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,---~ & // ,;:--/'1:::-. ,". ~~:::~ ;.....:'1-.'..' ,. " "I' w I~ ' - 0 Associates, Ir:c. May 27,1992 >,- '-::L.~ - ...., ~....:<l '-, _,,::.j' ~C;;;:;'.:<2' "CI-," -':4S-2.:JO .oj. ~..; Dr. John Liou, P.E., P.L.S. Fecieral Emergency Management Agency Region VIIT - Denver Federal Center Bu.1ding 710 - Box 25267 DelVer, Colorado 80225-0267 Mr. Larry Laag, P.E. EXl:cutive Director Co.lorado Water Conservati,on Board 72:. Centennial B uildiag 13.3 Sherman Street Deaver, Colorado 80203 II/I. Ken Bruch City Manager Ci.ty Hall 3, :H) Wl:st Main F,orence, Colorado 81226 REF: 89240 - OAK CREEK LMMP - 100- YEAR fL' )ODPLAIN Dear Sirs: The draft version of the 100-year floodplain has been celineated for Oak Creek from its confluence wi':h the Arkaasas River to the irrigation ditch CroSSi:lg upstream of the Denver iUld Rio Graade Western Railroad (D&RGW) and was presented ill the" May 13 meetiag attended by the individuals listed above as addressees as well as representatives tmm the U.S. Army Corps of Engineers aad other interested persons. The 100-year flow rate,lsed. for this analysis is 16,000 cfs. The following is a description of the hydraulic aaalysis use:! to delineate the 100-year floodplain. South of :he D&RGW, at the upstream study limit, '2940 cfs of the total 16,000 cfs flow splits frcm the main channe:l aad flows east along the irrigatlQn ditch alignment. This split flow becomes hydraulically disconnected from the main channel. T:ll: main channd flow is reduced by 2940 cfs which leaves approximately 13,060 cfs in the main ,,~hatUlel upstre:am of the D&RGW. At the railroad the 13,060 cfs is split as follows: 4250 cfs is-c:onveyed through the bridg'~ opening; 8310 cf~; is conveyed over the railroad tracks, aad 500 cfs SIJlits to the east along the railroad. The flow split to th,~ east (500 c:fs) combines with the 2940 cfs :lowillg along the irrigation c:anal aligrunem. Tbs combined flow of 3440 cfs continues east along: the: railroad to the low point in the tracks at about Fra:cier Avenue:. The flow spills continuously o/,~r ttle railroad, along this length of track. Downstream of the D&RGW, the 100-year floodp12.im is delineated using thtee HEC-2 models: mun chmlOel, right overbank area, and left ovelllbatlk area because the an:as of flow are hydraulically disconnected. The flow in the left ovej'bank area is estimated as that portion of the flow over the railroad from a point 200' west of the ',bridge crossing. This point was selected by an alysis of the downstream topography. The left o'!'l:rbank flow was estimated using a sloping w,:ir equation aad the water surface elevation at eros'" se(:tion L given in the main channel model (s'~ Figure 1). The flow in the left overbank area is <pproximately 2720 cfs.