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(I)~M.A~~~ ~ ~ ~ " ~ ~ > o , Federal Emergency Management Agency Washington, D.C. 20472 February 26, 2002 CERTIFIED MAIL RETURN RECEIPT REQUESTED IN REPLY REFER TO: Case No.: 02-0S-024R The Honorable Chairman Ted Strickland Chairman Adams County Board of Commissioners 450 South 4th Avenue Brighton, Colorado S060 I Community: Adams County, CO Community No.: OSOOO I 104 Dear Mr. Strickland: ,. This responds to a request that the Federal Emergency Management Agency (FEMA) comment on the effects that a proposed project would have on the effective Flood Insurance Rate Map (FIRM) and Flood Insurance Study (FlS) report for your community in accordance with Part 65 ofthe National Flood Insurance Program (NFIP) regulations. In a letter dated October 22, 2001, Mr. Douglas C. Weber, P.E" MKCfPTG, requested that FEMA evaluate the effects that the proposed E-470 highway crossing of Big Dry Creek would have on the flood hazard information shown on the effective FIRM and FIS report. The proposed project consists of two parallel E-470 highway bridges crossing Big Dry Creek, an on-ramp north of the highway, an off-ramp south ofthe highway, a new bridge at 152nd Avenue, improvements to York Street, and channelization of Big Dry Creek at E-470, The bridges crossing Big Dry Creek will be constructed with two bridge spans, supported by a pier and the abutments. All data required to complete our review of this request for a Conditional Letter of Map Revision (CLOMR) were submitted with letters from Mr. Weber. We reviewed the submitted data and the data used to prepare the effective FIRM for your community and determined that the proposed project meets the minimum floodplain management criteria of the NFIP. The submitted existing conditions HEC-2 hydraulic computer model (corrected effective model), dated January 19, 2002, based on updated topographic information, was used as the base conditions model in our review ofthe proposed conditions model for this CLOMR request. We believe that, if the proposed projett is constructed as shown on Figures S, 9, and 11-15 of the report entitled "Conditional Letter of Map Revision Application, Big Dry Creek (ADCO)," dated October 2001, by MKCfPTG, and the data listed below are received, the floodplain boundaries of the flood having a I-percent chance of being equaled or exceeded in any given year (base flood) and a 0.2-percent chance of being equaled or exceeded in any given year will be delineated as shown on Figure 10 of the same report. The corrected effective model was developed by utilizing the model used to produce the effective FIS and updating it with new cross-section data based on new topographic information. Our comparison of the corrected effective model revealed increases in the base flood elevations (BFEs) when compared to the duplicate effective model. The maximum increase in BFE, approximately I.S feet, occurred just downstream of 156th Avenue. The width of the Special Flood Hazard Area (SFHA), the area that would be inundated by the base flood, increased a maximum of approximately 500 feet at approximately 1,500 feet downstream of 156th Avenue. The width of the SFHA decreased a maximum of approximately 200 feet at approximately 500 feet downstream of 156th Avenue.