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<br />. <br /> <br />based on a 1"=100' map scale and one-foot contour interval. This mapping was <br />developed using aerial photography and flown December 2000, The aerial mapping <br />was supplemented with field mapping in September 2001. The new project mapping <br />references NGVD-88, All elevations presented in this CLOMR, for the new <br />interceptor channel, continue to reference the NGVD-88 datum, At the request of <br />FEMA, the elevations can be converted and presented on the NGVD-29 datum in the <br />final request for Letter of Map Revision (LOMR), <br /> <br />5. <br /> <br />HYDROLOGY <br /> <br />The hydrology used in this revision request is consistent with that used to develop <br />the effective flood hazard information for the Pawnee Creek Overflow (City of <br />Sterling FIS), The total flow for each of the four frequencies are 1,250 cfs, 2,400 <br />cfs, 5,000 cfs, and 11,000 cfs, for the 10-, 50-, 100-, and 500-year floods, <br />respectively. <br /> <br />6. HYDRAULICS <br /> <br />6.1 HYDRAULIC MODELING <br /> <br />. <br /> <br />The revised hydraulic analyses for the Pawnee Creek Overflow were performed <br />using the U.S, Army Corps of Engineers Water Surface Profile computer program <br />HEC-RAS, Version 3,0. Digital copies of the models discussed below are contained <br />on the CD in Appendix A (Form 4), <br /> <br />Effective Models <br /> <br />The effective floodplain and floodway models were developed by Charles F, Leaf, et <br />aI., as part of the "Flood Plain Information and Drainage Plan for Sand Creek and the <br />Pawnee Creek Overflow at Sterling, Colorado", A copy of this report was obtained <br />from the City of Sterling. This report provided detailed floodway and floodplain <br />maps that clearly identified the flood-inundated areas from each of the South Platte <br />River, Sand Creek, Pioneer Drainage Ditch, Pawnee Creek Overflow, and local <br />drainage through the City of Sterling, The models associated with this study were <br />not modified as part of this analysis, since a new channel is proposed to intercept the <br />flows before they reach the historic Pawnee Creek Overflow location, A request for <br />additional technical backup information was submitted to Michael Baker Jr., rnc, <br />However, they did not have any additional technical backup data for this area, A <br />copy of their response to our request is included in Appendix B, <br /> <br />Duplicate Effective Models <br /> <br />Duplicate effective models were not developed as part of this submittal since a new <br />channel is proposed to be installed and the historic channel will be eliminated, <br /> <br />. <br /> <br />4 <br />