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Cottonwood Est-CS191111O’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 November 11, 2019 Mr. Kumar Gali, P.E., CFM, D.WRE Assistant Director of Public Works City of Coppell 255 Parkway Coppell, TX 75019 RE: Comments for 125 Cottonwood Drive Lot Improvements, Coppell, Texas OEI Job number: 347.003 Dear Mr. Gali: O’Brien Engineering, Inc. is providing hydraulic analysis on behalf of the client The Holmes Builders for the new home construction and reshaping of the existing pond at 125 Cottonwood Drive in Coppell, TX. The initial analysis was provided to the City of Coppell on September 22, 2019, and Kimley Horn provided comments on behalf of the City of Coppell, dated October 9, 2019. The review comments are summarized below in bolded italics and our responses are noted in plain regular font. 1) Prepare pre-project and post-project HEC-RAS models using 100-year ultimate condition flows from the City’s Storm Water Management Study by Halff Associates, Inc. dated 1991 and confirm the proposed improvements adhere to the City of Coppell Floodplain Management Ordinance. The 100-year ultimate condition flows have been added from the City’s Storm Water Management Study (SMS). The 2-year flows were added to the model from the City provided HEC-RAS model, the 100-year ultimate flow from this file also matched the City’s SMS. 2) Cross section 8584 is shown on the hydraulic workmap but is not included in the provided HEC-RAS models. Cross section 8621 is included in the HEC-RAS models but not shown on the hydraulic workmap. Revise the modeling and mapping to correct this inconsistency and update the report appropriately. Cross sections from the model are reflected on the workmap. 3) Provide workmaps that show the 100-year existing and post-project floodplain delineations using FEMA FIS flows and City fully developed flows, respectively, in addition to cross section locations and existing condition topography. The workmap has been updated to show the floodplain delineations requested. The approximate location of additional cross sections were added to the map based on a workmap from the SMS 4) Provide workmaps that show proposed topography, cross section locations and 100-year proposed and post-project floodplain delineations using FEMA FIS flows and City fully developed flows, respectively, in addition to cross section locations and proposed condition topography. See Sitemap in addition to workmap updated for response #4. Mr. Kumar Gali, P.E., CFM, D.WRE November 11, 2019 Page 2 of 4 5) Pre-project cross sections 8823 to 8651 include a blocked obstruction within the onsite existing pond at elevation 469.46, which is assumed to be the pond’s normal water surface elevation and outfall elevation; however, per the topography shown on the hydraulic workmap provided, the outfall elevation seems higher than 469.46. Provide supporting documentation and justification regarding the assumed normal water surface elevation of 469.46 and confirm the HEC-RAS modeling is consistent with the pond’s normal water surface elevation. The blocked obstruction was modified to a top elevation of 470.5 feet, which is the invert of the existing outlet based on available contour data. This revised obstruction elevation in addition to the revised Manning’s n-values in the right overbank, represent the flow through the right overbank. Due to the high bank immediately adjacent to the channel flow is split between the main channel and the right overbank. Manning’s n-values have been adjusted to better maintain continuity of flow thought the split. Using cross section 8863 as the controlling 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 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 near the location of these cross sections. 6) There are locations where post-project HEC-RAS geometry seem inconsistent with proposed grading. Verify geometry in the HEC-RAS modeling is consistent with the proposed grading, including but not limited to cross sections 8823 and 8651. HEC-RAS geometry for the proposed modeling has been updated. 7) An impoundment is proposed near cross section 8783. It is unknown where the cross sections are in relation to proposed grading. Confirm that proposed condition HEC-RAS cross sections are placed at the toe and top of banks of this proposed impoundment to accurately model the hydraulic effects of the impoundment. Provide detailed design calculations to the City of Coppell to show that erosion will not occur due to this proposed impoundment. The berm, referred to as an impoundment, has been removed from the proposed improvements. 8) The report stated that Manning’s “n” values are 0.04 at the inlet and outlet of the existing and proposed pond due to the increased friction at these locations. The pre-project and post-project HEC-RAS model doesn’t include an “n” value of 0.04 at cross section 8621. Revise the modeling and report to correct this inconsistency. Manning’s n-value of 0.04 at the inlet of the pond, section 8863, is maintained in the pre- and post- project conditions. Manning’s n-values have been revised through the right overbank of the project to maintain the split flow as described above for comment #5. 