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<br />.; <br /> <br />:, <br /> <br /> <br />elevations for the St. Vrain Creek at the confluence of <br />the three streams. <br /> <br />The flooding in Loomiller Basin is in the form of sheet <br />runoff, in which velocities are low, depths less than <br />1. 0 foot. <br /> <br />The discharges for the 50-, 100-, and 500-year floods <br />along Qry Creek are not contained by the Dry Creek chan- <br />nel. Excess flows for these frequencies will be directed <br />north to the St. Vrain Creek along most of the study <br />reach. ' The water-surface profiles for this stream were <br />therefore computed by manual calculation methods using <br />control sections at culverts to reflect this condition. <br /> <br />Locations of selected cross-sections used in the hydraulic <br />computations in the detailed studies are shown on the <br />Flood Boundary Maps and Floodway Maps. Channel roughness <br />factorq (Manning's "n") for these computations were <br />assigned on the basis of field inspection of the flood <br />plain areas and engineering judgment. Bridge geometry <br />and elevation information was obtained from the Colorado <br />State Highway Department and the City of Longmont, when <br />available, and measured in the field. <br /> <br />Flood profiles (Exhibit 2) were drawn showing computed <br />water-~urface elevations to an accuracy of 0.5 foot for <br />floods iQf the selected recurrence intervals. <br /> <br />All elevations are measured from mean sea level datum, <br />and elevation reference marks used in the study are <br />shown an the maps. Detailed cross section data for the <br />St. Vr~in Creek was obtained from the Omaha office of <br />the u.S!. Army Corps of Engineers and supplemented with <br />additional cross sections taken from a 1 inch = 400 feet, <br />, <br />2.0 foo:t contour map also prepared by the U. S. Army Corps <br />of Engineers (Reference 2). Detailed cross sections for <br />Left Hand Creek, Dry Creek, and Spring Gulch were field <br />surveye:d in September of 1975. The cross sections were <br />located at close intervals above and below bridges and <br />culverts in order to accurately compute backwater effects <br />at thes~ structures. U.S. Geological Survey topographic <br />mapping' enlarged to a scale of 1:600~ with a contour <br />interval of 10 feet was used to supplement field survey <br />data (R~ference 17). <br /> <br />The hydraulic analysis completed in this study did not <br />account for probable debris blockage of bridges and <br />culvert~. The potential for blockage exists, and plan- <br />ning for development in or near the flood plain should <br />recogni~e the limitation of this data if structures be- <br />come inoperable and fail. <br /> <br />17 <br /> <br />I <br /> <br />............,~,_,I <br /> <br /> <br />L_~, <br />