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Cottonwood Est-SY 190911O’BrienEngineering, Inc. 2340 E. Trinity Mills, Ste 220, Carrollton TX 75006 972.233.2288 Ph ⋅ 972.233.2818 Fx obrieneng.com Texas Firm ID # F-3758 SDVOSB | Texas HUB | TxDOT SBE Firm | Since 1987 September 11, 2019 Mr. Kumar Gali, P.E., CFM, D.WRE Assistant Director of Public Works Department of Engineering City of Coppell 265 E. Parkway Blvd. Coppell, TX 75019 RE: 125 Cottonwood Drive Lot Improvements OEI Job # 347-003 Dear Mr. Gali: The Holmes Builders (Holmes) is constructing a new home at 125 Cottonwood Drive in Coppell, Texas. The new home will not be located within the effective floodway or floodplain; however, the proposed reshaping of an existing pond onsite lies largely within the FEMA floodway of Cottonwood Branch Creek. The existing pond is the shape of a boot, which is 200 feet long – roughly following the flow path of the floodplain – and 165 feet wide – roughly perpendicular to the floodplain flow path. A small portion of the pond bank projects onto the neighboring property to the northeast, although the actual pond does not. The projection of the bank result in terrain that, for the purposes of mowing and other maintenance, is impassable. The intent of reshaping is twofold: 1) to bring the pond with its bank, fully onsite to facilitate maintenance access, and 2) aesthetics. Holmes had begun regrading the pond and were notified by the City that analysis we needed before the work could proceed. Holmes engaged O’Brien Engineering, Inc. (OEI) to evaluate the impacts of the regrading on the floodplain. Coppell Floodplain Ordinance Section 16-4-24 6.a. states: Encroachments are prohibited within designated floodways, including fill, new Mr. Kumar Gali, P.E., CFM, D.WRE September 11, 2019 Page 2 of 3 construction, substantial improvements and other development unless certification by a registered professional engineer is provided, demonstrating that equal conveyance encroachments do not result in any increase in flood levels at any floodplain location and that all other provisions of this ordinance are met during the occurrence of the base flood discharges. Although there is no encroachment proposed and therefore equal conveyance encroachments are precluded, it is still necessary to demonstrate that the proposed modifications will not result in any increase in flood levels. The City provided a hydraulic model database of Cottonwood Branch, which we used to analyze the existing and proposed conditions. Several pieces of creek and different conditions were represented in the database, none of which, by themselves provided adequate or representative coverage of the subject area. Therefore, we joined the lower channel cross sections 6710 and 8000 from the Duplicate Effective - FP model to the LOMR - Sandy Lake - AsBuilt model to represent the City’s Model for the subject area for Effective conditions. The City’s model was then used as the basis for the Pre-Project model by adding 6 cross sections in the immediate project area (8621, 8651, 8771, 8783, 8823, and 8890), using TNRIS to adequately define the transitions in the floodplain due to the existing pond. Parameters in the new cross sections – contraction, expansion and friction – were held equal to those that are in the effective model for corresponding portions of upstream and downstream cross sections, with the exception that at the inlet and outlet of the pond, the friction in sections 8621 and 8890 was reduced to 0.04 to reflect localized increased friction slope due to the presence of the pond. The Pre-Project model was used as the basis for the Post-Project model. All model parameters that were developed for the Pre-Project model were held in the Post-Project model including blocks, friction, and expansion and contraction coefficients. The section geometry was modified to reflect the proposed reshaping (regrading). Due to the increased conveyance resulting from the wider pond at sections 8771 and 8783, a low Mr. Kumar Gali, P.E., CFM, D.WRE September 11, 2019 Page 3 of 3 berm is proposed on the west side of the pond centering on sections 8771 and 8783. This configuration is shown on Exhibit 1 and the model results for this configuration are contained in Table 1, demonstrating no projected increases in BFEs. The models used and developed for this evaluation will accompany the email that this letter is transmitted in. We request that the City review the attached data and exhibit and approve the proposed pond modifications. Sincerely, O’BRIEN ENGINEERING, INC. TBPE FIRM 3758 Jim O’Brien, PE, CFM, F.SAME President attachments xc: Mr. Kent Collins, Director of Public Works Mr. Terry Holmes, The Holmes Builders 9/11/2019 470475470475 480 4 8 5 490 4 6 5 475470480475485470465475480 475465465 480475475475 4754854804754704754654754754804804854754 8 0 4754758621 8651 8823 8783 8771 8890 FULLY DEVELOPED CONDITIONS WATERSHED MAP OEI JOB #: 347.003 EXHIBIT The Holmes Builders CITY OF COPPELL, DALLAS COUNTY, TX WORKMAP ±0 100 20050 Feet Date: 9/11/2019 © OpenStreetMap (and) contributors, CC-BY-SA NOTES: Contours developed from TNRIS LiDAR elevation data 2340 E. Trinity Mills Rd. Ste 220, Carrollton, Texas, 75006 p: 972.233.2288 f: 972.233.2818 www.OBrienEng.com Texas Registered Engineering Firm F-3758 Legend OEI Cross Sections Property Boundary Proposed Contours Index 5ft Contour Intermediate 1ft Contour TNRIS Contours Index 5 FT Contour Intermediate 1FT Contour !!!Stream Centerline FEMA Flood Hazard Layer 1 PCT ANNUAL CHANCE FLOOD HAZARD 0.2 PCT ANNUAL CHANCE FLOOD HAZARD FLOODWAY Document Path: P:\Current Jobs\347.003 125 Cottonwood Drive - Coppell - The Holmes Builders\GIS\Workmap.mxdO'BrienEngineering, Inc. Project Site VICINITY MAP NO SCALE N Table 1: Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch Description Cross Section Number Effective Pre-Project Post-Project Post- Project WSEL - Pre- Project WSEL Post- Project Channel Vel - Pre- Project Channel Vel 1% AC WSEL Channel Velocity 1% AC WSEL Channel Velocity 1% AC WSEL Channel Velocity 1% AC WSEL Channel Velocity Units ft fps ft fps ft fps ft fps Column Number ID (1) (2) (3) (4) (5) (6) (5)-(3) (6)-(4) 10130 482.35 7.51 482.35 7.51 482.35 7.51 0.00 0.00 10120 482.30 7.60 482.30 7.60 482.30 7.60 0.00 0.00 9890 479.99 11.17 479.99 11.17 479.99 11.17 0.00 0.00 Sandy Lake Road (U/S) 9590 478.16 9.31 478.23 9.22 478.23 9.22 0.00 0.00 Sandy Lake Road (D/S) 9475 478.02 7.19 478.09 7.11 478.09 7.11 0.00 0.00 9233 477.47 6.41 477.58 6.18 477.58 6.17 0.00 -0.01 Immediately U/S of Pond 8890 - - 475.66 6.72 475.64 6.77 -0.02 0.05 Inside U/S edge of Pond 8823 - - 475.52 4.62 475.49 4.68 -0.03 0.06 Inside Pond U/S of Transition 8783 - - 475.34 4.88 475.32 4.85 -0.02 -0.03 Inside Pond D/S of Transition 8771 - - 475.31 4.57 475.30 4.27 -0.01 -0.30 Inside D/S edge of Pond 8651 - - 474.92 5.13 474.92 5.15 0.00 0.02 Immediately D/S of Pond 8621 - - 474.88 3.62 474.88 3.62 0.00 0.00 8000 473.28 5.09 473.28 5.09 473.28 5.09 0.00 0.00 6710 470.13 4.87 470.13 4.87 470.13 4.87 0.00 0.00