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<br />I <br />I <br />I <br /> <br />Regression analyses of the s tatis tical parameters versus various <br /> <br /> <br />physiographic parameters were made for both types of events and tested <br /> <br /> <br />for statistical significance. The only relationships found to be <br /> <br /> <br />significant were between the mean discharge and drainage area. Since <br /> <br />no valid regression equation could be found for the standard deviation <br /> <br /> <br />or the skew, the values determined for the Bear Creek gage at Morrison <br /> <br />were used for Turkey Creek, Deer Creek, and the other reaches of Bear <br /> <br />Creek. <br /> <br />routed to the downstream sites using a reach travel time method. The <br /> <br />hydrographs routed to a point were combined with the subarea runoff <br /> <br /> <br />hydrograph generated for this point, and the peak total discharge at <br /> <br />the desired location was determined. <br /> <br />I <br />I <br />I <br /> <br />The results of the hydrologic analyses are displayed graphically <br /> <br />on the 100-Year Discharge Profiles shown on Figures 2 through 4. The <br /> <br /> <br />details of the regional frequency analysis and the CUHP analysis can <br /> <br />be found in References 8 and 9, respectively. <br /> <br />I <br />I <br /> <br />With the mean discharge calculated from the regression equation <br /> <br /> <br />and regional values of standard deviation and skew, the discharges for <br /> <br /> <br />rainfall and for snowmelt events for several return periods at <br /> <br />selected sites were then calculated using the procedures from <br /> <br /> <br />Bulletin 1117 published by the Water Resources Council (Reference 6). <br /> <br /> <br />The two resultant frequency curves at each site were then statistical- <br /> <br /> <br />ly combined to produce a composite frequency curve which incorporated <br /> <br />both rainfall and snowmelt events. This curve was then adjusted based <br /> <br /> <br />on the ratio of the results of the log-Pearson Type III analysis for <br /> <br />Bear Creek at the Morrison gage to the results obtained at this site <br /> <br /> <br />using the regional frequency analysis approach. The 100-year return <br /> <br />period discharges at the selected sites in the study reaches were then <br /> <br /> <br />obtained from their respective frequency curves. <br /> <br />Hydraulic Analysis <br /> <br />I <br />I <br />I <br />I <br /> <br />To determine the discharges at selected sites on the study <br /> <br /> <br />reaches, the drainage area of the stream was divided into subareas and <br /> <br /> <br />runoff hydrographs were generated for each subarea at these points. <br /> <br />The generated hydrographs for upstream subareas were sequentially <br /> <br />Hydraulic analyses were conducted to determine the water surface <br /> <br /> <br />elevations for the 100-year frequency discharge. The elevations were <br /> <br /> <br />computed using the Corps of Engineers' HEC-2 Water Surface Profiles <br /> <br /> <br />computer program (Reference 10). The topographic maps were prepared <br /> <br />at the scale 1 inch equals 100 feet by Landmark Mapping Co. using <br /> <br /> <br />photogrammetric methods and most of the cross sections were digitized. <br /> <br />Dimensions and survey notes for the major structural crossings were <br /> <br /> <br />provided by UD&FCD and additional cross sections were measured on the <br /> <br />topographic maps to describe all the significant hydraulic structures. <br /> <br /> <br />Estimates of channel and overbank roughness coefficients were made <br /> <br />after field investigation of the study area. Typical roughness values <br /> <br /> <br />used for this study range from 0.03 to 0.06 for the channel and from <br /> <br /> <br />0.03 to 0.07 for the overbanks. Higher values were used in Morrison <br /> <br /> <br />and Idledale to reflect the density of development in the floodplain. <br /> <br /> <br />Critical depth has been adopted as a reasonable depth of flooding in <br /> <br /> <br />the channel for areas where, in a natural channel, supercritical flow <br /> <br /> <br />might occur. Most areas along the reaches studied belong to this <br /> <br /> <br />category and, consequently, roughness coefficients will have Ii ttle <br /> <br />effect on the flood boundaries. The 100-year flood boundaries, <br /> <br /> <br />typical cross sections, and flood profile are shown on Sheets 2 to 34. <br /> <br /> <br />Bridge sections are shown on Sheets 35 and 36. Computed 100-year <br /> <br /> <br />flood elevations at representative locations, are tabulated in Tables <br /> <br />1 through 7. <br /> <br />I <br />I <br />I <br /> <br />I <br /> <br />~olorado Urban Hydrograph Procedure Analysis <br /> <br /> <br />The flood discharges of 100-year return periods at selected sites <br /> <br /> <br />in the study reaches of Mt. Vernon Creek, Sawmill Gulch, and Parmalee <br /> <br />Gulch were calculated using the CUHP approach. A detailed description <br /> <br /> <br />of CUHP is given in Volume I of the "Urban Storm Drainage Criteria <br /> <br />Manual" (Reference 7). The design rainfall used in this analysis was <br /> <br /> <br />supplied by the Urban Drainage and Flood Control Dis trict. Values <br /> <br />selected for infiltration losses and depression and detention losses <br /> <br /> <br />were selected in conformance with the recommended values presented in <br /> <br />the Criteria Manual. <br /> <br />I <br />I <br />I <br /> <br />-6- <br />