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<br />~ <br />ENGINEERING,INC <br /> <br />RUNOFF ANALYSIS <br /> <br />ICON is under contract to develop hydrology and hydraulics for the drainage basins of Leach Creek and <br />Horizon Drive Channel (also know as Ranchmans Ditch) in the City of Grand Junction. Horizon Drive <br />Channel is a left bank tributary of Leach Creek, with the confluence immediately north of the business loop <br />ofI-70 southwest ofthe Mesa Mall. The Walker Field Airport is located in these basins. <br /> <br />In addition to developing discharges for the hydraulics models, the hydrology models will be used to size <br />regional storm water detention ponds. The results of our study will be submitted to FEMA in the form of a <br />CLOMR to get comments on the impacts the storm water detention ponds have on the Special Flood Hazard <br />Areas in these basins. <br /> <br />In addition to the above analysis of rainfall data, we have compared the results of our preliminary rainfall- <br />runoff models using both the NOAA Atlas 2 24-hour rainfall and the 2-inch rainfall (determined by HMS in <br />1992 and ICON in 2001) to regional regression equatiotts. The regional regression equations include I) <br />equations developed by ICON Engineering, Inc. using stream gage data from gages with generally less than <br />100 square miles drainage area in Mesa, Delta, Montrose, and Garfield Counties, 2) an equation published by <br />the USGS for northwest Colorado, and 3) equations for the Colorado River basin published by the CWCB. <br /> <br />Rainfall-Runoff Model <br /> <br />The rainfall-runoff model has been developed using the U.S. Army Corp of Engineers HEC-I computer <br />program. The methodology includes use of a 24-hour storm; the NRCS Type-II rainfall distribution; an initial <br />rainfall abstraction using the NRCS equation based on a curve number; the Green and Ampt equation for <br />rainfall abstractions once the initial abstraction has been satisfied; the Colorado Unit Hydrograph Procedure; <br />and hydrograph routing based on both the kinematic wave and Modified PuIs. Appendix B contains <br />supporting information for this analysis. <br /> <br />The Leach Creek and Horizon Drive drainage basins are complex since there are several locations where split <br />flows occur, and portions of the runoffhydrographs may leave the basin(s). The analysis is incomplete atthis <br />time due to the desire to resolve the rainfall depth-duration-frequency issue prior to addressing the split flow <br />locations. However, the information included herewith is considered valid for the purpose of the rainfall <br />Issue. <br /> <br />Regression Analysis <br /> <br />ICON Engineering, Inc. performed an analysis of 28 USGS streamflow gages in the vicinity of Grand <br />Junction to develop regression equations, per recommendation by John Liou. The largest drainage area used <br />in this analysis is 102 square miles. Curves were developed based on a linear best fit as well as an <br />exponential best fit to the I OO-year computed values for the gage data. The discharge-frequency CUrves for <br />the gage data was developed using the USGS computer program PEAKFQ Version 4.0, which is based on <br />"Guidelines for Determining Flood Flow Frequency," 1981 (Bulletin 17-B). The linear equation is QIOO = <br />113.5+25.14A. The exponential equation is QIOO = 35.14AO.9214, where A is the area to the point of <br />interest in square miles. Figure 3 displays the results of this analysis. <br /> <br />The USGS published equation for the northwest region of Colorado, which includes the Gunnison and <br />Colorado Rivers, is Q 1 00 = 104.7 A 0.624 <br />