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<br />r <br />I <br />I <br />I <br /> <br />, <br /> <br /> <br />I <br /> <br />, <br /> <br /> <br />I <br />f <br />I <br />, <br />I <br />i <br /> <br />l' - <br /> <br />I: <br /> <br />1 <br /> <br />I <br /> <br />~ <br />I <br />n <br />I'~ <br />I- <br />I <br />Iw <br />1-- <br />~,;; <br />r'~ <br /> <br /> <br />,\'-' <br /> <br /> <br />I~ <br /> <br />The Flood Insurance Study for Fruita (3) lists the 10-year and <br />50-year discharges for only Little Salt Wash. To determine peak <br />discharges for Big Salt Wash for these frequencies, the 100-year <br />and 500-year discharges were plotted on probability paper and <br />values, for the 10-year and 50-year floods were extrapolated. <br />(See Figure 7.) <br /> <br />Results of the previous FIS and this new study are tabulated <br />below. <br /> <br />10-year <br /> <br />50-year 100-year 500-year <br /> <br />Big Salt Wash <br />Little Salt Wash <br /> <br />1,900 <br />1,500 <br /> <br />5,500 <br />3,170 <br /> <br />7,900 <br />4,300 <br /> <br />16,700 <br />8,100 <br /> <br />C. Horizon Drive Channel <br /> <br />The City of Grand Junction's report on Horizon Drive Channel (9) <br />estimates only the 100-year discharge. Discharges at different <br />points along the city's storm sewer system were estimated by <br />using the HEC-l model. Input to the City's model included <br />precipitation estimates from the NOAA Atlas (11), runoff <br />characteristics determined from SCS soil types, and routing <br />information including size and releases from the detention pond <br />and types and sizes of storm sewers. A base channel discharge of <br />60 cfs, which represents normal flow in the ditch for summer <br />irrigation, was added to the computed 100-year discharge to more <br />accurately reflect flood conditions. <br /> <br />To determine flood peaks for the 10-year and 50-year floods for <br />these on-going Flood Insurance Studies, the same HEC-l model was <br />used, with appropriate precipitation values obtained from the <br />NOAA Atlas. <br /> <br />The precipitation values used are tabulated below. <br /> <br />6-hour Precipitation <br /> <br />10-year <br />50-year <br />100-year <br /> <br />1.2 <br />1.6 <br />1.8 <br /> <br />The NOAA Atlas does not give precipitation values for the 500- <br />year storm. Therefore, a frequency-precipitation curve for the <br />stream was plotted using the 10-, 50-, and 100-year values, and a <br />curve extrapolated to SOO-years. (See Figure 8.) This <br />precipitation value was used in the HEC-l model to determine the <br />500-year flood value. <br /> <br />The development of flood peaks used in the Flood Insurance Study <br />for Grand Junction (5), is discussed in the Flood Hazard <br />Information Report for Grand Junction (2). Unit hydrographs were <br />derived by the Snyder method and a standard project storm was <br /> <br />-12- <br />