FLOOD10516 (2) - Laserfiche WebLink

Home Browse Search

. . . II. HYDROLOGY Hydrology for the current study, the 1999 & 2001 LOMRs, and the 2005 Removal Action LOMR were developed by the USACE [USACE 1988] and provided the basis for the hydraulic analyses along the Poudre River completed as part of the effective FIS for the City of Fort Collins (March 18, 1996). Results of the 1988 hydrology study have been provided in Appendix A. For the current study reach, the peak discharges reported in the Fort Collins FIS match the results of 1988 hydrology study. The peak discharges for the current study reach have been provided in Table 2.1. Table 2.1 Cache la Poudre River FIS Hydrology (USACE 1988, FJEMA 1996). Location Peak Discharge (cfs) 10-Year 50-Year 100-Year SOO- Year Bluff Line Gage 6,490 11,800 15,100 26,300 Upstream of Dry Creek Confluence 5,370 10,200 13,300 24,100 Downstream of Dry Creek Confluence 6,700 12,700 16,600 30,100 The hydraulic analyses completed for the Fort Collins FIS, and the various LOMRs utilized more detailed discharge profiles than those reported in the table. In addition to potential inflows to the Cache la Poudre River from Dry Creek, the USACE hydrologic study l::onsidered the attenuation of flood peaks along the river. The detailed results of the USACE study were incorporated into the effective FIS analyses as indicated by the variation in discharge:s given as input to the HEC-2 models. The detailed discharge profiles used in the current study are based on the QT c:ard values in the effective HEC-2 models from the original FIS; these profiles are given in Tablc~ 2.2. The profiles given in Table 2.2 represent the QT card values from the original 1996 FIS HEC-2 files. The Table 2.2 discharges typically represent the total discharges for the river, prior to the consideration of any split flows with the exception of the Vine Drive 500-year split of 1,100 cfs. It should be noted in the 500-year profile at the Linden Street Bridge that 500 cfs is removed from the discharge profile. It is unclear, due to the lack of available documentation, whether this decrease in the 500-year discharge is attributed to in-chmmel attenuation or flow leaving the main chmmel upstream of the bridge. Therefore, based on other discharge reductions in the original profile it is assumed that the 500 cfs removed at Linden Street is a result of attenuation. Similarly, at the B.N.R.R. Bridge 1,100 cfs is removed from the 500-year profile. Based on notes in the 1994 hydraulic model, an offline rating curve determined that 1,100 cfs split from the main channel along the north side of Vine Drive and the B.N.R.R. tracks. The 1,100 cfs Vine Drive split was removed from the USACE's 1988 baseline hydrology between RN.R.R. Bridge and Dry Creek prior to determining the QT values in the 1994 hydraulic models for the Cache la Poudre River. COFC2003-1O Oxbow FINAL LOMR.doc 2.1 ANdERSON CONsulTiNG ENGiNEERS, INC.