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<br />D. Hydraulics <br /> <br />Water surface elevations for the 10-, 50-, lob~, and 500-year floods were <br />computed using the U.s. Army Corps of Engineers' step-backwater computer <br />program HEC~2. The version used wss the HEC-2 release dated November 1976, <br />updated August 1977, error correction - 01, 02, modification - 50, 51, 52, <br />53. Manning's "n" values for the channel were computed using the formula <br />"n" a .222(S+0.001)0.33 where S is the channel slope. This formula was <br />extracted from a paper by V.V. Golubtstov in "Soviet Hydrology: Selected <br />,Papers", published in 1969 by the American Geophysical Union. Resultant <br />"n",'~alues were 0.075 and 0.080 for the channel of James Creek and Little <br />James Creek respectively and 0.080 for the overbanks of both streams. The <br />results obtained from the HEC-2 computer model were verified by comparing <br />them to ground photographs of the 1969 flood through Jamestown. <br /> <br />Water surface elevations for James Creek were started at normal'aepth. <br />Water surface elevations on Little James Creek were started at the flood <br />elevations on James Creek resulting from coincident discharges. Bridges <br />in this study were analyzed using a blockage criteria dependent upon bridge <br />. construction and water depth. Concrete and steel bridges were assumed <br />unobstructed until the upstream water surface elevation reached the bridge <br />"low steel" elevation at which time the bridge was assumed fUlly obstructed. <br />Wooden bridge decks were assumed destroyed due to debris. This type of <br />bridge was assumed unobstructed at all discharges with winRWalls and abut- <br />ments in place but the deck removed. Head losses at fully obstructed <br />bridges were determined by weir computations. Unobstructed bridge losses <br />were computed by using the normal bridge routine in HEC-2. Flood boundaries <br />for the 100- and 500-year floods were determined by plotting the respective <br />water surface elevations on 2' contour mapping provided by the Colorado <br />Water Conservation Board. Because of high velocities the IOo-year flooded <br />area will be designated as a floodway. <br /> <br />Floods on James Creek are <br />or general storm systems. <br />Mayor June by melting of <br /> <br />produced by intense rainfall during <br />This rainfall runoff is frequently <br />the mountain snowpack. <br /> <br />either isolated <br />augmented in <br /> <br /> <br />E. Flood History <br /> <br />"Damage from flooding along James Creek has occurred in 1894, 1913, 1955, 1965, <br />and 1969. The following ie a brief description of some of these events. <br /> <br />June 1894 - Heavy rains along with high spring runoff caused a flood -which <br />damaged most of the low lying areas of the town of Jameatown. Most of the <br />houses on the north side of Main Street were damaged or completely destroyed <br />as was much of the road through Jamestown. <br /> <br />August 1913 - Flooding damaged or destroyed moet of the houses along James <br />Creek. All wagon and footbridges were destroyed. The '~oad to Jamestown was <br />out for two weeks. <br /> <br />3 <br />