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Westhaven-CS120718 (3)M F1Kimley -Horn and Associates, Inc. July 18, 2012 Mr. Kenneth M. Griffin, P.E. Director of Engineering and Public Works 255 Parkway Boulevard Coppell, Texas 75019 Re: Response to Comments — June 25, 2012 Westhaven Residential Development CLOMR Denton Creek Dear Mr. Griffin: This letter addresses comments received from Halff Associates, Inc. ( Halff), dated June 25, 2012, in regard to the above referenced CLOMR. Our responses to the comments are as follows. Comment 1: Be advised that the duplicate effective HEC -2 models should be imported into HEC -RAS and executed before any modifications (additional cross - sections) for the project are made. This will allow the submitting engineer to explain why differences in water surface elevations (if any) are occurring due to changing between computer models only. Furthermore, this will allow the submitting engineer to provide a consistent comparison as required in the MT-2 Form 2 "Riverine Hydrology & Hydraulics Form ", Section B. Hydraulics, for Effective and Proposed/Revised Water Surface Elevations. Tables 3 and 4 within the report should also reflect the results of all models executed (duplicate effective — HEC -2, duplicate effective — HEC -RAS, revised existing, and proposed). Please revise and submit. As requested, the duplicate effective HEC -2 FEMA and City models have been imported into HEC -RAS and run without any modifications. Tables 3 and 4 from the report have been updated to include columns for the duplicate effective HEC -2 results and the duplicate effective HEC -RAS results. In addition to updating tables 3 and 4, a table comparing only the duplicate effective HEC -2 and HEC -RAS results is attached to this response letter. The method for calculating conveyance in HEC -RAS was modified from "At breaks in n values only" to "Between every coordinate point' which is consistent with the methodology used in HEC -2. The attached table shows the maximum water surface elevation difference between the duplicate effective HEC -2 and HEC -RAS models is 0.05 feet for the FEMA model and 0.06 feet for the City model. These differences are minor and likely due to smaller differences ( +/- 0.01 feet) propagating upstream as the computations go from downstream to upstream. ■ TEL 972 770 1300 FAX 972 239 3820 ■ 12750 Merit Drive Suite 1000 Dallas, Texas 75251 C Kimley -Horn and Associates, Inc. Mr. Ken Griffin, P.E., July 18, 2012 Comment 2: Please address the following comments concerning valley storage. a. The report states that a valley storage gain of 0.9 acre-feet occurs due to the proposed project; however, supporting calculations were not provided. Further explanation of how the storage calculations were developed for each of the conditions (revised existing and proposed) should be provided. b. Be advised that based on a comparison of volumes generated by HEC- RAS for both revised existing and proposed conditions that a loss in valley storage occurs to the proposed project. c. Per City of Coppell Floodplain Management Ordinance, please provide the amount of valley storage gained / reduced for the standard project flood. The existing and proposed condition valley storage volumes were determined using a Civil 3D surface analysis of the subject reach. Existing and proposed water surface profiles were overlaid on top of existing and proposed condition ground surfaces to determine the net difference in valley storage. The valley storage is represented as "fill volume" on the attached Civil 3D screenshots. Per request from Russell Erskine during a phone call on July 16, the valley storage calculations should be provided for the entire width of the floodplain within the subject reach. The Civil 3D surface analysis discussed above has been updated to reflect the entire width of the floodplain and updated water surface elevations based on other comments that have been addressed as part of this comment response. The new existing and proposed floodplain volumes are 585.0 and 591.0 acre -feet, respectively. The