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I I I I I I I I I I I I I , I I I I I and SWMM (Reference 7) runoff models. MITCAT was used to model the 400 square mile mountainous area upstream from Golden, and SWMM was u~ed to model the lower 175 square mile plains basin. Rainfall values used in the models were obtained from the Precipitation-Fre- quency Atlas of the Western United States, Volume III, Colorado, published by the National Oceanic and Atmospheric Administration in 197~ (Reference 8). Discharge-probability profiles for the 10-, 50-, 100-, and 500-year frequency storms are provided in Figure 3. Peak dis- charges for the lOa-year frequency storm at selected design points are listed in Table I. The 100-year flood elevations computed as part of this re- port are different from those published in the Adams County Flood Insurance Study (Reference 2) due to the revised discharges and more detailed topographic mapping. The elevations in this report also differ from those published in the Clear Creek F.P.I. (Refer- ence 3) for the same reasons. A detailed, technical appendix with all hydraulic calculations is available at the offices of the Urban Drainage and Flood Control District. - FUTURE FLOODS - Channel - 0.025 to 0.045 Overbank - 0.040 to 0.070 Flood Frequency and Discharge Floods of the same and greater magnitude as those that have occurred in the past will occur in the future. With development in the basin, flooding will occur more frequently. The discharges reported in Table I for the 100-year flood frequency represent the relative extent and impact of this flood event. The discharge information is usable not only for floodplain regulation but also for planning, engineering and floodplain management. The 100-year flood event, which can be expected to occur at any time in a given area, based upon recorded historical precipi- tation and other valid data, has a one percent chance of being equalled or exceeded during anyone year. The 100-year flood event is considered by the Urban Drainage and Flood Control District, the Colorado Water Conservation Board and the Federal Emergency ~~nagement Agency as the flood magnitude for which floodplains should be designated for regulatory and im- provement purposes. In Colorado, the 100-year floodplain repre- sents an area of state interest as defined in House Bill 1041. The la-year flood magnitude has also been computed at various design points along Clear Creek. This flood magnitude represents an event which can cause damage but would not inundate the area characteristic of a 100-year event. The peak flows for the 10-year event should be used in the planning and engineering of improve- ments where the higher risk of failure or damage is economically feasible and the hazard to life and property is lower or nonexistent. Hydraulic Analysis Hydraulic analyses were completed to determine the water-sur- face elevations for the 10- and 100-year storm events. The eleva- tions were computed using the Corps of Engineers' HEC-II Water Sur- face Profiles computer program (Reference 9). Valley cross sections for the length of Clear Creek in Adams County were measured on 1"=100' scale, 2-foot contour interval topographic mapping which was reduced and is included with this report. Valley cross sections for the Jefferson County portion of the study were compiled through photogrammetric methods by Bell Mapping Company, as the topographic maps included with this report were prepared. Estimates of channel and overbank roughness factors were made after a field investiga- tion of the study area. Typical roughness or Manning's "n" values used in this study are: The computed flood profiles are plotted on the Flood Hazard Area Delineation profile sheets (Sheets 22-29) included in the Drawings section of this report. Also, the computed flood eleva- tions for Clear Creek are tabulated at each reference point in Table I. -5-