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Cottonwood Est-SY 200124O’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 January 24, 2020 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 was notified by the City that analysis was 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 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. Mr. Kumar Gali, P.E., CFM, D.WRE January 24, 2020 Page 2 of 4 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 was 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, the lower channel cross sections 6710 and 8000 from the Duplicate Effective - FP model were joined 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 (8863, 8836, 8813, 8793, 8675, and 8640). TNRIS elevation data was then used to create a terrain for cross section ground data to adequately demonstrate the transitions in the floodplain due to the existing pond. Contraction and expansion parameters were then held equal to those in the effective model for corresponding portions of upstream and downstream cross sections. A blocked obstruction is added to the onsite pond with a top elevation of 470.5 feet, which is the invert of the existing outlet based on available contour data. Roughness coefficients in the right-of-bank (ROB) section were then updated in order to rebalance the continuity of flow in the ROB. Due to the high bank immediately adjacent to the channel flow a split exists between the main channel and the right overbank. Manning’s n-values have been adjusted to better maintain continuity of flow through the split. Cross section 8863 is used to control inflow to the right overbank split flow (of approximately 5,000 cfs). Manning’s n-values for cross sections 8836 through 8640 have been adjusted to maintain similar right overbank flows. While sections 8640 and 8675 have slightly higher right overbank flows, these sections appear to have more connectivity to the channel through lower sections of the high bank located near these cross sections. Manning’s n-values at section 8863 have been set to either 0.05 to represent the potentially tall grasses that can grow in this area, or 0.04 to represent the increase friction slope into the pond (not increased friction). Manning’s n-value at the inlet of the pond, section 8863, is maintained in the pre- and post-project conditions. 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 obstructions, friction, and expansion and contraction coefficients. Mr. Kumar Gali, P.E., CFM, D.WRE January 24, 2020 Page 3 of 4 Exhibit 1: Workmap (attached) shows a workmap for Cottonwood Branch site included including cross sections from the workmap for the 1989 Master Drainage Study for City of Coppell, also attached. The Sitemap for project site is displayed in Exhibit 2: Sitemap (attached). Flows in the 100-year ultimate flows for the hydraulic model reflect the HEC-2 model results for Cottonwood Branch taken from City’s 1989 and 1991 studies (attached). These studies were also used as a basis for the 50% AC flows. A comparison of the FEMA flow results from the HEC-RAS model are listed in attached Table 1: Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch FEMA Flows. Similarly, Table 2: Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch Ultimate SMS Flows, provides a comparison of the Ultimate SMS flow results. The proposed project would have no projected increases in BFEs and approximately 0.1% or less reduction of valley storage for both FEMA and City SMS 100-year flows. As Cottonwood Branch is approximately a 6 square mile drainage basin this reduction in storage would meet the City’s allowed 15% reduction in storage. 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 Stephen Bolster, PE, CFM, MS Water Resources Task Manager xc: Mr. Kent Collins, Director of Public Works Mr. Terry Holmes, The Holmes Builders Attachments: Exhibit 1 – Workmap Exhibit 2 – Sitemap Table 1 – Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch FEMA Flows Mr. Kumar Gali, P.E., CFM, D.WRE January 24, 2020 Page 4 of 4 Table 2 – Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch Ultimate SMS Flows Workmap from the KHA of City-Wide Storm Water Management Study for the City of Coppell, Texas Digital Copy of Hydraulic Model !!!!!! ! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!! ! !!!!!!!!!