Arkansas River Division 2 Design Report

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Arkansas River Design Report for Canon City Whitewater Kayak and Recreation Park in the Ark ans • VII. Statute Compliance There are a number of permit requirements associated with working in a waterway. For the WKRP required permits are the US Army Corps 404 permit, the State of Colorado 401 Water Quality Certificate, and the Federal Emergency Management Agency Map Revision. These permits and the associated coordination with natural resource managers have been initiated with the Feasibility Design phase. The US Army Corps 404 permit is expected to be submitted July 10th, 2008. The 404 application will be forwarded to the CDPHE for 401 review. A Letter of Map Revision is being coordinated with the Canon City floodplain manager. VIII. Floodplain Issues The proposed project does result in increases of the Base Flood Elevations less than 0.2 feet compared to existing conditions. A Floodplain Information Report(FIR)dated June, 1982,prepared by Camp Dresser and McKee Inc. for the Town of Florence, Canon City, Fremont County and the Colorado Water Conservation Board,presents Base Flood Elevations (BFE)through the project site based on field surveys and the use of the HEC-2 floodway model developed by the U.S. Army Corps of Engineers. As applied to the Arkansas River,the HEC-2 model used in the 1982 FIR represents ten channel cross-sections and three bridges over the project • reach to describe channel geometry. These HEC-2 data were transformed into a HEC- RAS model to create a duplicate effective model. The duplicate effective model was compared to the most current published Flood Insurance Study(FIS)published in September 2007. To evaluate potential effects of the hydraulic structures on flood profiles for the Arkansas River, existing conditions cross-sections were added to the developed HEC-RAS model based on field surveys conducted by RiverRestoration.org in 2007. These existing conditions cross-sections represent the physical changes to the channel geometry that resulted from construction of three bridge structures post the FIR 1982 floodway model. The HEC-RAS model was then run with the existing cross-sections and results were compared to the duplicate effective model results. The draft results of the existing conditions analysis shows that the three bridge structures, installed after the 1982 FIR model, do have a significant affect on the floodplain and Base Flood Elevations. Therefore, the City of Canon City is initiating the map revision process to correct the inconsistencies between the 1982 FIR modeling and the existing conditions. Proposed channel modifications at the Nessman and Flytrap were compared to the existing conditions for the affects of the 100 year flood. The HEC-RAS model results (Table 2) indicate water surface elevations(WSEL)would increase less than 0.2 feet • ` DIVE 1 8 of 16