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<br />Starting Water Surface Elevations <br /> <br />In Georgetown, starting water-surface elevations were established from the <br />elevation of Georgetown Lake. This elevation was derived from the overflow <br />rating curve for the spillway at the lake. <br /> <br />Starting elevations for Clear Creek in Idaho Springs were taken at critical depth <br />below the Interstate Highway 70 bridge, east of the city. <br /> <br />The starting water-surface elevations for Clear Creek in Silver Plume and the two <br />re-studied reaches were based on the slope-area method of HEC-2. Channel <br />slopes were obtained from topographic mapping. <br /> <br />Roughness Factors <br /> <br />Roughness coefficients (Manning's "n" values) were selected for the HEC-2 and <br />HEC-RAS computer programs. These computations were assigned on the basis <br />of field inspection of flood plain areas, comparison with streams of known <br />roughness (Reference 16), and from computations relating to the USGS stream <br />gage at Lawson. Roughness coefficients for the Silver Plume restudy were <br />determined according to field observations and swface photography. The <br />method given in the report "Determination for Roughness Coefficients for <br />Streams in Colorado" (Reference 9) was also used to help determine the values. <br />The channel and over bank "n" values for the streams studied by detailed <br />methods are shown in Table 4. <br /> <br />Table 4, "Manning's "n" Values" <br /> <br />Stream Name <br /> <br />Roughness Coefficients <br />Channel Over banks <br />0.013 - 0.150 0.013 - 0.200 <br />0.050 - 0.095 0.013 - 0.050 <br />0.065 - 0.095 * <br />0.060 0.013 - 0.055 <br />0.040 - 0.050 0.035 - 0.080 <br />0.020 - 0.040 0.013 - 0.040 <br /> <br />Clear Creek <br />Chicago Creek <br />Fall River <br />Soda Creek <br />South Clear Creek <br />Virginia Canyon <br />* Data not available <br /> <br />The hydraulic analyses for this study were based on unobstructed flow. The <br />flood elevations are thus considered valid only if hydraulic structures remain <br />unobstructed, operate properly, and do not fail. Blockage of flow from buildings <br />and other unnatural obstructions were simulated in the over bank calculations to <br />predict the impacts these features have on flood elevations. Ten percent <br />blockage of bridge openings due to debris was assumed. <br /> <br />3.3 Vertical Datum <br /> <br />All FIS reports and FIRMs are referenced to a specific vertical datum. The <br />vertical datum provides a starting point against which flood, ground, and <br />structure elevations can be referenced and compared. Until recently, the standard <br /> <br />10 <br />