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. APPENDIX B ROARING FORK RIVER AND FRYINGPAN RIVER HYDRAULIC CALCULATIONS The water surface elevations for the various floods discussed in this report were computed by means of the U.S. Corps of Engineers HEC-2 Water Surface Profile computer program (Reference 4). Sixty-four cross sections were digitized and input into the computer in order that water surface elevations could be calculated for the 10-, 50-, 100- and 500-year floods along the study reaches of the Roaring Fork and Fryingpan Rivers. The results of these computer runs are shown in Appendix C, which contains a listing of all com- puter runs performed for the Roaring Fork and Fryingpan Rivers. Section numbers on the computer printout correspond to the digitized cross sections shown on the flooded area maps and the flood profile plates. . Since the digitized cross sections indicated the water surface elevations rather than the true channel invert elevations, the cross sections had to be corrected. The cross sections were corrected in the following manner: A field survey was conducted to determine the depth of flow (D) at each cross section on June 13, 1977, The gage heights and corresponding rates of flow for the Roaring Fork and Fryingpan rivers on November 18, 1975 and June 13, 1977 were supplied by the U.S,G.S. office in Grand Junction, Colorado. The aerial photographs were taken on November 18, 1975, so that the gage heights for that date correspond to the relative water surface elevations on the topographic maps, which were used to digitize the cross sections. Knowing the depth of flow (D) on June 13, 1977, the corrected elevations for the channel inverts were determined by taking the difference (d) between gage heights on June 13, 1977 and November 18, 1975, and then subtracting this difference from the depth, 0, (diff = D-d). This change in depth (diff) represents the amount that the channel invert should be lowered at a given cross section. This change in depth (diff) was com- puted for every cross section, and thus, the corrected elevations for each channel invert were determined. .