosures." 3) Because of weaknesses in the City of Coppell's past administration of the provisions of its floodplain management ordinance, we feel it necessary to remind you of the requirement to address other concerns with the project; these are: a) An~ project or activity within an identified floodplain must be processed through the City's permitting system to ensure that all provisions of the ordinance applicable to the specific project will be addressed; the fact that the project is a city project would not exempt it from these requirements. Mr. Wayne Ginn, P. £. b) A concession building falls within the definition of "Structure" as defined by the National Flood Insurance Program (NFIP) regulations. Thus, if it is placed within a floodplain area it must be elevated so that the lowest floor is at or above the currently effective base flood elevation; or, it must be floodproofed in accordance with the criteria set forth in the City's ordinance and certified by a licensed professional engineer as meeting that criteria. The use of "enclosures" as provided in the report ~ou)d necessitate that the specific design criteria for enclosures as set forth in the City's ordinance be incorporated and that a air ' a licensed professonal en-~- ......... ~ .. it b~ certified b specifically relating to ~u~s?ee~lng =ne ordinance criteri~ I wish to emphasize that one appropriate use for a flood hazard area would be a community park or recreational area. However, whenever buildings or other structures are placed within the floodplain, it is necessary that all floodplain management requirements be addressed. Since the technical report submitted b PA - does not involve a r ...... , ~ ..... Y _ WA Winkelmann and Map Revision, it is not b~ ....... ndlt~onal or Final Letter of · · . ~,-g r~.rw~raeo to our technical review staff. However, ~t ~s th~s Agency s pos~t~on that ~ta_~r clar, f~cation of the location of +~ .~y.[?er support,ng technical · ' ~-: )~ruc~ures is necessary before one ~y of Coppell can safely accept the report as sufficient documentation of "no impact on the lO0-year water surface elevation or floodway widths." f.laps should be included identifying the location of both the existing floodplain and floodway boundaries, and the location of any obstruction (such as tennis court fences, etc.) or buildings must be a part of the report. For any obstruction such as tennis court fences within the regulatory floodway, design criteria must be included which will allow the City to safely verify that they would not cause any obstruction to the flow of the floodwaters· Please contact Mrs. Cunningham at 817-898-9161 if you need further assistance in this matter. Because our recent evaluation of the City's floodplain management program identified problems with past implementation and enforcement, we request that you, in your capacity as City Engineer, address the above expressed concerns and provide us with the resolutions reached. Copy to: Shohre Daneshmand City of Coppell Carl Anderson, p. E. PAWA-Winkelmann & Associates Sincerely, Alton S. Ray, Acting Chief Natural and Technological Hazards Division GINN, INC. November 2, 1987 Mr. Alton S. Ray, P.E. Acting Chief, Natural and Technological Hazards Division Federal Emergency Management Agency - Region VI Federal Center 800 North Loop 288 Denton, TX 76201-3698 Re: Improvements to Andrew Brown Community Park Denton Creek Floodplain - Coppell, TX Dear Mr. Ray: This letter is in response to your letter of October 6, 1987, relating to a report submitted on September 21, 1987, to support improvements to Andrew Brown Park. 1) 2) 3) The engineering analysis of hydrologic and hydraulic data established that there was "NO IMPACT" on the base flood elevations, floodway boundaries, or carrying capacity by construction of the proposed improvements. We understood that FEMA approval was not needed; but, the City of Coppell considered the report necessary to make the determination that there was "NO IMPACT". The submittal to the Federal Emergency Management Agency, although not required, was done as a courtesy and indication that implementation of the floodplain management program is being provided. The "concession building" reference that was made in this report was an afterthought, since it is not a part of the Brown Park Improvements but, a separate project. a. This project has been processed through the City. report is a result of City review and comments. This b. The lowest floor elevation of the concession building will be 463.25 which is at or above the current effective base flood elevation of 463 ±. It does not have to be "floodproofed',. We hope that this provides clarification that the structure meets the floodplain management requirements. The City of Coppell has accepted the report as sufficient documentation of "no impact on the 100-year water surface elevation or floodway width". There are no tennis court fences or buildings which will be an obstruction within the regulatory floodway. As City Engineer and floodplain administrator for the City of Coppell, we feel we have addressed your concerns. Should you have any further questions or concerns, please contact us. Sincerely, · Wa Ginn, P.E. GF/HWG/dsp cc- Maryetta Cunningham - FEMA Shohre Daneshmand - City of Coppell Carl Anderson - PAWA-Winkelmann Steve Goram - City of Coppell Alan D. Ratliff - City of Coppell Richard Diano - City of Coppell Lee Richardson - City of Coppell John Karlsruher Gabe Favre File 87365 attachment