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2 Mr. Guetig's HEC-2 hydraulic computer models of existing conditions, dated October 23, 1996, were used as the base conditions models in our review of the proposed conditions models for this CLOMR request. We believe that, if the proposed project is constructed as described in the report entitled "Request for Conditional Letter of Map Revision for the Big Thompson River, City of Loveland, Larimer County, Colorado," dated August 1996, and as shown on the preliminary plans entitled "Centennial Village Shopping Center, Rough Grading Plan Exhibit," dated October 1996, both prepared by Galloway, Romero & Associates, the floodplain boundaries of a flood having a I-percent chance of being equaled or exceeded in any given year (base flood) and a 0.2-percent chance of being equaled or exceeded in any given year will be delineated as shown on the topographic work map entitled "Big Thompson River CLOMR, Existing Conditions Topographic Data and Floodplain Mapping, Exhibit 3," dated August 1996 and revised October 29, 1996, also prepared by Galloway, Romero & Associates. When compared to the model developed by RCI, the submitted model of existing conditions along the main channel of the Big Thompson River indicates increases in the base flood elevations (BFEs) for the reach from West First Street to approximately 2,950 feet upstream of Taft Avenue and decreases in the BFEs for the reach from approximately 2,950 to 3,350 feet upstream of Taft Avenue. The maximum increase in BFE, 2.1 feet, occurs approximately 2,400 feet upstream of Taft Avenue, and the maximum decrease in BFE, 0.3 foot, occurs approximately 3,200 feet upstream of Taft Avenue. The existing conditions model indicates increases in the width of the Special Flood Hazard Area (SFHA), the area inundated by the base flood, in some areas and decreases in others compared to the SFHAs shown on the effective FIRMs for the City of Loveland and the unincorporated areas of Larimer County. The maximum increase in SFHA width occurs along the southwestern overbank approximately 500 feet downstream of West First Street where updated topographic information reflects numerous existing gravel mining pits southeast of the intersection of West First Street and Taft Avenue. During the base flood, all of these gravel mining pits will be inundated by floodwaters. The existing conditions model indicates a decrease in floodway width compared to the floodway shown on the effective FBFM for the unincorporated areas of Larimer County. The maximum decrease in floodway width, 900 feet, occurs approximately 2,400 feet upstream of Taft Avenue. The majority of the proposed fill will be placed outside the floodway indicated by the existing conditions model with a minor portion of the fill in the floodway. Mr. Guetig provided two HEC-2 models of proposed conditions. The first model reflects only the f111 that will be placed outside the floodway and the second model reflects both the f1ll outside the floodway and the minor fill in the floodway. There are no increases in BFEs resulting from the fill in the floodway primarily because this area is controlled by backwater from the Taft Avenue bridge crossing. The proposed placement of fill will reduce the amount of split flow just upstream of Taft Avenue, resulting in an increase in the base flood discharge in the main channel downstream of Taft Avenue. The BFEs will increase from just upstream of West First Street to approximately 1,550 feet upstream of Taft Avenue, with a maximum increase in BFE of 0.57 foot occurring approximately 150 feet upstream of Taft Avenue. As a result of the proposed f11l placement, the SFHA north of West First Street will decrease from just upstream to approximately 700 feet upstream of Taft Avenue. The submitted analysis affects the flood hazard information for both the City of Loveland and the unincorporated areas of Larimer County. The effective FIRMs for these communities do not reflect the updated corporate limits of the City of Loveland. As part of any future map revision for the above- referenced reach of the Big Thompson River, we will obtain an up-t<Hlate corporate limits map and identify more clearly the reach within the City of Loveland.