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Westhaven-SY120625 ••• HALFF June 25, 2012 AVO 28875 Mr. Kenneth M. Griffin, P.E. Director of Engineering and Public Works City of Coppell PO Box 9478 Coppell, Texas 75019 Re: Review of Floodplain Study Submittal by Kimley-Horn and Associates, Inc. — Conditional Letter of Map Revision (CLOMR) for Westhaven Residential Development—Denton Creek Dear Mr. Griffin, We have completed an initial review of the Conditional Letter of Map Revision (CLOMR) for Westhaven Residential Development — Denton Creek prepared by Kimley-Horn and Associates, Inc., dated May 3, 2012, and received on June 1, 2012. This review was authorized by the City of Coppell dated May 31, 2012, and received by Halff Associates, Inc. on June 5, 2012. This general review consists of checking the submittal package for completeness and compliance with the City of Coppell floodplain ordinances, regulations, and policies. The following comments are a result of this initial review: 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 Propose/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 resubmit. 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 provide. Further explanation of how the storage calculations were developed for each of the conditions (revised existing and proposed) should be provided. HALFF ASSOCIATES,INC. 1201 NORTH BOWSER ROAD TEL (214)346-6200 WWW.HALFF.COM RICHARDSON,TX 75081-2275 FAX(214)739-0095 ••• HALFF 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 due 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. 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. 4. Please check the channel Manning's n-value at cross-section 37702. Revise and resubmit. 5. Floodplain and floodway topwidths within the HEC-RAS model do not match the floodplain topwidths shown on the workmap. Please provide and explanation or revise and resubmit. 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. 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. 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.