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FIRM-890703��NGY MA W m 0 0 Federal Emergency Managemen Washington, D.C. 20472 JUL 3 1989 The Honorable Lou Duggan Mayor of the City of Coppell P.O. Box 478 Coppell, Texas 75019 Dear Mayor Duggan: Community: City of Coppell, Dallas and Denton Counties, Texas Community No.: 480170 5W This is in reference to a January 12, 1989 letter from Mr. Russell R. Doyle, P.E., City Engineer and Floodplain Administrator for the City of Coppell, in which he requested that the Federal Emergency Management Agency (FEMA) revise the September 22, 1988 preliminary Flood Insurance Study (FIS) and Flood Insurance Rate Map (FIRM) for the City of Coppell to include updated corporate limits information and the results of an untitled technical report prepared by Kimley -Horn and Associates, Inc., consulting engineers for the City of Coppell. This report, transmitted to the City of Coppell by Kimley -Horn and Associates, Inc., with a January 5, 1989 letter, contained the following: HEC -2 hydraulic modeling representing the 100- and 500 -year floods and the 100 -year floodway for the Elm Fork of the Trinity River, Grapevine Creek, Denton Creek, and Cottonwood Branch; and topographic mapping of the entire community delineating the proposed 100- and 500 -year floodplain boundaries and the proposed floodway configuration. We have reviewed the submitted technical data for each stream and have identified a number of concerns, enclosed separately as review comments, that must be resolved before FEMA will revise the preliminary FIS and FIRM for the City of Coppell. Please direct all responses to these review comments to our technical evaluation contractor, Dewberry & Davis, 8401 Arlington Boulevard, Fairfax, Virginia 22031, to the attention of Mr. Dave Lushbaugh, Management Engineering and Technical Services Division. Our processing of this request to revise the preliminary FIS and FIRM for your community will be considered inactive pending receipt of your community's response to the review comments. Should you have any questions regarding this matter, please do not hesitate to contact the Chief, Natural and Technological Hazards Division of the Federal Emergency Management Agency in Denton, Texas, at (817) 898 -9127 or Matthew B. Miller of our Headquarters staff in Washington, D.C., at (202) 646 -3461. Sincerely, Jt*, Matticks C isk Studies Division Federal Insurance Administration Enclosure cc: /Mr. Russell R. Doyle, P.E. Mr. Ronald Morrison, P.E., Kimley -Horn and Associates, Inc. Mr. Jerry Parche, P.E., Jerry Parche Consulting Engineers City of Coppell, Texas Map Revision Review Comments 1. The submitted HEC -2 hydraulic models did not analyze the 10- and 50 -year floods for each stream. Since this will result in the deletion of these profiles from the Flood Insurance Study (FIS) and would deviate from standard Federal Emergency Management Agency (FEMA) guidelines, please verify whether or not this was your intended course of action. If not, please provide HEC -2 models representing the 10- and 50 -year floods. 2. In a technical report previously submitted to FEMA, entitled Floodplain Hydraulics Study, Grapevine Creek, Moore Road Bridge Replacement, City of Coppell, Texas, dated 1987, prepared by Kimley -Horn and Associates, Inc., the HEC -2 hydraulic model for Grapevine Creek contains more detailed information than the HEC -2 hydraulic model submitted with this request. Specifically, the previous model contains more detailed bridge geometry for the Beltline Road bridge, the most downstream St. Louis Southwestern Railway bridge, and the Moore Road bridge, as well as eleven additional cross sections between cross sections 18650 and 25086. We feel that the more detailed HEC -2 hydraulic model should be used. Please revise the submitted model or justify its use in place of the previously submitted model. 3. The starting water - surface elevations for Grapevine Creek and Denton Creek were determined using the HEC -2 tributary option. This option should be used only when two streams are subject to coincidental peak flooding conditions which normally occurs only with streams of equal drainage area. Please either justify the use of this option or use the slope -area method for the starting water - surface elevation, which may be more applicable to this modeling situation. Please note that when determining the floodway configuration, the effects of backwater flooding from the Elm Fork of the Trinity River should not be included. The tributary option does include the effects of backwater flooding and thus the floodway models should be revised, as appropriate. 4. We have received a request for a conditional Letter of Map Revision for the construction of the proposed Parkway Boulevard bridge, which is located near the confluence of Cottonwood Branch and Denton Creek. This February 24, 1989 request was also from Mr. Doyle and was transmitted to us by our Region VI office. With this request, Mr. Doyle submitted a technical report prepared by Jerry Parche Consulting Engineers that included the following: HEC -2 hydraulic models representing existing and proposed 100 -year flood and floodway conditions for Denton Creek and Cottonwood Branch; topographic mapping delineating the proposed 100 -year floodplain boundary and proposed floodway configuration; and plotted cross sections for Denton Creek and Cottonwood Branch. A comparison of the existing conditions hydraulic models for Cottonwood Branch and Denton Creek from this submittal and the hydraulic models from the Kimley -Horn and Associates, Inc., submittal indicates differences in the technical analyses that yield different results. These differences are listed below. W a. Kimley -Horn and Associates, Inc., has modeled the two streams with an assumed common floodplain from their confluence upstream to a point approximately 1,220 feet upstream of Denton Tap Road. Jerry Parche Consulting Engineers has modeled the two streams with an assumed common floodplain from their confluence upstream to a point approximately 475 feet upstream of Denton Tap Road. b. The 100 -year flood discharges utilized in the submitted HEC -2 hydraulic models for Denton Creek are outlined below. 100 -Year Flood Discharges Kimley -Horn. and Jerry Parch6 Cross Section Associates, Inc. Consulting Engineers 28870 (below Cottonwood 21300 - - - -- Branch) 32150 21300 21300 32780 (above Cottonwood 21300 21300 Branch) 33145 21300 14900 36970 15600 14900 39440 (at Interstate Route 15600 15600 121) 39540 15400 - - - -- Please note that the flood discharges listed above are for the 100 -year frequency storm only and that there may be similar discrepancies in the 10 -, 50 -, and 500 -year frequency storms. Before FEMA will revise the preliminary FIS and Flood Insurance Rate Map for the City of Coppell, these concerns with Denton Creek and Cottonwood Branch must be resolved. The City of Coppell must resolve these discrepancies and decide upon a single HEC -2 hydraulic model for Denton Creek and for Cottonwood Branch that most accurately represents the floodplain and floodway configura- tion for Denton Creek and Cottonwood Branch for their entire lengths within the community.