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<br />- 17 - <br /> <br />-18 - <br /> <br />FuTURE FLOODS <br /> <br />As mentioned previously, storm-producing events in the Blue <br />River basin can come from either a snowmelt condition or a <br />cloudburst or thunderstorm in the basin. Both conditions <br />were investigated and the type of activity producing the <br />highest runoff for the recurrence interval being investi- <br />gated was chosen as the design runoff for that interval. <br /> <br />The peak discharge from the snowmelt condition occurs <br />primarily in mid-June and was determined by preparing flow <br />frequency curves from past streamflow records. Considera- <br />tion was given to climatic conditions as they relate to <br />snowmelt and to the runoff characteristics of the basin. <br />Flooding derived from cloudbursts or thunderstorms occurs <br />primarily after the middle of June and usually during July <br />or August. Due to the lack of available precipitation data <br />in the basin, it was not possible to evaluate major thunder- <br />storm events that had occurred ~n the past. Because of the <br />lack of data, synthetic storms were derived from statistical <br />analysis of rainfall data presented in Technical Paper 40 <br />of the U.S. Department of Commerce, Weather Service. It was <br />found that the runoff generated from the snowmelt gave <br /> <br />Floods of the same magnitude as those that have occurred <br />in the past can recur in the future, and even larger floods <br />are possible. The discussion of future floods in this report <br />is limited to those designated as the Intermediate Regional <br />and Standard Project Floods. The Standard Project Flood <br />would be larger and occur less frequently than thc Intermed- <br />iate Regional Flood. ~he Standard Project Flood is a rare <br />event, but could reasonably be expected to occur in the <br />future. In addition to determining the Intermediate Regional <br />Flood and the Standard Project Flood, the 10 and 25-year <br />recurrence floods have also been identified at various des- <br />ign points along the Blue River into Dillon Reservoir. <br />Floods larger than the Standard Project Flood are also poss- <br />ible, but the probability of their occurrence becomes <br />increasingly remote. <br />Determination of the runoff for these floods was based <br />on hydrologic computation, which included analysis of avail- <br />able records of past floods and consideration of pertinent <br />meteorologic and physiographic conditions. Snowmelt floods <br />on the river and cloudburst floods in the tributary bilsins <br />create the more severe flood conditions in the study area. <br /> <br />higher peak discharges fer the :rcqucnt runoff events, such <br /> <br />FREQUENCY OF FLOODING <br /> <br />as the lO-year, whereas the runoff from a cloudburst type <br />storm gave higher peak discharges for the Intermediate <br />Regional and Standard Project Floods. Peak flows for the <br />four recurrence interval floods are shown in Table 3 for <br />selected locations along the Blue River: <br /> <br />The Intermp(liate Regional Flood is ont> that could occnr <br />about once in 100 years on the il.verage, il.lthough it hil.s il. <br />OUt! p<Hr.;"nl t'rul>duiliLy of L...ing ""<.juall""cl ur exc""",L1",cl clu~iny <br />anyone year. 'rhe runoff from a Standard Project Flood is <br />that generated by a SeVere combination of meteorological <br />conditions r"il.sonably characteristic of thOi' l'1ydrologic <br />region excluding extrr~fnely rare combinati.ons. It is diffi- <br />cult to assign a frequency to the recurrence of a storrr of <br />such magnitude but for purposes of this study Cil.n be <br />regarded as having recurrence of about once in 500 years. <br /> <br />(\) <br /> <br />l!oo"",R~eCOf1Su'tn1Qii':~;nQ'..1"S. oe <br /> <br />(\) <br /> <br />le1'l,~R;c!Gonsu:tI~W,:er['li~W! In.: <br />