proposed project results in a net increase of 6.0 acre -feet of valley storage. KHA acknowledges that the City of Coppell Floodplain Ordinance indicates that valley storage must be computed for the Standard Project Flood (SPF). KHA reviewed readily available information for Denton Creek, including the "City -wide Storm Water Management Study," but was unable to find a flow associated with the SPF for Denton Creek. KHA also followed up with Russell Erskine regarding this comment. During the discussion, he indicated that the Floodplain Ordinance requirement was probably intended for development along the Elm Fork of the Trinity River. With the lack of information regarding the SPF flow, he recommended not performing the SPF valley storage analysis. Due to the lack of available information and the fact that this development does not impact the Trinity floodplain elevation, KHA is requesting that the SPF valley storage requirement and Comment 2c above be dismissed. KADAL_Hydro \64447701\Docs \Comment Response Letter City.docx -� KimleyHorn and Associates, Inc. Mr. Ken Griffin, P.E., July 18, 2012 Comment 3: The report states "The existing and proposed condition geometry modeled by KHA in the fully developed models is identical to the existing and proposed condition geometry modeled by KHA in the FEMA models. "; however, based on our review, we found the following discrepancies between the models: a. Manning's n- values differ at cross - sections 39490, 39540, and 40650. b. Contraction and expansion coefficients differ at cross - sections 33145, 33470, 34260, and 34850. c. Cross - sectional geometry differs in the right overbank for the following cross - sections 33145, 33470, and 34260. KHA's modifications to the City fully developed and FEMA models are located between cross sections 349 +50 and 394 +36. The discrepancies described in this comment occur in cross sections located outside of the limits of the KHA modifications. Comment 4: Please check the channel Manning's n -value at cross - section 37702. Revise and resubmit. The Manning's n -value for the channel section have been modified from 0.1 to 0.04 at cross section 377 +02 in the proposed condition models. Comment 5: Floodplain and floodway topwidths within the HEC -RAS model do not match the floodplain topwidths shown on the workmap. Please provide an explanation or revise and resubmit. The topwidths included in the HEC -RAS output subtract the interior portions of the cross section that are raised above the floodplain elevation. If mapped, these raised portions would be shown as "islands" within the floodplain. The effective FEMA FIRM panels for the subject reach did not remove the raised areas from the mapped floodplain, and KHA is also not proposing to remove the raised areas from the effective floodplain. Comment 6: Please provide an explanation on why the Manning's n -value in the right overbank area was changed from 0.055 in the revised existing condition models to 0.05 in the proposed condition models when the proposed Westhaven project is located entirely on the north side of Denton Creek. Or revise and resubmit. KHA verified that there was a discrepancy between Manning's n- values between revised existing and proposed condition models at cross sections 349 +50 and 364 +65. The revised existing Manning's n- values have been revised from 0.055 to 0.05 at each cross section. KADAL_Hydro \64447701\Docs \Comment Response Letter_City.docx and Associates, Inc. Mr. Ken Griffin, P.E., July 18, 2012 Comment 7. The following comments pertain to the proposed pedestrian bridge crossing Denton Creek. a. Due to potential hydraulic impacts, the rails on the bridge should be modeled. Detailed design for the pedestrian bridge has not occurred at this time. KHA assumes that a 4 -foot handrail will be included in the bridge design; therefore, the proposed condition models have been updated to include a solid 4 -foot handrail. The solid handrail represents a fully clogged handrail and yields the worst case scenario with respect to decreased conveyance in the floodplain