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Creekview Add PD-CS 871229GINN, IN . 1g 7 CONSLLTING ENGINEERS CITY MANAGER TO: Shohre Daneshmand DATE: December 29, 1987 SUBJECT: Creekview Addition - Variance Request We have reviewed the above referenced subject and related correspondence (attached) with Mr. Carl Anderson for conformance to the City's Floodplain Ordinance. We recommend approval of this variance request based on review of the engineering data submitted by NDM Consulting Engineers on 12/18/87 and 12/21/87 and the comments provided by Mr. Carl Anderson on 12/23/87 (copy attached). Please place this item on the January 12, 1988 City Council agenda per their request. We will be available at the 1/12/88 council meeting to answer any questions that the Council may have regarding this issue. Also, please enclose a copy of this recommendation in the council packets. Thank you and please call if you should have any questions. Alan D. Ratliff Dennis Johnson Wayne Ginn Taryon Bowman Carl Anderson !7103 Preston Road · Suite 10,0 · lB 118 · Dallas. Texa; 75248 · Phone 214,'248-4900 FILE COPY Kimley. Horn and A&~ociate~ Inc. 12660 Colt Road, Suite 200, Dallas, Texas 75251 ,, (214) 386-7007 "~ Raleigh, Durham, Charlotte, Nashville, Virginia Beach, Washington, Dallas, December 23, 1987 w., Palm Beach, Tampa, Orlando, Ft. Lauderdale. Veto Beach, Ft. Myers, Phoenix Mr. Kevin Peiffer Ginn, Inc. 17103 Preston Road - Suite 100 Lock Box 118 Dallas, Texas 75248 re: Creek ¥iew Addition - Floodplain Study Grapevine Creek - Coppell, Texas Dear Mr. Peiffer: We have completed a review of the submittals by Nathan D. Maier Consulting Engineers, Inc. (NDM) on the above project. Submittals reviewed include data from HEC-2 computer models of 12/18 and 12/21/87 as well as their earlier models. As you requested, these reviews have been made in light of their petition of 12/18/87 for a variance to the new City of Coppell Floodplain Management Ordinance. To summarize our reviews, the project as proposed, will have little effect on the Grapevine Creek floodplain. That is, the impacts on the effective flood insurance study (FIS)base flood are nil; the impacts on the ultimate conditions 2-yr flood event are nil; and, although they are evident, the impacts on the ultimate conditions base flood (the design base flood) are felt to be minimal and within acceptable limits. Design base-flood velocities increase at four locations, in areas already having erosive velocities, but in reaches having large floodplain limits to accommodate erosion and meandering which may result from those future velocity increases. Other design base-flood velocity increases are less than 0.1 fps in locations of non-erosive velocities, assuming 8 fps is the erosion threshold velocity. Further, the design base-flood water surface elevations increase from less than 0.1 ft to less than 0.3 ft at seven Building client relationships since 1967 December 23, 1987 - Page 2 Mr. Kevin Peiffer locations within the Creek View study reach of Grapevine Creek. The increases in design base-flood elevation are at locations with existing home finish floor elevations two feet or more above the calculated water surface elevation. It is our feeling that NDM has done all that is reasonablY possible in order to meet the conditions of the new ordinance while attempting to meet the desires of nearby homeowners to minimize environmental/aesthetic impacts and still make the project economically feasible. For these reasons we recommend staff approval of the proposed variance request for the Creek View project, as presented by NDM in their computer models which include data of 12/21/87, with the following provisions: 1. That the floodplain/floodway Ii,nits shown in the drawings be considered as acceptable regions of possible erosion or stream meandering in the erosive velocity reaches; 2. That the finish floor elevations of all existing homes in the Creek View reach of Grapevine Creek are above the design base-flood elevations, as would occur with the proposed development of Creek View, by an amount equal to or greater than elevations otherwise stipulated in the new Floodplain Management Ordinance; and 3. That all other provisions of the Floodplain Management Ordinance are met. If you need further information or have any questions before the City Council meeting to address the variance request, please call. Sincerely, KIMLEY-HORN & ASSOCIATES, INC. Carl V. Anderson, PE Senior Hydrologist NATHAN D. MAIER ONSULTING