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<br />~ <br />ENGINEERING, INC <br /> <br />There are four equations published by the CWCB for the Colorado River basin. The most applicable <br />equation for the Grand Junction area is the equation for the Lower Gunnison River. As you know, the <br />equation for the Lower Colorado River is not available from CWCB at this time. The Lower Gunnison River <br />equation, QIOO = 180.8AO.578, is based on tributaries of the Gunnison River downstream of the City of <br />Gunnison, including Mesa County, using drainage areas between 2 and 8,000 square miles. Of the CWCB <br />equations, this equation results in the largest estimate of 100-year discharges for the range of area sizes <br />applicable to the Leach Creek/Horizon Drive drainage basin. In addition, this equation is an upper envelop of <br />the ICON developed and USGS published equations. Figures 4 and 5 display the four equations discussed <br />above and the results of our preliminary rainfall-runoff models for the Leach Creek/Horizon Drive drainage <br />basins. Clearly the lower rainfall results in discharges that compare more favorably with results of all the <br />regression equations. Appendix C contains supporting information for this analysis. <br /> <br />CONCLUSION <br /> <br />Figures I or 2 clearly indicate the NOAA Atlas 2 depth-duration-frequency values are significantly higher <br />than the rainfall data support. The Log-Gumbel curve for the 24-hour data is a very good fit. The results of <br />the rainfall-runoff modeling also indicate the NOAA Atlas 2 rainfall depths are high compared to the <br />published regional regression curves. <br /> <br />As stated above we have met with John Liou, to present this information to him. John indicated he was <br />comfortable with the analysis, but he did ask for a letter of concurrence from the CWCB. <br /> <br />We would appreciate your expeditious review of this information. If you have any questions please give me a <br />call. <br /> <br />Sincerely, <br />ICON Engineering, Inc. <br /> <br /> <br />Mike Vinson, PE. <br /> <br />cc: Mr. Trent Prall, P.E. <br />City of Grand Junction, CO <br />