FLOOD10516 (2) - Laserfiche WebLink

Home Browse Search

. . . 3.1.2 Starting Water Surface Elevations and Roughness Coefficients Starting water surface elevations for all Lower Reach floodplain models (LO'WQ99 and LOW599) and the one-foot rise floodway models (lFTFW99 and IFTFL WAY) wer,e obtained from the Larimer County FIS restudy [FEMA, November 17, 1993]. Starting water surface elevations for the 1994 effective Upper Reach floodplain models (UPPERQ, UPPER500, and LINe:) match those used in the 1994 study. The 2005 Removal Action LOMR models (RCUPR05, RCUPR505, and RCUPIF05) began at Lemay Avenue and utilized starting water surface elevations from the 1999 LOMR. Within the current study reach, the Mmming's n value for the Cache la Poudre River main chmmel was 0.035 and the Mmming's n values for the overbank areas ranged between 0.040 and 0.104. Along the LINC DFP upstream of Lemay Avenue, the Mmming's n value was 0.035 on all cross sections except those containing buildings where a 0.053 value was utilized. 3.1.3 Floodplain and One-Foot Floodway Results Effective water surface profiles were obtained from the plotted water surface profiles and floodway data tables published in the City of Fort Collins FIS [FEMA, 1996] and the Timberline LOMR [FEMA, 1999]. Additionally, effective water surface profiles were obtained from the pending 2005 Removal Action LOMR [ACE 2005, Revised 2006]. Table 3.2 presents a summary of the effective water surface elevations for the current study reach. The effective 100-, and 500-year floodplain delineations downstream of Linden Street from the original 1996 FIS workmap are shown on Sheet lA. The revised condition 100-, and 500-year floodplain delineations from the 2005 Removal Action LOMR are shown on Sheet lB. Effec:tive water surface profile plots are located in Appendix C.l. A floodway is the chmmel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood (i.e. 100-year flood) without cumulatively increasing the water surface elevation by more than a specified depth. FEMA administers a one- foot rise floodway criterion on the Cache la Poudre River. The effective one-foot rise floodway delineations downstream of Linden Street from the original 1996 FIS workmap are shown on Sheet lA. The revised condition one-foot rise floodway delineations from the 200:5 Removal Action LOMR are shown on Sheet lB. Effective FEMA floodway data tables are located in Appendix B.l. Based on a review of the effective FEMA workmaps and models, the following conclusions are drawn: COFC2003-1O Oxbow FINAL LOMR.doc 3.3 ANdERSON CONsulTiNG EI~GiNEERS, INC.