cross section. The top elevation of the handrail is above the 100 -year fully developed floodplain elevation. Therefore, if the handrail was raised during final design, it would not impact the 100 -year fully developed floodplain. b. According to City of Coppell Floodplain Management Ordinance, "All public utility equipment and bridges shall be designed to remain operable and accessible with a minimum of 2 feet of freeboard above adjacent FIS base flood elevations or the design base flood, whichever is greater". As currently modeled, the proposed pedestrian bridge is non- compliant. Please revise and resubmit. KHA met with Ken Griffin on July 12, 2012 to discuss this comment. KHA advised the City that the 2 -year fully developed flow will pass under the bridge and that the bridge foundations would be set approximately 7.5 feet beyond the top of bank to reduce erosion. Mr. Griffin agreed that the placement of the pedestrian bridge as shown in the updated Pedestrian Bridge Crossing Exhibit would be acceptable. c. As currently modeled, the structure is overtopped by the 100 year event by approximately 3 to 4 feet. Please provide an explanation on why "Energy Only" for High Flow Methods is used instead of "Pressure and /or Weir" or revise and resubmit. The modeling approach has been modified to "Pressure and/or Weir ". Comment 8: According to Appendix C, the proposed sanitary sewer aerial will have a diameter (width) of 18 inches; however, the proposed condition models reflect a width (deck width in direction of flow) of I foot. Please revise and resubmit. The deck width has been modified to 1.5 feet in both the fully developed and FEMA proposed condition models. K: \DAL_Hydro \64447701\Docs \Comment Response Letter_City.docx C Kimley -Ham and Associates, Inc. Mr. Ken Griffin, P.E., July 18, 2012 Please note, in addition to comments that have been addressed above, KHA modified the flowline elevations for the cross sections included in the previous submittal. The flowline elevations were adjusted to more accurately reflect natural depressions within the main channel. Additional survey was also obtained for more detail at the pedestrian bridge location. An updated crossing exhibit for the pedestrian bridge is included as an attachment to this letter. I trust the above responses and the updated attachments are sufficient to address Mr. Erskine's comments. If you have no further comments, please sign the appropriate location on FEMA Form 1 and return it to me. A scanned copy of the signed form is sufficient. If you do have additional questions or comments, please do not hesitate to contact me at (972) 776 -1757 or kyle.dickey@kimley- horn.com. Sincerely, KIMLEY- -HORN AND ASSOCIATES, INC. Kyle A. Dickey, P.E. cc: Russell P. Erskine, P.E., CFM; Halff Associates, Inc. Mark Harris, P.E.; Kimley -Horn and Associates, Inc. Attachments: Updated Report Pages 8, 9, and 10 Effective HEC -2 vs. Effective HEC -RAS Result Comparison Table Valley Storage Calculations Updated Pedestrian Bridge Crossing Exhibit Updated Floodplain Workmaps Updated FEMA Form 2, Page 2 Updated Annotated FIRM Updated HEC -RAS model K: \DAL_Hydro \64447701\Docs \Comment Response Letter_City.docx DENTON CREEK - WESTHAVEN RESIDENTIAL DEVELOPMENT CLOMR SUBMITTAL APRIL 2012 Table 3 summarizes the 100 -year water surface elevations in the subject reach. The results show that the proposed improvements will not cause increases in water surface elevations. The proposed FEMA tie -in elevations are shaded with a light blue for reference. Table 3: 100 -Year FEMA Water Surface Elevation Summary Cross Section FEMA Duplicate Effective Models KHA Models HEC -2 HEC -RAS Revised Existing Proposed Difference 349 +50 463.65 463.70 463.44 463.44 0.00 352 +53 - - - -- - - - -- 463.90 463.90 0.00 356 +14 - - - -- - - - -- 464.69 464.68 -0.01 358 +38 - - - -- - - - -- 464.90 464.85 -0.05 360 +34 - - - -- - - - -- 465.04 465.01 -0.03 362 +28 - - - -- - - - -- 465.09 465.08 -0.01 362 +48 - - - -- - - - -- 465.10 465.10 0.00 364 +65 - - - -- - - - -- 465.32 465.32 0.00 365 +42 - - - -- - - - -- 465.33 465.33 0.00 368 +37 - - - -- - - - -- 465.50 465.41 -0.09 369 +90 - - - -- - - - -- 465.52 465.43 -0.09 372 +01 - - - -- - - - -- 465.61 465.43 -0.18 373 +50 - - - -- - - - -- 465.62 465.45 -0.17 375 +39 - - - -- - - - -- 465.66 465.50 -0.16 377 +02 - - - -- - - - -- 465.80 465.57 -0.23 377 +27 - - - -- - - - -- 465.85 465.67 -0.18 377 +87 - - - -- - - - -- 465.81 465.67 -0.14 379 +69 - - - -- - - - -- 466.03 465.90 -0.13 381+94 - - - -- - - - -- 466.19 466.00 -0.19 383 +81 - - - -- - - - -- 466.30 466.10 -0.20 385 +73 - - - -- - - - -- 466.31 466.12 -0.19 387 +81 - - - -- - - - -- 466.33 466.16 -0.17 389 +88 - - - -- - - - -- 466.31 466.16 -0.15 392 +81 - - - -- - - - -- 466.46 466.24 -0.22 394 +36 - - - -- - - - -- 466.83 466.46 -0.37 394 +90 466.71 1 466.72 466.97 466.64 -0.33 4.0 Hydraulics conKimley -Horn and Associates, Inc- 8 K: \DAL_Hydro\64447701 \Dots \CLOMR Report.docx Copyright © 2012 Kirnley -Horn and Associates, Inc. DENTON CREEK - WESTHAVEN RESIDENTIAL DEVELOPMENT CLOMR SUBMITTAL APRIL 2012 5.0 Fully Developed Condition The City of Coppell requires floodplain improvements to be designed to convey the fully developed 100 -year flow. This section summarizes the analysis performed in order to meet the City Criteria. The 100 -year fully developed discharges for the subject reach were obtained from the City effective hydraulic model of Denton Creek. The 100 -year fully developed flow used for the subject reach is 16,681 cfs. Based on conversations with City staff, KHA understands the City effective hydraulic model for the subject reach of Denton Creek has not changed since the 2002 CLOMR, described in Section 4.1, was performed. Like the FEMA effective model, the City effective model was originally performed in HEC -2, but KHA converted the modeling to HEC -RAS v.4.1.0 for this project. The City effective model extends from cross section 331+45 to cross section 569 +40. Effective cross sections 349 +50 to 394 +40 were replaced with KHA cross sections 349 +50 to 394 +36. The existing and proposed condition geometry modeled by KHA in the fully developed models is identical to the existing and proposed condition geometry modeled by KHA in the FEMA models within the subject reach. Table 4 below summarizes the 100 -year fully developed water surface elevations in the revised existing and proposed condition City models. Table 4: 100 -Year Fully Developed Water Surface Elevation Summary Cross Section City Duplicate Effective Models KHA Models HEC -2 HEC -RAS Revised Existing Proposed Difference 349 +50 464.74 464.80 464.52 464.52 0.00 352 +53 - - - -- - - - -- 464.83 464.83 0.00 356 +14 - - - -- - - - -- 465.56 465.56 0.00 358 +38 - - - -- - - - -- 465.78 465.71 -0.07 360 +34 - - - -- - - - -- 465.90 465.86 -0.04 362 +28 i - - - -- - - - -- 1 465.96 465.94 -0.02 Kimley -Horn and Associates, Inc. K TAL_Hydro\64447701 \Dots \CLOMR Report.docx Copyright © 2012 Kimley -Hom and Associates, Inc. DENTON CREEK - WESTHAVEN RESIDENTIAL DEVELOPMENT CLOMR SUBMITTAL APRIL 2012 Cross Section City Duplicate Effective Models KHA Models HEC -2 HEC -RAS Revised Existing Proposed Difference 362 +48 - - - -- - - - -- 465.97 465.95 -0.02 364 +65 - - - -- - - - -- 466.15 466.15 0.00 365 +42 - - - -- - - - -- 466.16 466.15 -0.01 368 +37 - - - -- - - - -- 466.29 466.22 -0.07 369 +90 - - - -- - - - -- 466.30 466.23 -0.07 372 +01 - - - -- - - - -- 466.37 466.24 -0.13 373 +50 - - - -- - - - -- 466.38 466.25 -0.13 375 +39 - - - -- - - - -- 466.41 466.29 -0.12 377 +02 - - - -- - - - -- 466.52 466.35 -0.17 377 +27 - - - -- - - - -- 466.56 466.43 -0.13 377 +87 - - - -- - - - -- 466.52 466.42 -0.10 379 +69 - - - -- - - - -- 466.67 466.59 -0.08 381+94 - - - -- - - - -- 466.81 466.68 -0.13 383 +81 - - - -- - - - -- 466.91 466.77 -0.14 385 +73 - - - -- - - - -- 466.92 466.78 -0.14 387 +81 - - - -- - - - -- 466.94 466.81 -0.13 389 +88 - - - -- - - - -- 466.94 466.81 -0.13 392 +81 - - - -- - - - -- 467.06 466.90 -0.16 394 +36 - - - -- - - - -- 467.42 467.07 -0.35 394 +90 467.40 467.40 467.54 467.22 -0.32 The results indicate that