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<br />I <br />I <br />I <br /> <br />The operating criteria described above explain what the reservoir stage will be prior to a <br /> <br />large 1100d event: 6,920 feet for a large May snowmelt and 6,924 feet for a large rain-on-snow or <br /> <br />general rainllood event. To evaluate the maximum stage at McPhee that could occur during a 1% <br /> <br />intlow hydrograph, three scenarios - snowmelt, rain-on-snow, and general faintlood - and <br /> <br />simulated reservoir response are discussed below. <br /> <br />10.1 Jlvpothetical Routings of Ilisloric Floods. In order to estimate the I % reservoir <br /> <br />stage at McPhee Reservoir, two historic hydrographs at Dolores, the snowmelt hydrograph in <br /> <br />1941 and the rain-on-snow hydrograph in 1949, were modeled using the IIEC-5 computer model <br /> <br />to simulate reservoir operation. The peak tlows at the Dolores gage for those events were 8,070 <br /> <br />cfs (in 1941) and 8,140 cis (in 1949). For infonnation on the lIEC-S program. see Reference 4-p. <br />From discussion with the dam operator. the reservoir routing used to estimate the t % reservoir <br /> <br />stage at McPhee Reservoir did not include diversions for irrigation or for other purposes because <br /> <br />it \....as felt thai. during an extreme event, dO\....nstream facilitics would be unable to handle <br /> <br />additional runofT. Additional inflow from other McPhee tributaries was not considered for Ihe <br /> <br />snowmelt season simulation because these tributaries arc allo\ver altitudes. The assumption was <br /> <br />made that the snowmelt from these lower tributaries \....ould have occurred prior to snowmelt from <br /> <br />the Dolores River. <br /> <br />Operational constraints for reservoir operation for snowmelt season are: (I) making a <br /> <br />maximum rate of change release of SO cfs per hour (for both increasing and decreasing releases), <br /> <br />(2) allowing a maximum release of 4,000 cfs, so as not to exceed downstream channel capacity, <br /> <br />and (3) avoiding releases over the spillway or the use of spillway gates, ifat all possible. For a <br /> <br />spring with high snowmelt forecasted, the spill/till criterion keeps the reservoir slagc al or below <br /> <br />6,920 feet until the end of May. This criterion was included in the HEC -5 model for the large <br /> <br />I <br /> <br />40 <br />