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FlOOD HAZARD AREA DELINEATION SCOPE OF WORK BOULDER AND ADJACENT COUNTY DRAINAGEWAYS The scope of \~ork for this study was prepared under the current format of the District's Flood Hazard Area Delineation Reports. AUTHORIZATION This report was authorized under an "Agreement" with the Urban Dra inage and Flood Control District, the City of Boulder, and the County of Boulder. The report is in partial fulfillment of Agreement Number 83 - 1.1 with the afore- mentioned local governments. This report addresses the objectives of the established work program for 1983 (Resolution No., 49, Series of 1982) which includes master planning and delineation of flood hazard areas. This study consists of a floodplain analysis for the IO-year and IOO-year events along 26.8 miles of streams within the City and County of Boulder. The streams under study are all tributary to Boulder Creek, with Fourmile Canyon Creek being the northernmost tributary and Bear Canyon Creek the southernmost tributary. Included in this report are Flood Hazard Area Maps which show the areas that would be flooded by a potential lOO-year event. Water surface profiles are included for both the IO-year and IOO-year flood events and show the elevation of flood flows related to Mean Sea Level (MSL) datum. The study identifies floodprone areas along eleven drainageways within the City and County of Boulder. The names of the drainageways with their respective lengths are shown in Table 1. The floodplain information presented herein for the drainageways of this report can be used to facilitate floodplain management programs and drainageway improvements within the study area. TABLE 1 Drafnageway HaES and Study Reach lengths Mileage 3.3 2.1 0.9 0.9 1.2 1.2 3.5 2.0 0.9 4.1 ~ 26.8 Drainageway Bear Canyon Creek Skunk Creek Bluebell Canyon Creek Ki ngs Gul ch Gregory Creek Sunshine Canyon Gulch Goose Creek Twomile Canyon Creek Elmer's Canyon Creek Wonderl and Creek Fourmile Canyon Creek A floodway was also computed to designate those areas that must be kept free of encroachment of convey the 100-year flood event along each of the drainageways. Floodways are defined as high hazard floOdplain areas that should be kept free from encroachment to pass large flood events without serious damage. In this case~ encroachment into the floodplain was considered until the elevation of the energy grade line increased by no more than one-hal f foot. The areas left free from encroachment are labeled as the' floodway. In addition, all areas where depths exceed 2 feet and/or velocities exceed 2 feet per second, are considered as floodway. The aforementioned parameters are City of Boulder Criteria. A floodway is determined for each of the eleven streams in this study wherever the 100-year floodplain warrants its use. The flood profiles and flooded areas presented are based on fully developed basin conditions considering present channel development. Floodplain information for seven streams (Goose Creek, Twomile Canyon Creek, Elmer's Twomile Creek, Wonderland Creek, Fourmile Canyon Creek, and the lower portions of Bear Canyon Creek and Skunk Creek) were taken from the "Flood Insurance Study for the City of Boulder and Vicinity" by the MSM/SP Group (now Greiner Engineerng Sciences, Inc. J. Flood- plain information for the remaining streams in thlS study were prepared by Greenhorne, & O'Mara, Inc., Denver, Colorado. 1