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<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 SOO-year floods <br />alo~g Dry 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 />therefo~e cocputed by manual calculation methods using <br />control sections at culverts to reflect this condition. <br /> <br />'0 <br /> <br />Locations of selected cross-sections used in the hydraulic <br />co~putatio~s in the detailed studies are shown on the <br />Flood Boundary ~aps and Floodway Maps. Channel roughness <br />factors (r;,j,anning IS "n") for these computations were <br />assigned on the basis of field inspection of the flood <br />plai~ areas and engineering judgment. Bridge geometry <br />and elevation in=or~ation was obtained from the Colorado <br />State Highway Department and the City of Longmont, when <br />availa~le, and measured in the field. <br /> <br />Flood pro=iles were d~awn showing cOEputed water-surface <br />eleva~io~s ~o a~ acc~racy of 0.5 ~oot for floocs of the <br />selec~eG recurrence i~tervals. <br /> <br />A:l elevations are ~easured from mean sea level datlli~, <br />a~d elevation ~eference marks usee in the study are <br />shown on the ~aps. Detailed cross section data for the <br />St. Vrain Creek was obtained froffi the Omaha office of <br />t~e C.S. Arny Corps or Engineers and sapplemen~ed with <br />additional cross sections taken from a 1 inch = 400 feet, <br />2.0 foot cor-to~r ~a? also prepared by the U.S. Army Corps <br />of E~sineers (Reference 2). Detailed cross sections for <br />Left Hane Creek, Dry Creek, and Spring Gulch were field <br />s~rveyed in Septe~er of 1975. The cross sections were <br />loca~ee at close intervals above and below bridges and <br />culverts in order to accurately compute backwa~er effects <br />at t~ese structures. U.S. Geological Survey topographic <br />napping enlarged to a scale of 1:6000 with a contour <br />interval of 10 feet was used to supplement field survey <br />data (Reference 17). <br /> <br />~he hydraulic analysis completed in this study did not <br />accoun~ :or proba~le debris blockage of bridges and <br />c~lverts. ?he potential for blockage exists, and plan- <br />n~ng for developmen~ in or near the flood plain should <br />recognize t~e li~itation of this data if structures be- <br />co~e inoperable and =ail. <br /> <br />17 <br />