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" " . . . " Federal Emergency Management Agency Washington, D.C. 20472 AUG 0 7 2000 REceIVED AUG 1 0 2000 CERTIFIED MAIL RETURN RECEIPT REQUESTED IN REPLY REFER TO: Case No.:.,QJlf!l$;';1@4R,., "_~.",,,.,, ,.,,_.{ c". Colorado Watsr ConsslValion Board The Honorable Nancy Heil Mayor, City of Westminster 4800 West 92nd Avenue Westminster, CO 80031 Community: City of Westminster, CO Community No.: 080008 104 Dear Mayor Heil: 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) for your community in accordance with Part 65 of the National Flood Insurance Program (NFIP) regulations. In a letter dated February 9, 2000, Ms. Sheila Beissel, P.E., Senior Civil Engineer, Department of Community Development, City of Westminster, requested that FEMA evaluate the effects that the following proposed improvements along Big Dry Creek from approximately 400 feet downstream of Interstate Highway 25 (1-25) to just downstream of Huron Street would have on the flood hazard information shown on the effective FIRM and FIS report: enlargement and paving of the compound trapezoidal bridge waterway at 1-25, with an improved inlet and a riprap-stabilized downstream channel from approximately 200 feet downstream to just upstream ofI-25; placement of erosion-control fabric and riprap on the north bank of Big Dry Creek and construction of an overflow bench on the south bank from approximately 1,200 feet downstream to just downstream of Huron Street; and construction of a 15-foot-wide low-flow crossing structure approximately 300 feet downstream of Huron Street. This request also incorporated the effects of more detailed topographic information from approximately 200 feet downstream ofI-25 to just downstream of Huron Street. Because this request for a Conditional Letter of Map Revision (CLOMR) also affects the City of Thomton, a separate CLOMR for that community was issued on the same date as this CLOMR. All data required to complete our review of this CLOMR request were submitted with letters from Ms. Beissel. 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-RAS hydraulic computer model, dated December 14, 1999, based on updated topographic information, was used as the base conditions model in our review of the proposed conditions model for this CLOMR request. We believe that, if the proposed project is constructed as shown on the submitted topographic work map entitled "Big Dry Creek Floodplain," dated November 8, 1999, and as described in the submitted reports entitled "Big Dry Creek Improvements, Hydraulic Design Report," dated November 1999, and "Big Dry Creek Improvements," dated February 2000, all prepared by the Sear-Brown Group, and the data listed below are received, a revision to the FIRM would be warranted. Our comparison of the existing conditions model to the effective model revealed that the elevations of the flood having a I-percent chance of being equaled or exceeded in any given year (base flood) increased as a result of the more detailed topographic information. The maximum increase in base flood elevation (BFE), 2.5 feet, occurred just downstream of Huron Street.