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<br />Channel roughness factors (Manning's "nil) for these computations <br />were assigned on the basis of field inspection of flood plain <br />areas, comparision with streams of known roughness (Reference 12), <br />and from computations relating to the u.s. Geological Survey stream <br />gage at Lawson. The following is the range of values used for <br />Manning's "nll: <br /> <br />Flooding Source <br /> <br />Manning's <br />Channel <br /> <br />Hnll Values <br />Overbanks <br /> <br />Clear Creek <br />Chicago Creek <br />Virginia Canyon <br />Soda Creek <br /> <br />0.013 to 0.055 <br />0.050 <br />0.020 to 0.040 <br />0.060 <br /> <br />0.013 to 0.050 <br />0.013 to 0.050 <br />0.013 to 0.040 <br />0.013 to 0.055 <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). <br /> <br />Starting elevations for Clear Creek were taken at critical depth <br />below the Interstate Highway 70 bridge, east of Idaho Springs. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks used in the study <br />are shown on the maps. <br /> <br />Floodwater elevations in the streams studied are often unstable, <br />due to very high stream velocities. Another complicating factor is <br />the possibility of debris collection at bridges obstructing flows. <br />This is a concern because of the heavily forested area above Idaho <br />Springs. Although this possibility exists, especially at the <br />smaller bridges, any attempt to include it in the analysis would be <br />speculative. Therefore, the flood elevations as shown on the <br />profiles are valid only if hydraulic structures, in general, remain <br />unobstructed. <br /> <br />No attempt was made to quantify the amounts of sediment deposits <br />that are likely to occur at the base of Gilson Gulch and the Gulch <br />above 8th Avenue. The geologic and soils information and other <br />data required to estimate the rate and volume of displaced materials <br />washed from the mountain slopes to the valley floor are not available <br />and are beyond the scope of this study to develop. <br /> <br />Shallow flooding designations (Zone B) on Gilson Gulch and the <br />Gulch above 8th Avenue are derived from different sources. Gilson <br />Gulch deposits sediment material washed from the mountain sides on <br />the valley floor at Idaho Springs. As this material is deposited, <br />an alluvial fan is created and the flow depth and direction becomes <br />indeterminate. <br /> <br />8 <br />