1997-06-13_REPORT - M1981302 (16)

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1 1 FEMA's reported regulatory 100-year flood discharge for South Boulder Creek at U.S. Highway 36 is noted to be 6,160 cfs as reported in the Flood Insurance Rate Study for Boulder County (FIS) published in June, 1995. As previously noted, the FEMA FIS was ' based upon he G&O 1986 floodplain report. No new studies were performed by FEMA in order to verify the data supplied in the G&O Report. It was at the request of Boulder ' County and the City of Boulder that the G&O report was published as the regulatory ' floodplain study for this stream. ' The flood hydrology of South Boulder Creek utilized in the Love & Associates review and analyses was obtained from the regulatory model (G&O). According to the G&O ' FHAD, the hydrology of South Boulder Creek was developed by the U.S. Army Corps of Engineers (USACE). ' REVIEW OF THE GREENHORNE & O'MARA HYDRAULIC MODELS A hard copy of the G&O regulatory HEC-2 hydraulic models of South Boulder Creek in ' the study area was obtained from Boulder County and verified to be the same model used in the FEMA FIS. The G&O HEC-2 models, within the study reach, consists of a main ' channel model and a split flow model. In order to insure that the same model as used in the regulatory study is the model provided to us in the G&O study, these models were re- created and run on the PC version of HEC-2. The models and the floodplain delineation shown within the FHAD were reviewed for conformance with the re-created models. The Love & Associates resulting re-creation did in fact match the G&O and regulatory HEC-2 models as published. A copy of this re-created HEC-2 regulatory model is provided in the Technical Appendix of this report. During the review process, inconsistencies and errors were discovered in the G&O (regulatory) models. Flow separations modeled by G&O were found to be hydraulically connected within portions of the study area. Additional topographic mapping was ' -11-