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<br />Federal E=gency Management Agency - Letter of Map Revision <br />Two-Mile Canyon Creek - Boulder, Colorado <br />Page 2 <br /> <br />2.1 Communi~ Description <br /> <br />The City of Boulder is located in central Boulder County along the foothills of the <br />Front Range in nonhero Colorado. <br /> <br />The Two-Mile Canyon Creek basin origin.ates in the foothi1ls west of thl~ City of <br />Boulder. The terrain of the upstream sub..basins is steep and rocky and consists of <br />topsoils in the C and D hydrologic soils group as classified by the U.S. Soils <br />Conservation Service, (Reference 1). Vegetation wnsists of evergreen and <br />deciduous trees with occasional areas of natural grasses and weeds. <br /> <br />The channel reach is located within an UIhanized area consisting mostly of single <br />family residences with occasional open space and park: areas. Terrain consists of <br />relatively steep slopes ranging from 1.7% at the confluence to 5.5% at the upper <br />study limit. <br /> <br />A well defined channel exists at the upstream reaches with little or no definiti.on of the <br />channel downstream of Broadway. <br /> <br />2.2 Principal Flood Problems <br /> <br />Urbanization and lack of channel definition in the lowe:r study reach has created a <br />wide shallow flooding area with the majority of the flows being conveyed within the <br />streets. <br /> <br />Several drainage culverts exist at the street crossings within the study reach, but it has <br />been determined that they are considerably undersized for conveyance of a major <br />storm event. <br /> <br />Two-Mile Canyon Creek has frequently overtopped its banks with the worst flooding <br />occurring in September of 1933. Flooding has also occurred in 1909, 1941, 1942, <br />1949, and 1965. <br /> <br />2.3 Flood Protection Measures <br /> <br />Urbanization has encroached upon drainageways and reduced stream storage, thereby <br />increasing runoff peaks and volumes. Several undersized culverts are present within <br />the study area as previously stated. <br /> <br />A small detention/siltation pond is located at the upstream study limit, just upstream <br />of Linden Drive. This pond will control moderate flows, although it does not <br />attenuate the 100-year peak: flow. <br />