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<br />Equation Cakulations <br />There were many stream gages that did not have any available information. For these <br />gages, it was necessary to use the "Guidelines for Determining 100- Y ear Flood Flows for <br />Approximate Floodplains in Colorado" as a tool for estimating the 100- Y ear flood <br />discharges for approximate floodplains where detailed engineering analysis are limited or <br />unavailable. The state of Colorado is divided into major drainage basins and each of <br />these are then divided into preliminary geographiclhydrologic subregions. These <br />subregions are assigned a regression equation to help estimate the 100-year flood flows. <br />The regression equations are to be used for unregulated natural streams. Some <br />subregions are areas of "No Regression" due to lack of information and data. The use of <br />the regression equations is restricted by drainage basin size in each of the subregions. <br />Therefore, there were many stream gages that were too small or too large and were <br />exempt from using the regression equation. Once the drainage area was determined, <br />these regression equation calculations are very straightforward. <br /> <br />HEC-FFA Prol!ram Calculations <br />One of the most effective methods for determining the 100 and 10-year flood flows was <br />using a computer program to determine the flood flow frequency analysis. In order to use <br />this program, the water year and stream flow information, which was compiled off of the <br />USGS website, was needed. This information, as well as an interpolation of the <br />generalized skew coefficient oflogarithms of annual maximum stream flow, was <br />produced in a formal table. The table was then used as an input for the program which <br />would produce an output with the desired information. It is evident with some of the <br />stream gages; the calculations are not very accurate with an unrealistic 100-year flood <br />flow. In other cases, there was an insufficient amount of data available in order to do the <br />calculation. <br /> <br />Site Visit Calculations <br />Site visit trips to Glenwood Springs and Aspen allowed first hand calculations to be made <br />by determining the area and velocity of the stream and using the continuity equation to <br />compute the flood flow of the river. The Roarihg Fork River at Basalt, Aspen, Difficult <br />Creek, and Glenwood Springs were all surveyed and estimated for a minimum and <br />maximum flood flow. These values were then compared with the other means of <br />computing the 100 and 10-year flood flows to determine its accuracy. <br />