ENGINEERS, INC. December 18, 1987 FILE COPY Hr. Kevin Pief£er Cinn, Incorporated 17103 Preston Road Suite lO0, LB 118 Dallas, TX 75248 Re: Creek~iew Variance NDM No.: 87-6-062 Dear Mr. Peiffer: Pursuant to our ~eeting of December 14, 1987, we provide you, the City of Coppell and lit. Carl Anderson, with this request for a v. yA,r~I, AIIGACo the new floodplain ordinance for the Creekview project. We vould like to be included on the agenda for the ~-n--~ 12, 1988 council ~etin~ Additional changes have been~ade to the plans and we have again changed the hydraulic models to reflect additional co~promise fro~ those plans previously submitted. Fro~ ou~ December 7, 1987 letter, ~htle not our ~reference, we have incorporated cba~nelization of the overbank along the west ~nd of the ~oJect and aciually reduced the 100-year ultimate water surface upstream from the site. Basic changes that were made to the December 7, 1987 hydraulic model, include widening the floodplain at sections 1.0, 22600, 3.0, and 4.0, a distance of 20', 10', 10', and 10', respectively by moving the fill limit line back farther from the Creek, changed the fill boundary to a 3:1 sloped encroachment rather than a vertical encroacl~ent, and lowering the right overbank 'n' valu~ to account for underbrush clearin~ on the backside of several of the lots. Tabulations of water surface elevations and velocities are enclosed for your review. AC chin point, we have taken all reanonable measu~en to comply with the present ordinance. The fill limit boundary on ~ost of the lots fronting the Creek has been reduced to a 'bare mini~u~', and will cer~ainly li~i~ the building altetmatives on these lots. ~hil~ we ~mst ank. ~ar a v~?snee du~ t~ ~a some slight technical deviations to the new oral-pence.. ~ne. perceive9 \_~- throu the resiA~tial area is not thouiht ti r-. $ - ' - Three NorthPark/8800 N. Central Expwl;./Suite 300/Dallas, Texas ?S231/(214) 739-4741 Mr. Kevin Pleffer December 18, 1987 Page 2 Working with you, John Kahrlsruher, Carl Anderson and Shohre Daneshmand, we have together provided for some very significant improvements to the previous models. These include: Lowering water levels and velocities in certain parts of the study reach up to 0.21 feet. Water levels increase up to 0.27 feet with this project in certain areas, however, no existing structures will be flooded. The maximum water surface increase with the enclosed hydraulic model has been reduced from 0.42 (12/7/87 model) to 0.27 feet. Similarily, the maximum channel velocity increase has been reduced from 0.82 to 0.56 feet per second. The velocity increases generally occur at straight sections of the Creek (e.g. sections 1.0, 22600 and 4.0). The north overbankvelocities along the toe of slope of the existing homes, is generally 2.05 .to 4.31 feet per second, which is not erosive and should not threaten existing structures. Actually reducing ultimate lO0-year flood waters upstream from Denton Top Road by up to 0.20 feet. The project will have no impact on the 2-year flood for ultimate watershed build-out conditions. Being in compliance with all FEMA regulations, and in compliance with the City of Coppell ordinance in effect at the time the project was conceived and largely designed. Lots along the north side of Bethel Road will be filled by each home builder and architect, only to the extent necessary to desisn a home. To this end, maximum fill will not occur on most of the 26 lots alonS Grapevine Creek, especially since each owners motivation will be the preservation of the large trees that presently occupy most-of this area. As you are aware, the models, however detailed, cannot account for these situations, and conditions will surely be benefited by these accommodations. The fill limit boundary on each of these lots has been revised to the absolute minimum without requiring slsnificant channelization of the Creek, loss of trees and vegetation, and without requiring a major redesign of the already approved final plat. Hr. Kevin Pieffer December 18, 1987 Page 3 We feel we have made every effort to achieve a workable solution, given the status and timing of Creekview. The mitigating factors described above, coupled with the fact that we are in compliance with federal (FE~A) criteria, allows us to request your support of this request. Please feel free to call should you have any questions regarding the new model or this request. Sincerely, NATflAN D. MAIER CO~qDLTING ENDINEERS, INC. Dennis L. Johnson, P.E. DLJ/ld cc: Carl Anderson Shohre Daneshmand