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I I I I I I I I I I I I I I I I I I I Consequently. 1 ess rel i abl e means of "just ifyi ng" results were utilized. Two regression equations for use in Colorado Eastern Rocky Mountains are provided in a paper by John Liou of FEMA. (See Reference No. 10 in the Bibliography.) One is based on area and precipitation, which estimates 87 cfs per square mile for the 100- year event. Another is based on area and elevation, which estimates 112 cfs per square mile for the 100-year event. For the ent i re Gregory Gulch watershed up to the confl uence with North Clear Creek in Blackhawk, the Blackhawk Floodplain Study estimates the 100-year runoff is at 1240 cfs for 3.54 square miles, or 350 cfsjsquare mile. These values provide a large range of runoff per square mi 1 e, and cannot even be cons i dered as rel i abl e 1 imi ts within which a value should fall; nonetheless, they do provide a rough guideline to check results against. Incidently, this study results in a basin wide 100-year runoff of 282 cfs/square mile, despite a detention facility on Eureka Gulch which slightly reduces basin wide peak flows. The primary effort in trying to achieve realistic results was in selecting the best possible data based on field observations and past engineering experience. The better the original model, the less need for calibration changes. The second most relied upon "calibration", given the unfortunate lack of better data, was to run the HEC-l model for the 2-year storm event and compare culvert overtopping, gravel road and channel erosion potential, and deposition of sediment with frequencies known by City personnel. Lastly, engi neeri ng judgement was used based on channels i ze, uneroded mi 11 ta i 1 ings in channels, damaged or undamaged buil di ngs, photographs, and memories of 1 oca 1 res i dents. Although model changes were few based on this last category of "calibration", all changes resulted in higher peaks. V. HYDRAULICS A. General The flumes, which have been discussed as key components in Central City's drainage system, have not been thoroughly analyzed in previous studies. Capacities were investigated herein to determine subsurface stormwater quant i ti es. Surface flows that cannot be intercepted by inlets, culverts, or flumes, and conveyed by drainage systems must remain on the surface in hydraulic models. B. Criteria Culverts capacities were determined per guidelines presented in the Federal Highway Administration's HDS No.5. (See Reference No. 13 in the Bibliography.) Capacities for 4.0 foot diameter or less were reduced 20% to account for clogging, 5.0 foot culvert capacities were reduced by 10%, and culverts 8.0 feet in diameter were analyzed without clogging. Not only is clogging potential reduced because of the larger sizes, but the larger sizes are further downstream where major clogging potential will be somewhat reduced. Calculations are provided in Appendix "D". 7