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<br />. <br /> <br />. <br /> <br />. <br /> <br />The effective FIS for the City of Sterling appears to have adopted the floodplain <br />analysis for the Pawnee Creek Overflow presented in the "Flood Plain Information <br />and Drainage Plan for Sand Creek and the Pawnee Creek Overflow at Sterling, <br />Colorado" by Charles F. Leaf, P,E and Resource Consultants, Inc. This report was <br />used as the basis for our modifications to the FIRM maps, This report identifies the <br />areas within the City of Sterling that are inundated solely by the 100-year Pawnee <br />Creek Overflow and those inundated by 1 DO-year flows along Sand Creek, Pioneer <br />Ditch, the South Platter River, and other local drainage, A flood boundary map has <br />been complied from this report, Both the combined map and a copy of the report can <br />be found in Appendix G, <br /> <br />2. <br /> <br />STUDY LIMITS <br /> <br />The Pawnee Creek Overflow Interceptor Channel is proposed to be constructed <br />adjacent to County Road 22Y:. between County Road 35 and the South Platte River. <br />Since this is a new channel and the historic I DO-year Pawnee Creek Overflow will be <br />collected and rerouted, it is not appropriate to tie in to effective hydraulic models <br />downstream of the interceptor channeL Since the upstream limit ties into a Zone A, <br />with no technical backup available, a graphical tie in has been made to the effective <br />flood hazard information at County Road 22Y:., Revisions have been made to the <br />Flood Profile, Floodway Data Table, and FIRM maps to reflect the new location of <br />the interceptor channel and study limits, We recommend removing the effective <br />Pawnee Creek Overflow profile from the City of Sterling FIS and adopting a new <br />profile for the Pawnee Creek Overflow Interceptor Channel in the Logan County <br />FIS. Additionally, the BFE's and SFHA's have all changed as a result of the new <br />channel. <br /> <br />3. <br /> <br />PROPOSED CONDITIONS <br /> <br />The proposed conditions for the Pawnee Creek Overflow reflect the proposed <br />construction of an Interceptor Channel to collect the Pawnee Creek Overflows and <br />transport them directly to the South Platte River. This new channel will intercept <br />flows, up to the 1 DO-year event, and prevent them from entering the City of Sterling <br />and causing flood damage, The interceptor channel is proposed to consist of <br />approximately 12,000 linear feet of grass lined channel, with a bottom width varying <br />between 80-feet and 260-feet. Typical channel slopes vary between 0.15% and <br />0.3%, All channel sideslopes are anticipated to be a minimum of 3 (horizontal): 1 <br />(vertical), New bridges are proposed to cross the channel at US highway 6 and the <br />Union Pacific Railroad, Schematics of the bridge crossings can be found with the <br />FEMA forms, <br /> <br />4. <br /> <br />MAPPING <br /> <br />The workmaps for the effective floodplain were based on mapping with a 1"=200' <br />map scale and two-foot contour intervals, The mapping datum was NGVD- 29, The <br />project mapping associated with the construction of the interceptor channel was <br /> <br />, <br />y <br />