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<br />0 Fourmi 1 e Creek <br />0 Wonderland Creek <br />0 Twomil e Creek <br />0 Elmer's Twomile Creek <br />0 Goose Creek <br />0 Bea r Canyon Creek * <br />0 Skunk Creek ** <br /> <br />The CUHP model was used to develop hydrology for future basin conditions along <br />each drainageway. An investigation was completed by the MSM/SP Group to compare <br />existing vs full development hydrology. The results of this study showed that <br />the increase in discharges and associated water surface elevations for full <br />development was relatively small. Representatives of the City of Boulder, the <br />Urban Drainage and Flood Control District and the Federal Emergency Management <br />Agency reviewed the information and agreed that the use of future condition <br />hydrology was appropriate for the Flood Insurance Study as well as this study. <br /> <br />* Bear Canyon Creek from Boulder Creek to Yale Road. <br />** Skunk Creek from Bear Creek to King Avenue. <br /> <br />Frequency discharges were developed for each of the drainageways for the 2-, <br />5-, 10-, 50-, and 100-year storm events. The CUHP model was used to calculate <br />the 10-, 50-, and 100-year events for each stream included in the Flood Insurance <br />Study (Fourmile Creek, Wonderland Creek, Twomile Creek, Elmer's Twomile Creek, <br />Goose Creek, Bear Canyon Creek, and Skunk Creek). The 2-year and 5-year <br />discharges for these streams were determined by extrapolating the modelled <br />discharge information downward on a log probability graph. Frequency discharges <br />for the 2-, 5-,10-,50-, and 100-year events for the remaining streams (King's <br />Gulch, Bluebell Canyo~ Creek, Gregory Creek, and Sunshine Canyon Creek) were all <br />calculated using the CUHP method. <br /> <br />The floodplain information for the additional streams and stream reaches listed <br />below was completed as part of this study. <br /> <br />0 B 1 uebe 11 Canyon Creek <br />0 Ki ng' s Gu 1 ch <br />0 Gregory Cree k <br />0 Sunshine Canyon Creek <br />0 Bear Canyon Creek * <br />0 Skunk Creek ** <br /> <br />Hydrologic Analyses <br /> <br />Rainfall data was developed based on the revised guidelines dated September 15th, <br />1982, set forth in the Urban Storm Drainage Criteria Manual. Rainfall depths <br />presented in these guidelines were obtained from the Precipitation Frequency <br />Atlas of the Western United States (Reference 5). The total rainfall depths <br />for the flood events analyzed for each drainageway included in this study are <br />presented in Table 2. <br /> <br />* Bear Canyon Creek upstream of Yale Road. <br />** Skunk Creek upstream of King Avenue. <br /> <br />Historic stream flow measurements were not available for any of the streams <br />under study, and hence. hydrologic modell ing was used to develop frequency <br />discharge information. The Colorado Urban Hydrograph Procedure (CUHP), <br />(Reference 2,3), was used to determine the runoff hydrographs for each sub-basin. <br />Basins and sub-basins used in the analysis are shown on Figures 1 and 2. These <br />hydrographs were then lagged and routed through the basin using the HEC-I <br />computer model (Reference 4) to account for stream travel times and flow <br />attenuation due to channel or reservoir storage. <br /> <br />6 <br />