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<br />l <br /> <br />., <br /> <br /> <br />')U' '~ I <br />~ Lr---- <br />Federal Emergency Management Agency <br />Washington, D.C. 20472 <br />FEB 0 5 1997 <br /> <br />CERTIFIED MAIL <br />RETURN RECEIPT REQUESTED <br /> <br />IN REPLY REFER TO: <br />Case No.: 96-08,379R <br /> <br />RECEIVED <br />FEB 1 0 1997 <br /> <br />The Honorable Guillermo A. DeHerrera <br />Chairperson, Adams County <br />Board of Commissioners <br />450 South Fourth Avenue <br />Brighton, Colorado 80601 <br /> <br />Colorado Waler <br />Conservation Board <br />Community: Adams County, Colorado <br />Community No.: 080001 <br /> <br />104 <br /> <br />Dear Mr. DeHerrera: <br /> <br />This responds to a letter dated September 13, 1996, from Mr. Besharah Najjar, P.E., Floodpi@in <br />Administrator, Adams County, to the Federal Emergency Management Agency (FEMA) regarding the <br />effective Flood Insurance Rate Maps (FIRMs) and Flood Insurance Study (FIS) reports for Adams and <br />Arapahoe Counties and the City of Aurora, Colorado. With his letter, Mr. Najjar requested that FEMA <br />evaluate a revised hydrologic analysis of the Box Elder Creek watershed performed by CH2M Hill. The <br />hydrologic analysis incorporates the use of rainfall,runoff models for Box Elder Creek and Coyote Run, a <br />tributary to Box Elder Creek. All data required by FEMA to evaluate this request for a Conditional Letter <br />of Map Revision (CLOMR) were submitted with Mr. Najjar's letters dated September 13 and October 6, <br />. 1996.. CLOMRs Commenting on Mr. Najjar's request were also issued on this date for Arapahoe County and <br />the City of Aurora. ! '; :5<;1 <br /> <br />The rainf<iII'run~ffmodels prepared by CH2M.Hill use the Urban: Drainage and Flood Control DiStIjctY;V <br />Colorado Urban Hydrograph Procedure (CUHP) computer program to develop runoff hydrographs for each <br />subwatershed and the Urban Drainage and Storm Water Management Model to route the runoff hydrographs <br />through the stream network. The table below summarizes the peak discharge values computed by the <br />rainfall,runoff models for Box Elder Creek and Coyote Run. <br /> <br />.~ <br /> <br />'.:;-. <br /> <br />I..ocation <br /> <br />Drainage Area <br />(square miles) <br /> <br />Peak Base Flood Dischar~es* <br />Rainfall,Runoff Regional Regression <br />Model Equations <br /> <br />Coyote Run: <br />At Interstate 70 <br /> <br />16.9 <br /> <br />11,600 <br /> <br />12,200 <br /> <br />Box Elder Creek: <br />Just downstream of the confluence <br />with Coyote Run <br />Just upstream of the <br />confluence with Coyote Run <br />At the Arapahoe County <br />and Elbert County boundary <br /> <br />202.2 <br /> <br />14,900 <br /> <br />27,700 <br /> <br />173.5 <br /> <br />11,700 <br /> <br />25,800 <br /> <br />127.1 <br /> <br />10,800 <br /> <br />22,500 <br /> <br />*Cubic feet per second (cfs) <br /> <br />The report submitted with Mr. Najjar's September 13 letter states that the regional regression analysis, which <br />supports the flood hazard information along Box Elder Creek shown on the effective FIRM, overestimates the <br />peak discharge of the flood having a I-percent annual chance of being equaled or exceeded in any given year <br /> <br /> <br />