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'I. Name of study to be formally acted on: "Floodplain Information Report, City of Blackhawk, Colorado" dated September 2004. Studied Streams: North Clear Creek, Gregory Gulch, Chase Gulch, and Fourmile Creek Technical Issues: The update of the hydrologic analyses for North Clear Creek, Gregory Gulch, and Chase Gulch and Chase Gulch utilized the SCS computer program TR20. Flood peak discharges for the 10-, 50-, 100-, and 500-year events were produced. Flood peak estimates for North Clear Creek downstream of the City limits were computed using the area ratio equation by McCain, et al. (TM-l). The 100-year flood peaks for Fourmile Gulch were computed using the SCS methodology and the HEC-1 computer model at various design points and were previously approved by Mr. Lang of the CWCB staff. Updated 100-year floodplain delineations in the City for North Clear Creek, Chase Gulch and Gregory Gulch were accomplished through the use of the HEC-2 computer hydraulic model and were presented in a recent submittal to FEMA for a Physical Map Revision. The lower portion of Fourmile Creek was analyzed in detail using the HEC-2 computer model, however the 100-year flood is wholly contained in the storm sewer and thus there is no floodplain delineation for lower Fourmile Gulch. The upper portion of the Gulch was studied using approximate methods for the 100-year event only. Updated and current floodplain delineations for all studied streams are presented in the subject report. Community response: A letter from the CWCB was transmitted to the City of Blackhawk regarding the CWCB's proposed action for designation and approval of floodplain information for the City. The City was receptive to the CWCB's proposed action and sent an official letter requesting that the CWCB designate and approve the floodplain information now available to the community. The community was also contacted via telephone during the study process to obtain data and other general information pertinent to the study. Staff fmdings: The subject Floodplain Information Report for the City of Blackhawk contains the most current floodplain information (approximate and detailed 100-year floodplain delineations) for the City. CWCB staff has determined that the subject floodplain information for Blackhawk is in conformance with the CWCB's rules and regulations for floodplain designation and approval. Staff recommendation: Staff recommends that the Board: 1) designate and approve the 100-year detailed and approximate floodplain information contained in said report for the City of Blackhawk, and 2) authorize staff to prepare a floodplain resolution to be signed by the Director and transmitted to the City of Blackhawk, Gilpin County, and FEMA. This action is recommended in order to meet statutory requirements. . . 4) Consent Al!enda Item 2.d Floodplain Resolution Number: FPR 04-577 Affected Community: Town of Manas sa (Conejos County) Name of study to be formally acted on: "Floodplain Information Report, Town of Manas sa, Colorado" dated September 2004. Studied Streams: Conejos River (and North Branch Conejos River) Technical Issues: The Conejos River has a contributing drainage area of approximately 360 square miles at the hydrologic point of interest near Manassa. Flood peaks for the 10-, 50-, 100-, and 500-year evens were estimated using regional regression equations from the USGS for the Rio Grande basin. The Conejos River splits into two branches upstream of Manassa just west of highway 285. The northern most channel downstream of the split is known as the North Branch Conejos River. The 100- year flood flows will be carried in both branches of the river with more or less equal splits in the conveyance due to similar slopes and topography. The hydraulic analyses indicate flood depths of less . Flood Protection. Water Project Planning and Financing. Stream and Lake Protection Water Supply Protection. Conservation Planning 4