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<br />I <br /> <br />INTRODUCTION <br />A meteorologic and hydrologic study of the Boulder Creek waterHhed has <br />been performed. The objectives of the Htudy are to develop simplif:Led guid- <br />ance for mountain canyon flash flood warninq, and to assess the reliltive mag- <br />nitude of flood hazard from infrequent, severe storms that may occur in the <br />watershed. The study was performed by developinq a rainfall-runoff model of <br />the watershed and then applying both synthetic rainfalls and transposed his- <br />toric rainfalls to estimate the correspondinq flood magnitudes. Synthetic <br />rainfalls are based on the design rainfall criteria from the Urban Drainage <br />and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual. His- <br />toric rainfalls are based on the reconstitution and transposition of three <br />historic storms that have occurred alonq the front range. <br /> <br />The model development, model verification. and meteorologic analyses are <br /> <br /> <br />described in this report. The results are presented along with a graph of <br /> <br /> <br />peak discharge of Boulder Creek at the mouth of the canyon as a function of <br /> <br /> <br />maximum I-hour point rainfall in the watershed. The graph can be used to <br /> <br /> <br />estimate flood discharges that can be exp'ected from a forecast of rainfall. <br /> <br />MODEL DEVEWPI!lENT <br /> <br />The watershed was modeled using the HEC-l Flood Hydrology Program (U.S. <br />Army Corps of Engineers, 1988). The model development and selection of model <br />input is described in this section. <br /> <br />The watershed drainage area and subbasin delineation are shown in Figure <br />1. This map was provided by UDFCD. The subbasin areas and percent of <br />directly connected impervious area (RTIMP) are shown in Table 1. The areas <br />and RTIMP were obtained from file data that .'as obtained from UDFCD. An RTIMP <br />of 2 percent for all subbasins was also used in one model run. <br /> <br />The 10-year and 100-yeHr rainfalls were developed according to the proce- <br />dures in the UDFCD Urban Storm Drainage Criteria Manual. Rainfall depths were <br />determined for three elevat:on bands; the low elevation generally from 5,600 <br />ft to 7,500 ft (subbasins IO, 14 and 15). the middle elevation generally from <br />7.500 ft tp 8,500 ft (subbasins 2, 4, 6, 7, 8, 9, 12 and 13), and the upper <br />elevation generally above 8,500 ft (subbasins 3. 5 and II). The rainfall <br />depths were adjusted for arN reduction (based on NOAA Atlas 2 depth-area <br />