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<br />I <br />I <br />I <br />I <br />I <br />I- <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />sox aCER CREEK o.OMR <br /> <br />4. The UD FCD rainfall-runoff model was based on procedures established to provide consistent, <br />calibrated flood hydrology for the Denver area. <br /> <br />The rainfall-runoff model used the lJDFCD's Colorado Urban Hydrograph Procedure <br />(CUHP)for hydrograph generation and Storm Water Management Model (UDSWlV12) for <br />hydro graph routing. These models have undergone years of verification and calibration <br />based on regional rainfall and runoff da ta. They provide the standard methodology for <br />estimating flood discharges in the region and are accepted by all of the regional flood <br />management agencies. <br /> <br />5. The UD FCD Box Elder Creek rainfall-runoff rrwdel was developed with full coordination and <br />input from local FE,VfA rq;resentatives. <br /> <br />A project advisory conuruttee was formed at the outset of the UDFCD study to obtain input <br />from as many interested organizations as possible. The local FEMA office was one of the <br />agencies that was invited to participate on the project a<;ivisory conuruttee. As such, FEMA <br />staff were notified of project meetings and received meeting minutes. Seventeen meetings <br />were held between October 26, 1993 and June 20, 1995 to discuss results and solicit input <br />from the advisory conuruttee. <br /> <br />John Uou from the local FEMA office attended project meetings on January 4,1994, when <br />initial hydrology results were discussed, and February 16, 1994, when review comments <br />were provided on the draft hydrology report. John commented in this meeting that FElVIA <br />no longer used WS1680 to compute flood discharges, except when a very approximate <br />"ballpark" number was being looked for. In addition, a special meeting was held on March <br />14, 1994 at the local FE:VIA office (meeting notes are attached). This meeting was held at <br />John Uou's request and was attended by representatives of UDFCD, Arapahoe County, <br />Adams County, the City of Aurora, CWCB, and CH2."vl HILL. The purpose of the meeting <br />was to coordinate how CLOMR submittals regarding hydrology and floodplains would be <br />handled with FEvIA. The approach worked out at the meeting was to first submit for <br />FEMA acceptance the peak flood discharges based on the UDFCD rainfall-runoff model. <br />Revised floodplain mapping would be submitted as a subsequent package sometime after <br />the flood hydrology was accepted. <br /> <br />The specific hydrology results from the liDFCD study were reviewed with and concurred <br />with by local FEMA staff. The State's flood management agency, the Colorado Water <br />Conservation Board (CWCB), has formally adopted UDFCD's Box Elder Creek floodplain <br />results, including the hydrology. <br /> <br />In summary, the UD FCD rainfall-runoff model provides estimates of peak discharges that are <br />superior to the discharges predicted based on regi07UlI regression equations. The results of the <br />UDFCD rainfall-runoff model account for the long, narrow shape of the Box Elder Creek watershed <br />and compare favorably with the COE analysis for the Section 22 Study and with the estimated peak <br />discharge from the 1973 flood of record. The UD FCD rainfall-runoff model was based on procedures <br />established to provide consistent, calibrated flood hydrologtj for the Denver area and results were <br />reviewed with and concurred with by local FElvlA staff and the State's floodplain management <br />agenC'f. <br /> <br />If you have any questions regarding the information provided in this memorandum, please <br />call me at m-099 x5583. <br /> <br />OENIClCMRHYO.OOC <br /> <br />s <br /> <br />