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. . , '. RECEIVED Federal Emergency Management Age~ 03 19 Washington, D.C. 20472 97 F- E B 2 Colorado Waler 7 1997 Conservation Board Mr. Stephen T. Honey City Manager City of Boulder Post Office Box 791 Boulder, Colorado 80306 Dear Mr. Honey, This is in response to your letter dated October 10, 1996, regarding the Flood Insurance Rate Map (FIRM) for Boulder County, Colorado and Incorporated Areas, dated June 2, 1995. Specifically, you questioned the accuracy of the floodplain boundary delineations for the flood having a I-percent chance of being equaled or exceeded in any given year (base flood) along-South Boulder Creek. The base floodplain delineation shown on the effective FIRM is based on information developed for the South Boulder Creek Flood Hazard Area Delineation (FHAD), prepared by Greenhorne & O'Mara, Inc., dated July 1986. The FHAD was provided to the Federal Emergency Management Agency (FEMA) for incorporation into the FIRM as part of an appeal dated January 30, 1987, submitted by the City of Boulder and an appeal dated March 5, 1987, submitted by Boulder County. In letters dated August 20, 1987, and December 28, 1987, FEMA informed the City of Boulder and Boulder County, respectively, that the Flood Insurance Study reports and FIRMs for those communities would be revised to incorporate the submitted data. The information from the FHAD has been presented on the FIRM for Boulder County since July 15, 1988. In your letter, you stated that a recent study performed by Love & Associates, Inc., concluded that the base floodplain shown on the effective FIRM is incorrect due to errors in the analysis performed for the FHAD, The Love & Associates study indicated that the FHAD excluded areas from the floodplain based on a determination that an earthen embankment would contain flood waters along South Boulder Creek. Because the embankment is not a natural land form and has not been recognized by FEMA as providing protection from the base flood, you believe the portion of the Flatirons Property located landward of the levee and shown on the FIRM to be outside the base floodplain, is in fact subject to inundation by the base flood. The Love and Associates study also concluded that the split flow from South Boulder Cree\.( downstream of the levee would be greater than that identified in the FHAD. - You stated that the City of Boulder, Boulder County, and the Urban Drainage and Flood Control District have initiated an evaluation of the base floodplain along South Boulder Creek to redefine the floodplain boundaries as necessary. Although a formal map revision request will be filed when that evaluation is complete, you requested that FEMA use available information to revise the floodplain boundaries on the FIRM to include areas landward of the earthen embankment. You suggested two options: (I) use the base flood elevations (BFEs) shown on the effective FIRM to determine the extent of the base floodplain beyond the embankment or (2) use the flood hazard information from the 1979 FIRM, which was superseded by the information developed for the FHAD. Revisions to National Flood Insurance Program (NFIP) maps must be supported by hydrologic and hydraulic analyses of existing conditions. Although the Love & Associates study concluded that the FHAD was not accurate, it did not include a hydraulic analysis of existing conditions. Both conditions modeled by Love & Associates consider future conditions, either construction of a channel or improvement of the existing embankment. In addition, neither of the options for revising the FIRM that you suggested are supported by hydraulic analyses of existing conditions.