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 8000 6710 9590 9475 9233 8640 8675 8793 8813 8836 8863 10130 10120 9890 4754704804 8 5 490465495 500505 4 7 5 5054754754804804 80 480475475475475 465 4754804705 0 5 500475475470495 4754804854 7 0 480480490 485FULLY DEV ELOPED CONDITIONSWATERSHED MAP OE I JOB #: 347.003 EXHIBIT 2 The Holmes Builders CITY OF COPPELL, DALLAS COUNTY, TX WORKMAP ±0 300 600150 Feet Date : 11/11/2019 © OpenStreetMap (and)contributors, CC-BY-SA NOTES:Contours developed from TNRIS LiDAR elevation data Locations for cross sections 10130, 10120, 9890, 9590, 9475, 9233,8000, and 6710 are approximate. Either based on SMS workmapor located by reach lengths 2340 E. Trinity Mills Rd. Ste 220, Carrollton, Texas, 75006p: 972.233.2288 f: 972.233.2818 www.OBrienEng.com Texas Registered Engineering Firm F-3758 Legend !!!Stream Centerline Model Cross Sections Proposed C ontours Index 5 FT Intermediate 1 FT TNRIS Contours Index 5 FT Intermediate 1 FT Property Boundary Pos t-Projct FEMA 1% ACFloodplain Pre-Projct FEMA 1% ACFloodplain Pos t-Projct SMS Ultimate 1% ACFloodplain Pre-Projct SMS Ultimate 1% ACFloodplain Document Path: C:\Users\sbolster\Documents\347.003 125 Cottonwood Drive - Coppell - The Holmes Builders\GIS\Workmap.mxdO'BrienEngineering, Inc. Project Site VICINITY MAP NO SCALE N !!!!!!!! ! ! ! ! !!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !! ! ! !!!! Smal l m ound to be removed Lowered b y app roximately 0.4 FT 472470 471 468 467 469 473472 8793 8813 8640 8675 8836 8863 8000 470475 4 8 0475470475475475 475 475 470475 475475475475475475470475FULLY DEV ELOPED CONDITIONSWATERSHED MAP OE I JOB #: 347.003 EXHIBIT 2 The Holmes Builders CITY OF COPPELL, DALLAS COUNTY, TX SITEMAP ±0 60 12030 Feet Date : 11/11/2019 © OpenStreetMap (and)contributors, CC-BY-SA NOTES:Contours developed from TNRIS LiDAR elevation data Locations for cross sections 10130, 10120, 9890, 9590, 9475, 9233,8000, and 6710 are approximate. Either based on SMS workmapor located by reach lengths 2340 E. Trinity Mills Rd. Ste 220, Carrollton, Texas, 75006p: 972.233.2288 f: 972.233.2818 www.OBrienEng.com Texas Registered Engineering Firm F-3758 Legend !!!Stream Centerline Model Cross Sections Proposed C ontours Index 5 FT Intermediate 1 FT TNRIS Contours Index 5 FT Intermediate 1FT Property Boundary Pos t-Projct FEMA 1% ACFloodplain Pre-Projct FEMA 1% ACFloodplain Pos t-Projct SMS Ultimate 1% ACFloodplain Pre-Projct SMS Ultimate 1% ACFloodplain Document Path: C:\Users\sbolster\Documents\347.003 125 Cottonwood Drive - Coppell - The Holmes Builders\GIS\Sitemap.mxdO'BrienEngineering, Inc. Project Site VICINITY MAP NO SCALE N 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 9.32 478.85 8.42 478.84 8.42 -0.01 0.00 Sandy Lake Road (D/S)9475 478.01 7.19 478.24 6.95 478.24 6.96 0.00 0.01 9233 477.45 6.44 477.83 5.73 477.82 5.74 -0.01 0.01 Immediately U/S of Pond 8863 - - 475.84 5.72 475.67 5.98 -0.17 0.26 Inside U/S edge of Pond 8836 - - 475.66 6.69 475.54 6.24 -0.12 -0.45 Inside Pond U/S of Transition 8813 - - 475.45 6.42 475.38 5.71 -0.07 -0.71 Inside Pond D/S of Transition 8793 - - 475.38 5.56 475.32 4.83 -0.06 -0.73 Inside D/S edge of Pond 8675 - - 474.77 5.50 474.77 5.50 0.00 0.00 Immediately D/S of Pond 8640 - - 474.56 5.75 474.56 5.75 0.00 0.00 8000 473.27 5.11 473.29 5.09 473.29 5.09 0.00 0.00 6710 470.13 4.87 470.13 4.87 470.13 4.87 0.00 0.00 Table 1: Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch FEMA Flows 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 482.77 8.45 482.74 8.51 482.74 8.51 0.00 0.00 10120 482.71 8.58 482.67 8.64 482.67 8.64 0.00 0.00 9890 480.93 10.38 481.21 9.61 481.21 9.61 0.00 0.00 Sandy Lake Road (U/S)9590 478.68 10.61 479.59 9.39 479.59 9.39 0.00 0.00 Sandy Lake Road (D/S)9475 478.51 8.25 478.73 8.00 478.73 8.00 0.00 0.00 9233 477.95 6.81 478.31 6.16 478.31 6.17 0.00 0.01 Immediately U/S of Pond 8863 - - 476.24 6.06 476.09 6.31 -0.15 0.25 Inside U/S edge of Pond 8836 - - 476.07 7.16 475.96 6.73 -0.11 -0.43 Inside Pond U/S of Transition 8813 - - 475.85 6.98 475.78 6.32 -0.07 -0.66 Inside Pond D/S of Transition 8793 - - 475.78 5.91 475.73 5.22 -0.05 -0.69 Inside D/S edge of Pond 8675 - - 475.16 5.84 475.16 5.84 0.00 0.00 Immediately D/S of Pond 8640 - - 474.95 6.18 474.95 6.17 0.00 -0.01 8000 473.63 5.41 473.65 5.38 473.65 5.38 0.00 0.00 6710 470.45 5.01 470.45 5.01 470.45 5.01 0.00 0.00 Table 2: Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch Ultimate SMS Flows Description Cross Section Number Effective Pre-Project Post-Project O’Brien Engineering, Inc. 2340 E. Trinity Mills Road, Suite 220 Carrollton, Texas 75006 972-233-2288 | obrieneng.com TBPE Firm #F-3758 Since 1987 | Texas HUB / TxDOT SBE