9) A right overbank Manning’s “n” value of 0.06 seems high for the areas that include short grasses. Revise the pre- and post-project modeling to show a lower, more appropriate Manning’s “n” value or provide justification that 0.06 is representative of this area. Mr. Kumar Gali, P.E., CFM, D.WRE November 11, 2019 Page 3 of 4 Manning’s n-values at section 8863 have been lowered to 0.05 to represent the potentially tall grasses that can grow in this area, excluding of the lowered section of 0.04 to represent the increase friction slope into the pond (not increased friction). Elsewhere in the right overbank, Manning’s n-values have been updated to best represent the flow continuity required for the split flow as described above for comment #5. 10) The Manning’s “n” value along the normal water surface of the existing and proposed pond is set at 0.06. Reduce the Manning’s “n” value to be consistent with the normal water surface. We recommend using a value of 0.02.. Manning’s n-values have been updated to best represent the flow continuity required for the split flow as described above for comment #5. 11) Per the Post-Project HEC-RAS modeling, the normal water surface elevation of the proposed pond is assumed to be 469.50; however, the elevation near the outfall of the pond (cross section 8621) shows an elevation of 469.78, which is higher than the normal water surface of the pond. Per the proposed grading exhibit, the normal water surface elevation seems higher than 469.50. Revise the modeling and grading to show positive drainage at the outfall of the pond and provide more detail regarding the proposed pond’s outfall structure and elevation on the proposed hydraulic workmap. Also, confirm the normal water surface elevation is consistent at all HEC-RAS cross sections containing the proposed pond. No revisions are proposed at the pond outfall invert, which from elevation raster datasets for the area appears to be approximately 470.5 feet. As seen from historic aerials this pond has and will continue to hold water. No proposed outlet structure is planned and the historic and existing outfall will remain. 12) Due to the pond outfalling directly onto a neighboring property, show that the pond peak outflow is lower in post-project conditions than compared to pre-project conditions and erosive conditions will not be created at the proposed pond outfall. No revision is proposed at the pond outfall. Pond flows are controlled by the riverine flooding of Cottonwood Branch. Note that with the proposed updates the proposed project would have approximately 1% 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. Also during the updates to the hydraulic models, downstream reach lengths were updated based on the TNRIS elevation data as well as cross section numbering. We hope the above information provides sufficient detail to address the review comments and the City of Coppell can approve the proposed improvements. Once these comments are addressed satisfactory to the City, OEI will provide an updated report for the project. We appreciate your time and assistance in the matter. If you have further questions, please feel free to contact us. We may be reached at (972) 233-2288. Sincerely, O’BRIEN ENGINEERING, INC. Mr. Kumar Gali, P.E., CFM, D.WRE November 11, 2019 Page 4 of 4 Stephen Bolster, P.E., MS, CFM Water Resources Task Manager Attachments Tables Workmap Sitemap HEC-RAS Model xc: Mr. Kent Collins, Director of Public Works Mr. Terry Holmes, The Holmes Builders 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 9.32 478.84 8.43 478.84 8.43 0.00 0.00 Sandy Lake Road (D/S)9475 478.01 7.19 478.23 6.96 478.23 6.96 0.00 0.00 9233 477.45 6.44 477.81 5.75 477.81 5.75 0.00 0.00 Immediately U/S of Pond 8863 --475.97 5.44 475.76 5.78 -0.21 0.34 Inside U/S edge of Pond 8836 --475.82 6.34 475.65 6.04 -0.17 -0.30 Inside Pond U/S of Transition 8813 --475.65 6.04 475.50 5.54 -0.15 -0.50 Inside Pond D/S of Transition 8793 --475.47 6.73 475.41 5.65 -0.06 -1.08 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 8.00 478.72 8.01 -0.01 0.01 9233 477.95 6.81 478.31 6.16 478.30 6.18 -0.01 0.02 Immediately U/S of Pond 8863 --476.39 5.80 476.18 6.11 -0.21 0.31 Inside U/S edge of Pond 8836 --476.24 6.88 476.07 6.53 -0.17 -0.35 Inside Pond U/S of Transition 8813 --476.07 6.46 475.91 6.06 -0.16 -0.40 Inside Pond D/S of Transition 8793 --475.87 7.23 475.82 6.09 -0.05 -1.14 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 !!!!!! ! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!! ! !!!!!!!!!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 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 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