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<br />000337 <br /> <br />Draft- Do Not Cite <br /> <br />13 <br /> <br />August 2003 <br /> <br />2.3.1.2 Pitlick et a1. Model <br /> <br />In the Pitlick et al. (1999) model, suspended sediment transport was estimated using ~. <br />. . al 1.' hi.c d eak di 1 ads b f h . ~(.&.Ijoc.. ,,-\. t:J,~_>;. <br />separate empmc re atIons ps ~or pre- an post-p se ment 0 ecause o. ysteresl!! 'fl'o"''''' ,.<.; .-' <br />. _ _ ~ r 0- - ~<;).. <br />effects observed in plots of suspended load versus discharge. Peak flows for the ~ \)o..,l <br /> <br />Pitlick et al. (1999) model correspond to maximum. flows that occur during spring runoff. <br /> <br />Hysteresis effects occur because sediment loads are different for the rising and falling limbs of <br /> <br />the spring flood hydrograph. For the Gunnison River, suspended loads are typically higher on the <br /> <br />rising limb of the flow hydrograph than on the falling limb because of systematic differences in <br /> <br />pre- and post-peak sediment concentrations caused by a lag between the peak in sediment <br /> <br />delivery and the peak in runoff from melting snow (pitlick et al. 1999). <br /> <br />The pre-peak sediment relationship for the time period 1966 to 1993 is given by Pitlick et <br />~- ---=- ~ . <br /> <br />al. (1999) as: <br /> <br />Qsp = 16(Q _40)1.4 <br /> <br />(3) <br /> <br />where: Qsp is the sediment load in metric tons. <br /> <br />~st-peak s~~ads were estimated with the following relationship (Pitlick et al. 1999): <br /> <br />.~ 'Qsp = 0.2Q2 (4) <br /> <br />Figure 13 illustrates the behavior of the pre- and post-peak sediment loads predicted with <br />Equations 3 and 4. For flows greater than about 53 m3/s (1,850 cfs), the pre-peak sediment load <br />exceeds that of the post-peak. For flows less than 40 m3/s (1,410 cfs), the pre-peak relationship <br />is undefined. Because of this constraint, sediment loads for this study for flows less than 40 m3 Is <br />(1,410 cfs) were calculated using the post-peak relationship. For discharges between 40 m3/s <br />(1,410 cfs) and approximately 52.5 m3/s (1,850 cfs), the post-peak sediment loads are greater <br />than those predicted with the pre-peak relationship. For these conditions, the post-peak values <br />were used to avoid a discontinuity in load at 40 m3/s (1,410 cfs). In this way, the post-peak <br />period was extended from the time of the peak flow in the hydrograph to a time at which the <br />