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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 />MEMORANDUM <br /> <br />Attachment 1 <br /> <br />CHMHIU. <br /> <br />Box Elder Creek CLOMR <br /> <br />TO: <br /> <br />Scott Tucker/UDFCD <br /> <br />Ben Urbonas/UDFCD <br />Bill DeGroot/UDFCD <br />Besharah Najjar/ Adams County <br />Darren Duncan/ Adams County <br />Cindy Edwards/ Arapahoe County <br />Kevin Wegener/City of Aurora <br />Ron Degenhart/City of Aurora <br />Larry Lang/ CWCB <br />Tom Browning/CWCB <br />John Iiou/FElvlA <br /> <br />Jim Wulliman/CH2vl HIL~fw <br /> <br />March 7, 1997 IJ <br /> <br />COPIES: <br /> <br />FROM: <br /> <br />DATE: <br /> <br /> <br />Based on your request, we have prepared this memorandum to respond to the February 5, <br />1997 letter from the Federal Emergency lvlanagement Agency (FEMA) regarding a <br />Conditional Letter of lvlap Revision (CLOlvfR) for Box Elder Creek. Besharah Najjar, <br />Floodplain Administrator for Adams County, submitted a letter dated September 13, 1996 <br />requesting that FElvLA. issue a CLOlvlR to establish new hydrology for Box Elder Creek <br />based on a revised hydrologic analysis prepared as part of the Box Elder Creek Outfall <br />Systems Planning Study and Flood Hazard Area Delineation (FHAD) undertaken for the <br />Urban Drainage and Flood Control District (UDFCD), Arapahoe County, Adams County, <br />and the City of Aurora. <br /> <br />In its response letter FEMA informed Adams County that it does not agree that the <br />submitted discharges, based on detailed rainfall-runoff modeling, represent the base flood <br />more accurately than several regional regression equations, one of which was the basis of <br />the flood discharges for the effective Flood Insurance Rate Maps (FIlThfs). Just downstream <br />of Box Elder Creek's confluence with Coyote Run, the submitted base flood discharge was <br />14,900 cubic-feet-per-second (cfs) compared to 26,500 cfs for the effective FIR..'vI and 27,700 <br />cfs using a 1987 regression relationship developed by the US Geological Survey (USGS). <br />FEMA stated that before it could revise the effective FIR.c\1s to reflect the submitted <br />discharges, it must receive documentation demonstrating that the modeled discharges are <br />superior to the discharges determined using the regression equations. <br /> <br />This memorandum documents why the discharges determined using the regression <br />equations are not appropriate for Box Elder Creek and why the modeled discharges are <br />superior. <br /> <br />OENIo.O,,"HYO.OOC <br /> <br />109067 .A2 <br />