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Carter Phase 2-CS000406 · Coppell, Texas 75019 April 6, 2000 Brian Rathe Rathe Investments 2700 North Stemmons East Tower, Suite 403 Dallas, Texas 75207 RE: Carter Estates, Phase II Dear Mr. Rathe: I have received the preliminary information concerning your desire to construct an access road and a waterscape feature at the rear of Lot 2, Carter Estates, Phase II. Before any work is performed, a revised flood study will have to be submitted to this office. Depending on the nature of the improvements and the impact the improvements have on the floodplain, the study may also have to be submitted to FEMA. It should also be noted that the work as proposed is within a designated floodway of which there are stricter requirements in the City of Coppell's Floodplain Management Ordinance. There are review fees and submittal fees for these studies. The original flood study utilized the area in which you are proposing to create a waterscape feature as a swale for conveyance of water. This was necessary due to a rise in the water surface when fill was placed on the front of the lots. This area also acts as valley storage to meet our valley storage requirements. The impounding of water reduces the valley storage and the conveyance area. Your letter implies that you would like verbal approval to proceed with the work at your earliest convenience. Please be advised there will no approval of any work without a revised flood study showing the impacts of the work in the 100-year floodplain and floodway. If you should have any questions, please feel free to contact me at your convenience. Sincerely, Kenneth M. Griffin, P.E. Director of Engineering and Public Works 'x, l Tm I 4W. ST 4 TS March 29, 2000 03-30-00A08:58 RCVD Ken Griffin Ciq-' of Coppell 255 Parkway Blvd. P.O. Box 478 Coppell, TX 75019-4409 .~ !-..',-D Dear Ken: Recently, I purchased Lots 1 and 2 of Carter Estates Phase 11 subdivision. You may recall Bob Wright and l came and spoke to you regarding the building site on lot 2. Bob Wright will be contacting you shortly to request a permit for Fill on parts of lots 1 and 2. I am writing to discuss with you what I think is a much more informal request. As you are well aware, when Don Carter dix ided the lots in Phase Il a large portion of Lot 2 was excavated and the Fill was placed on the front of Lots I thru 5. The large excavated area is what l would like to discuss. I would like to install a roa~l coming off Moore Road that would go thru the center of the '~,a :aa~ ..~' excavated area, providing access to the back of Lot 2. 1 would build the road up so that it would .-' ~: -,-f, '0',-~. ; also act as a dam to create a small Waterscape. This \Vaterscape will be no deeper than 3 to 4 feet 3 a!... '~. ? and encompass an area no greater than the existing excavated area (about 3000 square feet of · ~' "':"'" ~ "- surface). The water source would be the drainage coming from the Beltline Road area. The road would be constructed of quality fill and crushed rock in order to avoid further erosion from heavy.-' rains. It would also include a main drain culvert and an overflow area in case of heavy rains. The road and \Vatersape design will insure that the bank of Grapevine Creek will not erode as it is now with the current drainage situation. At the same time the road is installed I will landscape the Moore Road frontage area x~ ith mature trees and shrubs, r-'~ ~'~ '~"-".' ~ '" ~x./.,~ '7i ' .-.~,.~C ~.--.'r~ , I do not envision using this road very often at all. I do no~_t need a curb cut from Moore Road. The purpose of the road is simply to gain access to the land when necessary and assist in creating a Waterscape. Please find enclosed rough drafts of the design. I would like to request 3'our approval at this point in order to proceed with the work. I will contact y.-'ou shortly.' to discuss in further detail. Thanks for your time and assistance and I will look forward to our next conversation. Sincerely. Brian L. Rathe 2700 North Stemmons East Tower Suite 403 Dallas, TX 75207 214.678.9898 I The Flood Plain Management Ordinance of the City of Coppell requires that the lowest i" floor of a structure be elevated to a minimum of two feet above the F.I.S. base flood elevation or to one foot above the design base flood elevation, whichever is higher. The Design Base I Flood is defined as the 100-year flood from a fully urbanized watershed. As shown in the Table on Exhibit A, the fully developed 100-year flood water surface elevations exceed the existing watershed 100-year flood water surface elevations by more than 1 foot at every cross section I studied. Therefore, minimum finish floor elevations in the Carter Addition will be based on the Ifully developed watershed 100-year flood levels. I PROPOSED FLOOD PLAIN RECLAMATION I j The area to be reclaimed from the 100-year flood plain is shown on Exhibit A. This area - - will be elevated so that finish floor levels of proposed houses on Lots 1-5 in the Carter Addition can be constructed a minimum of 1 foot above the fully urbanized 100:year flood elevation I computed at the upstream limit of each lot. Proposed fills are shown