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. ~: ;:<_/_J ~-~ '... <-.:,... --- -~ ,~"~--~..-..:. .'f a. . J _~ ; .$SCC:Cl.eS, nc. ,vJ'.]' '.;.o')cJfse ca"'l3ultanfs August 3, 1992 2OV:; Cenrer~:,*,n 20\..- ScL.~, 5<..;1:61.: aoulcer. Co:cr<::.::c 3OJS1.S':.21 ?hor:e:;f";J) <:~o-34J9 ~':"';';(203;~"2~ Mr. Tom Smith Michael Baker Jr., Inc.. 3601 Eisenhower Avenue Suite 600 Alexandria, Virginia 22304 REF: 8924D . OAK CREEK FLOODPLAIN AND FLOODWAY . FLORENCE, COLORADO. FEMA LMMP CONTRACT NO. EMW-90-C.3132 Dear Mr. Smith: Love & Associates, Inc. is currently updating the floodplain and flood way for Oak Creek in Florence, Colorado under FEMA's Limited Map Maintenance Program. The study reach for Oak Creek extends from the confluence with the Arkansas River to the irrigation ditch crossing upstream of the Denver and Rio Grande Western Railroad (D&RGW). This study was prompted by the recent re-evaluation of the hydrology for Oak Creek at the confluence with the Arkansas River which was prepared this spring by the U.S. Army Corps of Engineers. The re-evaluation reduced the 100-year peak flow from 22,500 cfs to 16,000 cfs. Love & Associates, Inc. has prepared a detailed hydraulic analysis for Oak Creek using the updated hydrology. Due to the complexities in the evaluation and delineation of the floodplain and floodway for Oak Creek, we originally requested a meeting with FEMA Central Office personnel and their technical consultants Michael Baker, Jr. Engineers to discuss the hydraulic analysis and the options for the delineation of the floodway. However, at your request, in lieu of a meeting we are sending you the enclosed information regarding the hydraulic analysis. Due to budget constraints, before we proceed with the fmalization of the floodplain and floodway, we request a review of the work to date and comments regarding how to proceed with the delineation of the floodway. Floodplain The following is a brief discussion of the hydraulic analysis for Oak Creek. The draft lOO-year floodplain is shown on the attached drawings for the study reach. The HEC-2 input and output are given in Appendix A and are on the enclosed floppy disk. South of the D&RGW, at the upstream study limit, 2,940 cfs of the totall6,000 cfs flow splits from the main channel and flows east along the irrigation ditch alignment. This split flow becomes hydraulically disconnected from the main channel. The main channel flow is reduced by 2,940 cfs which leaves approximately 13,060 cfs in the main channel upstream of the D&RGW. At the railroad the 13,060 cfs is split as follows: 4,250 cfs is conveyed through the bridge opening; 8,310 cfs is conveyed over the railroad tracks, and 500 cfs splits to the east along the railroad. The flow split to the east (500 cfs) combines with the 2,940 cfs flowing along the