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HomeMy WebLinkAboutAppendix 4 Need to Re-studyAppendix 4 Need to Re-study Letters to Jack Quarles, P.E. Civil Engineer FEMA Region VI I April 10, 2000 Mr. Jack Quarles, P .E. Civil Engineer FEMA Region VI Federal Regional Center 800 North Loop 288 Denton, TX 76201 RE: Lick Creek Watershed Flood Insurance Study COLLEGE STATION P. 0 . Box 9960 1101 Texas Avenue Tet. 409 764 3500 College Station, TX 77842 Brazos County September 1987 Dear Mr. Quarles, This is to follow up on our discussion from last week, and previous discussions with Dale Hoff, regarding soil parameters used in the Flood Insurance Study for Lick Creek. As discussed, the NuDallas hydro logic model (Exhibit 1) for this watershed used a ninety- percent sand condition for the entire watershed. The output from this model matches the flows published in the effective FIS (Exhibit 2). This study covers three other watersheds within Brazos County with similar input. A review of the Brazos County Soil Survey shows that the drainage basin primarily consists of Lufkin, Edge, and Tabor soils (Exhibit 3). All three of these soils are classified by the NRCS (SCS) as Hydro logic Group D, soils having very slow infiltration and high runoff potential. A field reconnaissance of the watershed confirmed the Soil Survey findings. A Preliminary analysis shows that changing the model from ninety-percent sand to forty-percent (based upon Soil Survey) would increase flows by more than fifty percent. The City of College Station is currently preparing a LOMR to extend this study upstream. We also believe hat the existing study should be revised if supporting material is not found to justify the effective study. The area within the City of College Station along Lick Creek is largely undeveloped at this time, but surrounded by growth. The city will be extending sewer service along this creek, which will open the area for development within a year. The city would like to ensure that this issue is resolved prior to the area experiencing development. We would like your assistance and cooperation in resolving this issue and updating the maps if necessary. Home of Texas A&M University If you have any questions, please call me at (979) 764-3570. Sincerely, 41£~-r~ Jeff Tondre, P.E. Civil Engineer Development Services xc: Dale Hoff, FEMA Region VI Ray Crow, P.E., Brazos County Engineering COLLEGE STATION April 10, 2000 Mr. William H. Black, P.E. Fort Worth District P. 0 . Box 9960 US Army Corps of Engineers P.O. Box 17300 Ft. Worth, TX 76102 RE: Backup Information Flood Insurance Study Brazos County Lick Creek, Sept 1987 Dear Mr. Black 1101 Texas Avenue Tet. 409 764 3500 College Station, TX 77842 The City of College Station is extending upstream a flood study of the Lick Creek watershed. The Army Corps of Engineers-Fort Worth District conducted the original study of Lick Creek in September 1987 as a Limited Detailed Study. Mr. Jack Quarles, with FEMA Region VI, directed us to you to request backup information from the existing effective study. We would like to request a copy of the following information: • study base contour map, • HEC-2 cross-section location map, • NuDallas drainage basin sub-area map, • supporting material regarding centroid location, slope, and length, • support material regarding soil types, urbanization, and impervious cover • available electronic NuDallas hydrologic analyses and HEC-2 hydraulic models. We have reviewed a paper copy of the NuDallas Hydro logic model used in the Flood Insurance Study. One input parameter used in the model concerns us, and we would like some classification regarding the bases of this parameter. Throughout the twenty square mile Lick Creek watershed, and the other watersheds included in the study, a soil factor of ninety-percent sand was used. It is our assumption that such a soil would have rapid infiltration and a low runoff potential. A review of the Brazos County Soil Survey shows that the drainage basin primarily consists of Lufkin, Edge, and Tabor soils. All three of these soils are classified by the NRCS (SCS) as Hydrologic Group D, soils having very slow infiltration and high runoff potential. A field reconnaissance of the watershed confirmed the Soil Survey findings. Home of Texas A&M University Unless we can find evidence of why the soil condition of ninety-percent sand is valid, we would like to revise the parameter used in the model. Preliminary findings show that a change from ninety-percent sand to forty-percent (based upon Soil Survey) would increase flows by more than fifty percent. Thank you in advance for your help and cooperation. If you have any questions, please call me at (979) 764-3570. Sincerely, ~~/~ Jeff Tondre, P.E. Civil Engineer Development Services xc: Jack Quarles, FEMA Region VI