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<br />Section 5 Site Flood Risk and Mitil!ation Analvsis <br /> <br />Site I Connected LakeslDuke Lake <br /> <br />Subsection 5.1 <br /> <br />Flood Photos <br /> <br />Aerial photographs of flooding at Connected Lakes in 1984 and 1993 and a photograph of present <br />conditions at the state park are included on the preceding two pages. <br /> <br />Connected Lakes and Duke Lake were entirely inundated in 1983 and 1984. Due to the <br />protection provided by the levee system constructed for Connected Lakes. in 1991 and by the <br />Pepsi-Cola Plant levee system and due to smaller flows, no flood inundation occurred during the <br />1993 flood event. That levee construction led to higher stream velocities. This reach of the <br />Colorado River was already subject to severe bank erosion and channel migration before the levee <br />construction. During the 1983 and 1984 floods part of the river was flowing through the site. <br />The constriction caused by the levee work has made those problems more severe because of the <br />higher flood stages and velocities. The entire area lies within the IOO-year floodplain. <br /> <br />During high stages in the river there is an additional risk at Connected Lakes and Duke Lake. <br />The groundwater table is higher than normal in those circumstances, leading to an increased risk <br />of high lake levels which will lead to overflow. <br /> <br />Subsection 5.2 <br /> <br />Field Inspection and Hvdraulic Determinations <br /> <br />The field survey conducted on January 19, 1995 at Connected Lakes State Park included an <br />examination of existing conditions along the river bank and an elevation survey of the levee <br />surrounding most of the state park. The elevation survey did not extend to the very downstream <br />(west) end of the Connected Lakes. Without such a survey it is not clear that the dike around <br />the Connected Lakes is above the IOO-year flood elevation all the way to the west end of the <br />park. Even if it is high enough, there is still a low area near Duke Lake where floodwaters could <br />enter the site. That is exactly what happened in 1983 and 1984, Once floodwaters entered Duke <br />Lake, they would have access to the Connected Lakes also. <br /> <br />The entire Connected Lakes/Duke Lake site was studied as part of the 1992 FEMA restudy for <br />Mesa County. The FEMA hydraulic analysis was used by the CWCB staff to evaluate the flood <br />risk at the site. Once a preliminary risk assessment had been made, it became clear that the <br />effects of regrading and levee construction in the stream reach since 1984 had to be considered. <br />There was visible evidence at the site of continuing streambank erosion. The upstream (east) end <br />of the state park is immediately adjacent to the outside of a bend in the Colorado River. The <br />erosive forces along the outside of that bend were severe in 1983 and 1984 and the potential for <br />future erosion remains great. <br /> <br />The field review and the review of the floodplain maps showed that the levee construction had <br />substantially reduced the width of the floodplain at Connected Lakes. The CWCB decided to <br /> <br />13.1 <br />