on the cross sections submitted with this study. The transition from proposed pad level to natural grade is accomplished by a 4:1 slope. i ' Initially, a computer model was prepared to determine the floodplain response with the ·. proposed fills only. The results of this model indicated that proposed fills alone were found to I' cause slight increases in the fully urbanized watershed 100-year flood level. Therefore an -j'". . excavated swale is proposed on the south side of the creek to provide conveyance area to compensate for the flood area lost due to filling. The size of the swale was carefully determined -7- at each cross section studied such that no rise resulted in the lO0-year flood level with fully urbanized conditions in accordance with City of Coppell requirements. The location of the proposed swale is shown on Exhibit A and the swale is shown in the cross sections submitted with this report. ! The computer model PROULT29 was prepared to analyze the proposed conditions in the I flood plain with the fully urbanized watershed 100-year flood through the project. Both the I proposed fill and swale are included in the model. A copy of the output data from this model is included in Appendix Iff. I The proposed f'flls and swale are input at cross sections 18650 through 19662 in model PROULT29. Manning's 'n' factor, cross section locations, channel bank stations, beginning water surface elevations and effective flow limits in model PROULT29 ~are similar to values used in the existing flood plain fully urbanized watershed 100-year flood model CARULT4. Also sections 18500 through 18615 and sections 20000 through 25085 are similar in both models. A comparison of the results of the 100-year flood levels determined in the proposed and existing condition models with a fully developed watershed in presented in Table 2. -8- $¢rry Parcha' Co~ulfing i.e. 466.39, at section 18615 and since this elevation is higher than the elevations computed at the next upstream section 18650 in both models, then the regulatory floodway elevation at section 18650 is 466.39 for both existing and proposed conditions. This is in accordance with FEMA criteria as stated above. Documentation is presented in Appendix IV to demonstrate compliance with additional requirements of Section C, Article 4 of the City of Coppell Floodplain Management Ordinance regarding velocity and flood plain storage impacts. HEC H models EX2YULT4 and PR2YUL29, for existing and proposed conditions respectively, presented in Appendix IV were prepared to evaluate impacts on channel flow velocity with a 2-year storm due to proposed improvements. A review of the velocities computed in both models indicates that velocity changes are very minor with the proposed improvements. During the 100-year flood a review of channel velocities computed in models CAREX3 and model PROEX29, which were discussed previously, for existing and proposed conditions respectively also indicates only minor changes in channel velocities due to proposed improvements. The maximum velocity decrease was -0.11 fps and the maximum increase was +0.08 f.p.s. Models EX4VOL and PRO29VOL included in Appendix IV were prepared to evaluate impacts on flood plain storage due to proposed improvements. In model EX4VOL, total storage in the existing flood plain through the project reach was computed to be 101.28 acre feet for the 100-year flood. This was computed by setting the computed water surface elevation from model CARULT4 at each cross section and omitting effective flow limits. Similarly, the total storage was determined for the proposed channel and flood plain condition with the 100-year flood in model PRO29VOL and computed to be 97.54. acre feet. Storage reduction is 3.7 % which is less than the maximum 15 % allowed by the city ordinance. CONCLUSION This study has presented an analysis of the hydraulic impacts due to the proposed construction of five pad sites for construction of homes on the north side of the Grapevine Creek Flood Plain just upstream of Moore Road in Coppell, Texas. Copies of the existing watershed condition flood plain models were obtained from FEMA's consultant, Michael Baker, Jr. and copies of the fully urbanized watershed condition flood plain models were obtained from the city's consultant. Computer models were prepared for this study to improve the existing condition models within the project reach and to evaluate hydraulic impacts of proposed reclamation. A swale is proposed on the south side of Grapevine Creek as a result of this analysis. The swale is needed to comply with the city's flood plain ordinance requirement that proposed reclamation not increase water surface elevations of the design base flood. We trust that information presented herein demonstrates that the proposed project is in compliance with the flood plain development regulations of the City of Coppell and the Federal ' Emergency Management Agency. On behalf of our client, we request that the City review and .: approve this report with attached exhibits, cross sections and computer models, for submittal to FEMA. We will then prepare FEMA forms to be submitted with the application and request i:' .{