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<br />. <br /> <br />. <br /> <br />. <br /> <br />4.1.1 Floodplain Analyses and Results <br /> <br />The 10-, 50-, 100-, and 500-year effective floodplain analyses were replicated for the <br />duplic.ate effective analysis. By definition, the duplicate effective floodplain models were re-run <br />in HEC-2 without any changes made to the models. The same starting water surface elevations <br />and roughness coefficients documented in the effective analysis were also utilized in the <br />duplicate effective analysis. A summary of the effective and duplicate effective FEMA water <br />surface profile results for the study reach is presented in Table 4.2. <br />Within the study reach, results of the floodplain analyses associated with th~: duplicate <br />effective HEC-2 modeling matched the water surface profile plots from the effective FIS and <br />LOMRs except at Cross Section AM:221702 (the upstream side of Lemay A venue). At Cross <br />Section AM:221702 the differences between the effective and duplicate effective 10- and 500- <br />year water surface elevations are +0.2 feet and -0.6 feet respectively. The 1999 Timberline <br />Road LOMR hydraulic models stopped at the downstream side of Lemay Avenue. However, the <br />plotted water surface profile provided by FEMA in the 1999 LOMR (See Appendix C.l.1) <br />indicated a 'Revised Reach' which extends upstream of Lemay Avenue. It appears that the <br />published water surface profiles were straight-line plotted from Cross Section AM:221702 to <br />Cross Section AN:222452 thus skipping over Cross Section 222377 (the downstream side of <br />Mulberry Street). Therefore, it was concluded that the floodplain models used as the basis for <br />the current study correctly reproduce the results from the effective FIS and LOMRs. <br />Discharge profiles for the duplicate effective 10-, 50-, 100-, and 500-year events are <br />shown on Table D.l in Appendix D. With in the current study reach, the original 1994 analyses <br />indicated that during the 10-, and 50-year events major spills did not occur from the Cache la <br />Poudre River main channel to the VINE, LINe, or LEMA YDS Divided Flow Paths. The <br />effective discharge profiles indicate that the 1 GO-year water surface elevations downstream of <br />Mulberry Street assumed that the Lemay Avenue embankment north of Mulberry Street did not <br />breach. <br />It should be noted in the 500-year profile between Cross Sections AS:228117 and 227437 <br />a drop in discharge due to the LINC DFP is not evident. This is a result of an error in the WS.3 <br />(WS Card, Field 3) input to the original HEC-2 hydraulic model named UPPERSOO [SLA, <br />1994]. The 1994 hydraulic model referenced Cross Section 228125 in the WS.3 field. instead of <br />Cross Section 228117. Therefore, the second LINC DFP lateral weir between main chmmel <br />Cross Sections AS:228117 and 227437 did not work properly. <br />It should also be noted that an approximately 3,000 cfs increase in the 500-year discharge <br />occurs under the Lemay Avenue Bridge (Between Cross Sections AM:221702 and 221637). <br />This is the result of an error assigning the correct value to a HEC-2 QT card during the 1999 <br />Timberline LOMR analyses [ACE, 1999]. <br /> <br />COFC2003-10 Oxbow FINAL LOMR.doc <br /> <br />4.2 <br /> <br />ANdERSON CONsulTiNG ENGiNEERS, INC. <br />