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I I I I I I I I I I I I I I I I I I I - AUTHORIZATION - This report was prepared under the current format of the District's Flood Hazard Area Delineation Reports. The scope consists of untilizing the COE's hydrology and hydraulics to prepare a report which includes: 1. Floodplain maps showing the boundaries of the 100- and 500-year fl oodp 1 a ins, 2. Flood profiles reflecting the water surface elevations for the 10-, 50-, 100- and 500-year flood events, 3. Discharge profiles showing the relative magnitudes of the four flood frequencies listed above, LOWER BOULDER CREEK FLOOD HAZARD AREA DELINEATION This report was authorized and contracted through the Urban Drainage and Flood Control District (UDFCD) as part of a program of identifying floodprone areas within the District's jurisdiction, The study was jointly funded by the UDFCD, Boulder County and the Colorado Water Conservation Board with the Omaha District Corps of Engineers providing technical services. This study identifies floodprone areas along Lower Boulder Creek within the unincorporated area of Boulder County from Valley View Road to the Boulder- Weld County Line. The floodplain information included in this report is provided by the Urban Drainage and Flood Control District' to enable the County to adopt and administer comprehensive floodplain management regulations. 4. Tabulations of pertinent floodplain and floodway data, and 5. Written text describing the flood history of Boulder Creek, various hazards associated with flooding and major drainage basin features. Upon acceptance of this report by the Urban Drainage and Flood Control District, 3nd the designation and approval of the flood hazard areas by the Colorado Water Conservation Board, this study can be used by Boulder County to regulate future floodplain construction, to guide future drainageway improvements and to plan flood protection measures along the channel and floodplain of Lower Boulder Creek. The report also summarizes the hydrologic and hydraulic procedures used by the COE including a discussion of floodways. All mapping and elevation data provided in this report are related to Mean Sea Level (MSL) Datum, also referred to as National Geodetic Vertical Datum (NGVD). - PURPOSE AND SCOPE - The floodway computed for this report is based on criteria of Boulder County and the Federal Emergency Management Agency (FE~~). FEMA is the agency that administers the National Flood Insurance Program and directs the flood insurance studies prepared under that program. FEMA guidelines allow a maximum increase in the IOO-year water surface of one (1.0) foot assuming equal encroachment from both sides of the floodplain. The floodway data is presented in Table IV of this report. This report was prepared to guide county officials in planning the use and regulation of the 100-year floodplain. The report includes information on past floods and the nature and extent of probable future floods along a 13-mile stUdy reach of Lower Boulder Creek. The report summarizes the hydrologic and hydraulic analyses completed by the Omaha District, Corps of Engineers (COE). The floodway is provided to designate those areas to be kept free from future encroachment and reserve an area for the conveyance of the IOO-year flood. Floodway criteria is designed to limit increases in flood stage which will result from future floodplain development. 1