FLOOD06304 - Laserfiche WebLink

Home Browse Search

. . . 2 . The effective HEC-2 model evaluated split flows in the area downstream of Cross Section 177628 (D) as follows. The total base flood discharge reaching Cross Section 177628 (D) is 4,725 cfs. Of this total, 2,032 cfs overtops the 1-25 roadway north ofLCR36 (approximate minimum elevation 4,833,1 feet), with the remaining 2,693 cfs passing though Cross Section 176343 (C). The 2,032-cfs split flow is modeled in the effective split-flow model (filename "I25sp1.dat," which previously was sent to you bye-mail) using the split-flow option ofHEC-2, The split-flow data is shown on the input data on the TW record as "WEIR FLOW OVER 1-25 176343-177628." The WC record defines the weir along the 1-25 roadway with a minimum elevation of 4833.1 feet approximately 850 feet north of LCR36, . The regulatory floodway shown on the effective FBFM east ofI-25 and north of LCR36 is based on the split flow leaving the effective model between Cross Sections 177628 (D) and 176343 (C). The modeling further shows that of the total split flow of2,032 cfs, approximately 1,707 cfs overtops the LCR36 roadway east of 1-25 and returns to the main 1-25 Divided Flow Path at Cross Section 173338 (A). The remaining flow, approximately 325 cfs, continues flowing to the east, just north of LCR36, and joins the main Cache La Poudre River floodplain. Therefore, we believe that the effective FIRM and FBFM accurately depict the effective HEC-2 modeling and do not require revision. However, as previously discussed, the FWDT is in error. To revise the effective regulatory floodway to reflect the conditions described in your December 7 letter, the following information must be provided: I. A revised floodway encroachment analysis to show that the 2,032-cfs split flow, if added to the effective HEC-2 floodway run for the main channel ofI-25 Divided Flow Path, would not cause floodway surcharges of more than 1.0 foot above the Base Flood Elevations of the unencroached run. For comparison purposes, the unencroached run would not include the additional 2,032 cfs. 2. All information to satisfy the requirements of Section 65.7 of the National Flood Insurance Program regulations Please send the required data directly to us at the address shown at the bottom of the first page. For identification purposes, please include the case number referenced above on all correspondence.