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<br />FUTURE FLOODS <br /> <br />Although flood producing storms of the same magnitude as those <br />that have occurred Tn the past could recur in the future, discussion <br />of future floods In this report is I imlted to those designated as the <br />Intermediate Regional and Standard Project Floods. The Standard Project <br />Flood would be larger and would occur less frequently than the Inter- <br />mediate Regional Flood. A Standard Project Flood would be a rare event, <br />but could reasonably be expected to occur in the future. Selection of <br />these floods was based on hydrologic computations, which Include analy- <br />sis of available records of past floods, and consideration of pertinent <br />meteorologic and physiographic conditions. As previously Indicated, <br />snowmelt floods on the river and cloudburst floods on the creeks would <br />create the more severe flood conditions in the study area. <br /> <br />During floods, debrTs collectIng on bridges could decrease their <br />flow carrying capacity and cause greater water depths (backwater effect) <br />upstream of those structures. Since the occurrence and amount of debris <br />are indeterminate factors, only the physical characteristics of the <br />structures were considered in preparino the maps and other Illustrations <br />showing the Intermediate Regional and Standard Project Floods. These <br />maps and I I lustrations reflect consideration of veoetation and si It <br />normally existing in the stream channel3 and show the backwater effect <br />of obstructive bridges, culverts, and the Pioneer Ditch diversion struc- <br />ture, but do not reflect increased water surface elevations that could <br />be caused by debrIs col lectin9 agaInst bridges, by further deposition <br />of si It in the stream channel under bridges, or by debris or silt plugs <br />in culverts. <br /> <br />19 <br />