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<br />In re-analyzing the Greeley gaqe, the 1981 Corps study <br />followed the analytical procedures of WRC Bulletin #17 using <br />7 more years of data. The result was a slightly lower 1% <br />flood discharge. In a variation of the analysis, the 1904 <br />flood (not gaged) was treated as a historical event, The <br />distribution was adjusted according to guidelines set forth <br />in WRC Bulletin #17 and the 1% flood again calculated. The <br />1981 Corps study concluded that the results indicated <br />adoption of the previous 1% flood of 9,400 cfs (Appendix 4). <br /> <br />ROUTING PROCEDURE <br /> <br />Note that the 1981 Corps study adopted <br />study's routing procedure for the plains <br />basin. The remainder of this discussion <br />methodology. <br /> <br />the 1973 Corps <br />portion of the <br />summarizes this <br /> <br />The lOa-year flood crest was routed through the basin using <br />PuIs' S+Q/2 method, Inflow hydrographs were developed for <br />the intervening sub-basins and added to the routed crest at <br />appropriate points along the rlver to develop the 1% <br />discharge profile. <br /> <br />The inflow hydrographs were developed by dividing up the <br />high plains portion of the cache La Poudre R,iver basin into <br />sub-basins drained by various tributaries, An analysis by <br />the Corps of similar basins in the area for which hydrologic <br />parameters were available provided Snyder's constants for <br />the region. These were used to develop synthetic unit <br />hydrographs for the sub-basins. <br /> <br />lOa-year rainfall data was taken from the U.S. Weather <br />Bureau's Technical Publication #40 and adjusted for <br />infiltration losses. The resulting runoff values were <br />applied to the synthetic unit hydrographs and adjusted for <br />expected probability before adding to the routed river crest <br />at their respective inflow points where they drained into <br />the Cache La Poudre River. <br /> <br />The resulting 1% discharge profile (Appendix 3) was then <br />used to create discharge probability relationships at these <br />inflow points along the river. This was done by plotting the <br />1% discharge at each point on probability paper and using an <br />adjusted standard deviation to find the 2%, 4%, and the 10% <br />discharge for each inflow. The adjusted standard deviations <br />were calculated by prorating the standard deviations of <br />0.232 and 0.457 calculated at t:he upstream and downstream <br />gages. The proration was linear and based on channel <br />distance (Appendix 3). <br /> <br />Page ~i <br />