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. .. . I. INTRODUCTION 1.1 Background The effective Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) information for the City of Fort Collins (COFC), Colorado was obtained from the revised FIS for the City of Fort Collins (March 18, 1996). The revised COFC Flood Insuranc:e Study is based, in part, on revised hydrologic and hydraulic analyses completed on the Cache la Poudre River (CLPR) by the United States Army Corps of Engineers (USACE), Omaha District, and Simons, Li & Associates, Inc. (SLA). The revised hydrologic analysis was complete:d in April 1988 [USACE 1988] and modified the peak-flood discharges on the Cache la Poudre River. The revised peak-flood discharges were used in conjunction with field-surveyed chmmel cross sections to modify and update the hydraulic models for the Cache la Poudre River through Fort Collins [SLA 1994]. The study limits for the revised hydraulic study, which was completed in May 1994, were from Horsetooth Road upstream to the south end of Watson Lake (near State Highway 28). However, it should be noted that the 1994 hydraulic study further separated the Cache la Poudre River into two separate HEC-2 modeling reaches through the COFC, the Upper Reach and the Lower Reach. The model limits for the Upper Reach extend from the south end of Watson Lake downstream to the Tinmath Reservoir Diversion Dam downstream of Lemay Avenue. The model limits for the Lower Reach extend from the Timanth Reservoir Diversion Dam downstream to Horsetooth Road. Figure 1.1 presents a vicinity map of the Cache: la Poudre River through the COFC and depicts the Upper Reach and Lower Reach model limits. In 2005, Anderson Consulting Engineers, Inc. (ACE) was retained by the City of Fort Collins to conduct hydraulic analyses in support of a Letter of Map Revision (LOMR) application for a portion of the Cache la Poudre River in the Upper Reach. The purpose of this LOMR analysis is to evaluate the improvements associated with the City of Fort Collins' Oxbow Levee Project. Figure 1.2 presents a site map for the current study. The Oxbow Levee proJect reach extends from the downstream side of the Linden Street Bridge downstream to the upstream side of the Lincoln Avenue Bridge. The project reaches include all proposed physical alterations within the regulatory floodplain. The study reach extends from the upstream side of College Avenue/U.S. Highway 287 (Cross Section AX:230897) downstream to the Timnath Diversion Dam (approximately one-half mile downstream of Lemay Avenue). The study reach corresponds to the limits of revised water surface elevations. The model reach encompasses both the study and project reaches and extends from the south end of Watson Lake downstream to Horsetooth Road because the Upper Reach model and the Lower Reach model were combined to form one continuous model for the Cache la Poudre River with in the City of Fort Collins. COFC2003-10 Oxbow FINAL LOMR.doc 1.1 ANdERSON CONsulTiNG ENGiNEERS, INC.