there are no increases in 100 -year water surface elevations within the subject reach. Output from the fully developed hydraulic models is included in Appendix F. The City of Coppell Floodplain Ordinance requires that projects which impact the floodplain may not decrease the 100 -year floodplain storage. KHA prepared a Civil 3D surface analysis comparing the storage for the entire width of the floodplain within the subject reach for existing and proposed conditions. The results of the analysis indicate a net gain of 6.0 acre -feet of floodplain storage with 585.0 acre -feet of existing floodplain storage and 591.0 acre -feet of proposed floodplain storage. 5.0 Ultimate Conditions Kimley -Horn C and Associates, Inc 0 KTAL_Hydro\64447701 \Dots \CLOMR Report.docx Copyright © 2012 Kimley -Hom and Associates, Inc. Cross Section FEMA 100 -Year City Effective 100 -Year Fully, Developed HEC -2 Original HEC -RAS Converted Difference HEC 2 Original HEC -RAS Converted Difference ` 25530 453.52 453.52 0.00 - -- - -- - -- 25770 453.97 453.97 0.00 - -- - -- - -- 26830 454.69 454.69 0.00 - -- - -- - -- 26880 454.71 454.73 0.02 - -- - -- - -- 27165 455.24 455.28 0.04 - -- - -- - -- 27449 455.35 455.39 0.04 - -- - -- - -- 27748 455.42 455.46 0.04 - -- - -- - -- 27992 455.48 455.52 0.04 - -- - -- - -- 28168 455.53 455.57 0.04 - -- - -- - -- 28492 455.57 455.61 0.04 - -- - -- - -- 28720 455.62 455.65 0.03 - -- -- - -- 28890 455.65 455.68 0.03 - -- - -- - -- 29124 455.70 455.74 0.04 - -- - -- - -- 29651 456.24 456.27 0.03 - -- - -- --- 29831 _ 456.33 456.36 0.03 - -- - -- - -- 30027 456.38 456.40 0.02 - -- - -- - -- 30384 456.74 456.77 0.03 - -- - -- - -- 30604 457.42 457.44 0.02 - -- - -- - -- 30958 457.93 457.94 0.01 - -- - -- - -- 311335 457.83 457.85 0.02 - -- - -- - -- 32150 < 457.91 457.93 0.02 - -- - -- - -- 32200 457.71 457.72 0.01 - -- - -- - -- 32230 457.91 457.90 -0.01 - -- - -- - -- 32250 460.31 460.30 -0.01 - -- - -- - -- 32305 460.15 460.14 -0.01 - -- - -- - -- 32725 461.24 461.24 0.00 - -- - -- - -- 33145 461.44 461.44 0.00 463.50 463.50 0.00 33470 461.72 461.74 0.02 463.71 463.70 -0.01 34260 462.54 462.59 0.05 464.11 464.10 -0.01 34850 462.56 462.61 0.05 464.38 464.40 0.02 34950 463.65 463.70 0.05 464.74 464.80 0.06 35700 464.39 464.41 0.02 465.44 465.50 0.06 36250 464.69 464.71 0.02 465.78 465.80 0.02 36970 464.59 464.64 0.05 465.75 465.80 0.05 37600 465.24 465.25 0.01 466.18 466.20 0.02 37800 465.27 465.28 0.01 466.18 466.20 0.02 37990 465.31 465.32 0.01 466.23 466.20 -0.03 38050 465.38 465.39 0.01 466.31 466.30 -0.01 38200 465.43 465.43 0.00 466.38 466.40 0.02 38550 465.58 465.59 0.01 466.49 466.50 0.01 39290 465.91 465.92 0.01 466.74 466.70 -0.04 39440 465.59 465.60 0.01 466.39 466.40 0.01 39490 466.71 466.72 0.01 467.40 467.40 0.00 39540 466.59 466.60 0.01 467.31 467.30 -0.01 39940 467.24 467.25 0.01 467.83 467.80 -0.03 40650 468.30 468.31 0.01 468.77 468.80 0.03 48000 470.77 470.78 0.01 471.89 471.90 0.01 51350 472.03 472.04 0.01 472.85 472.90 0.05 53350 472.59 472.59 0.00 473.38 473.40 0.02 55590 473.26 473.27 0.01 474.07 474.10 0.03 56940 474.51 474.51 0.00 475.29 475.30 0.01 Max Increase = 0.05 Max Increase = 0.06 Max Decrease = -0.01 Max Decrease = -0.04 linfxtnation 1pefnitlon'IAnalvsis', Statistics Statistics Value ® General Q TDI 8 Volume Base Surface Existng Total Comparison Surface Exstim WSEL Cut Factor 1.000 FA Factor 1.000 Cut volume (adjusted) 1582. % Cu. Yd. FA volume (adjusted) 943798.91 Cu. Yd. Net volume (adjusted) 942216.34 Cu. Yd. <FA> Cut volume (unadjusted) 1582.56 Cu. Yd. FN volume (unadjusted) 943798.91 Cu. Yd. Net volume (unadjusted) 942216.34 Cu. Yd. <Fl> ;Information l pefin * n IlMalYgs I Statistics i Statistics Value D General m TD! O Volume Base Surface Combo Total Comparison Surface Proposed WSEL Cut Factor 1.000 Fti Factor 1.000 Cut volume (adjusted) 1134.92 Cu. Yd. FA volume (adjusted) 953421.79 Cu. Yd. Net volume (adjusted) 952286.87 Cu. Yd. [Fi> Cut volume (unadjusted) 1134.92 Cu. Yd. FA volume (unadjusted) 953421.79 Cu. Yd. Net volume (unadjusted) 952286.87 Cu. Yd. <Fi> F— Help F OK