Loading...
Carter Phase 1-CS 961115dp Jerry Parch Consulting Engineers November 15, 1996 Mr. Kenneth Griffin, P.E. City Engineer City of Coppell P O Box 478 Coppell, Texas 75019 214/462-0022 I~d~: ~.~1~O~. 9~AIN ANALYSIS; ,,/"CARTER ADDITION~/PPELL, TEXAS Dear Ken: Pursuant to my conversation with Mr. Mike Martin of your office, the attached information has been I? prepared tO analyze the impacts of the proposed reclamation of approximately 0.93 acres of land from the flood plain of Grapevine Creek. The area is located on the north side of the creek just upstream of ! Moore Road and is a part of the proposed Carter Addition. This letter and the attached information are considered preliminary and are submitted to describe the proposed project and to request a preliminary review by the city. If approved, additional data will be prepared and the completed study will be submitted to the city requesting a Conditional Letter of Map Revision for this project. As shown on the attached exhibit, the Carter Addition lies upstream of Moore Road. Grapevine Creek flows easterly through the project. Carter Drive lies on the north side of the proposed development and provides access. The developer is proposing to subdivide the property into 5 lots for single family residential use. The proposed filling is designed to reclaim a part of the north overbank of the flood plain to yield a minimum of 115 ft. of depth south of Carter Drive on each lot that is above 100-year flood elevation. The current F.I.S. hydraulic computer models of Grapevine Creek were obtained from FEMA's consultant. In addition, we also obtained the computer models of the creek prepared by the city's consultant for a watershed management study conducted in 1989. The consultant's ~nodels include analysis of both the existing condition and fully developed watershed condition 100-year flood events. The cross sections in the study reach, sections 18500 through 21690, from the F.I.S. and the city models were compared and found to be similar. Reach lengths, 'n' factors, channel bank stations and other input data in both models were found to be similar in the study reach. Therefore, these models were used as the basis for the models prepared for this study. In addition to the cross sections included in the models we obtained from others for this study, three additional sections of the flood plain were taken within the property. These are sections 19034, 19320 and 19662 as shown on the exhibit. The cross sections used in our computer models from section 18565, which is on the upstream face of the Moore Road bridge, through 20540, which is approximately 700 feet upstream of the property, are shown on sheet numbers 1-6 attached. 1301 S. Bowen Road, Suite 300 · Arlington, Texas 76013 · Metro (817) 461-2433 Mr. Ken Griffin, P.E. November 15, 1996 JP No. 96003 Page 2 of 4 The city flood plain ordinance requires that determination of minimum finish floor levels consider both the existing watershed, or FEMA, flood plain elevations, and the fully developed watershed 100-year flood elevations ("design base flood"). A comparison of the fully developed and existing watershed models indicates that the 100-year flood elevations in the fully developed watershed model ranges from 1.4 feet to 2.9 feet higher than the existing watershed 100-year flood elevations through the property. Therefore the elevation of the proposed pads and minimum finish floor levels will be based on the fully developed watershed 100-year flood levels in accordance with city ordinance. For the purposes of this limited initial submittal, only models analyzing the 100-year flood plain with a fully developed watershed are included herein and discussed below. REVISED EXISTING FLOOD PLAIN MODEL - FULLY DEVELOPED WATERSHED MODEL CARULT3.DAT As shown on sheet number 1 of 6 attached, the 100-year flood elevation at Moore Road for the fully developed watershed condition does not overtop the bridge. The flood plain widens out very quickly upstream of Moore Road. Effective flow limits, however, will not expand as abruptly as the flood plain. In order to limit effective flow widths upstream of Moore Road due to the bridge, the HEC-II model we received from FEMA was modified to include effective flow limits at sections 18600, 18615 and 18650. The effective flow limits used are shown on these cross sections on sheet no. 2 of 6 and 3 of 6 attached. Cross sections 19160 (0.1 in FEMA model), and 19550 (0.2 in FEMA model) are located within the Carter Addition as shown on the exhibit. Sections 19034, 19320 and 19662 were surveyed and added to the FEMA model. 100-year flood discharges and beginning water surface elevations were obtained from the city watershed management study model for the fully developed watershed condition. The 100-year flood discharge at Moore Road is 16,250 c.f.s. The 100-year flood elevations computed in model CARULT3.DAT are shown in cross sections on Sheet Nos. lq5 of 6 attached. PROPOSED FLOOD PLAIN MODEL - FULLY DEVELOPED WATERSHED MODEL PROULT3.DAT Proposed fill areas, shown on the attached exhibit, are necessary to provide a minimum of 115 feet of lot depth above the 100-year flood elevation. According to the ordinance, lowest floor level shall be elevated a minimum of one foot above the design base flood elevation. Using the results from model CARULT3.DAT, pad elevations were set on each of the five proposed lots at 0.5 feet higher than the computed 100-year flood elevations at the upstream end of each lot. This fill elevation is extended to a point 115 feet south of Carter Drive from where the fill height is transitioned to natural ground on a 4:1 slope. Proposed fill areas are shown on cross sections 18650, 19034, 19160, 19320, and 19550 on sheet nos. 3-5 of 6 attached. Table 1 provides a comparison of the 100-year flood elevations computed using the HEC II models CARULT3.DAT and PROULT3.DAT for the existing flood plain and proposed flood plain respectively. Mr. Ken Griffin, P.E. November 15, 1996 JP No. 96003 Page 3 of 4 SECTION EXISTING CONDITIONS PROPOSED CONDITIONS MODEL CARIJLT3.DAT MODEL PROULT3.DAT 18500 466.83 466.83 18514 466.33 466.33 18575 466.25 466.25 18565 466.60 466.60 18566 466.47 466.47 18600 468.59 468.59 18615 468.67 468.67 18650 468.53 468.53 19034 469.67 469.65 19160 (0.1) 469.85 469.82 19320 470.16 470.18 19550 (0.2) 470.65 470.72 19662 471.62 471.68 20000 (0.3) 471.93 471.98 20540 472.94 472.97 20975 (0.5) 473.81 473.83 21365 (0.6) 473.63 473.66 21690 475.49 475.50 The amount of rise created by the proposed reclamation is extremely small, less than 0.1 foot. The existing flood plain limits will not be measurably affected outside of the fill areas. The only buildable properties affected are the site and the developer's property located immediately upstream of the site. In order to avoid disturbing the natural channel and overbanks in the addition, the developer's preference is to leave these areas in their current condition. However, to eliminate the small rise some measures to enhance the flow conveyance will have to be constructed. Mr. Ken Griffin, P.E. November 15, 1996 JP No. 96003 Page 4 of 4 On behalf of the developer, we request that the city evaluate these impacts and notify us whether the city concurs that the impacts are insignificant. Upon receiving direction from the city, we will complete the submittal of all flood plain data required for this project. If you have any questions, please call. Thank you. Sincerely,//~ /  ar~he, P.E. lP:ch Enclosures: HEC II Model CARULT3.DAT HEC II Model PROULT3.DAT Exhibit Cross Section Sheet Nos. 1-6 cc: Mr. Robert Wright, P.E./Needham Wright Engineers Inc.