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I I I I I II I I I I I I I I I I I I I Loss rate information for the hydrologic model was based upon infiltrometer studies by the U.S. Forest Service in forest areas, mixed forest and grassland areas, and grassland areas for mountain terrain in the South Platte River basin over a period of years. The results of these studies showed average infiltration rates of about 50% of infiltrometer studies for plains area. Based upon these studies, an infiltration rate of I-inch per hour was selected for the Boulder Creek basin. were also supplemented with cross sections taken from this mapping as needed. All bridge cross sections were obtained from actual field measurements. The determination of water surface profiles and flood boundaries was especially complicated along portions of Boulder Creek because of the extensive modifi- cation of the floodplain due to gravel mining operations. These operations have resulted in a floodplain typified by dikes and ponds of irregular shape and size. Because the integrity of these diking systems is unknown when subjected to floodwaters, the most conservative analysis was utilized. This consisted of assuming the diking system was intact for computation of flood water surface elevations. The results of the hydrologic analysis are presented graphically in Figure 3 - Discharge Probability Profiles. Hydraulic Analysis Water surface elevations for the 10-, 50-, 100-, and 500-year floods were computed using the U.S. Army Corps of Engineers step-backwater computer program HEC-2 (Reference 3). The version used was the HEC-2 release dated November, 1976, updated August, 1977, error correction 01, 02, modifications 50, 51, 52, and 53. No high water marks were located from which to calibrate the hydraulic model on Boulder Creek. Stream gaging data collected at the U.S. Geological Survey stream gage on Boulder Creek upstream from the mouth were used to compare results from the hydraulic model. Manning's "n" value of 0.040 was used in the channel for Boulder Creek. Overbank "n" values range from 0.070 to 0.150. Adjustments were necessary in the hydraulic model for overbank rough- ness and geometry due to the extensive use of the floodplains for gravel mlnlng. Starting water surface elevations for Boulder Creek were established from stage-discharge relationships developed for St. Vrain Creek. Bridges in this study were analyzed in accordance with the U.S. Department of Transpor- tation/Federal Highway Administration's publication entitled, "Hydraulics of Bridge Waterways" (Reference 10). Cross section data for Boulder Creek was obtained photogrammetrically by digitizing sections marked on the aerial photography of the stream and flood- plain. These sections were taken from the same photography flown for the topographic mapping used in this study (Reference 1). The digitized sections 8