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<br />e <br /> <br />, <br /> <br />. <br /> <br />,.. <br /> <br />e <br /> <br />. <br />" <br /> <br />. <br />, <br /> <br />e <br /> <br />~O~B.'a .1,)8.17 <br /> <br />~....~..Ol. Ill....'.. <br /> <br />. <br /> <br />... <br /> <br />EM 1110-2.1416 <br />15 Oct 93 <br /> <br /> <br />", <br /> <br />". <br /> <br />:U'7. <br /> <br />J"" ".... If"'. APE HAlf .;rUN .fUJ. AU", SIl. OCT ..OV P:II" <br /> <br />LC~~.O.T LaD rL~r"OT. Wrv.. T~IL BLBV <br />----- ltaAIlDOII ao&o t..:rt tLLI.gr. ..v... root. JlLSV <br /> <br />Figure 5-11. Stage hydrogrephs for the Illinois Waterwey at Lockport Tallwater and <br />Brandon Road Pool <br /> <br />and then 10 define shifts (seasonal, systematic, and ran. <br />dom) from the rating curve. The "shifted" rating curve is <br />then used to routinely derive discharge from stage with <br />the discrete flow measurements being the only solid data. <br /> <br />(I) The USGS defmes an "excellent" gaging station <br />as having 95 percent of the daily discharges within <br />:I: 5 percent of the true value. The departure of the mea- <br />surement from the rating curve is composed of the error <br />in the measurement and the true shift. The shift is man- <br />ually determined by attempting 10 isolate the error. The <br />records at upland stations where the bed slope is large <br />are usually good. On the otber hand. the records on <br />large rivers, where the bed slope is small and the dynam- <br />ics are large, are suspect. <br /> <br />(2) The ability 10 adequately determine the rating <br />curve shift depends on the frequency of discharge <br />measurement Long term trends of aggradation and <br />degradation are adequately defmed by even an infrequent <br />measurement cycle. However, unless several measure- <br />ments are taken during a flood event, it is unlikely that <br />the loop or a seasonal shift will be adequately defined, <br />resulting in an error. When modeling a river system, if a <br />gaging station is used as an upstream boundary, the error <br />results in an inconsistency in calibration between events <br />which cannot be reconciled. On the Middle Mississippi <br />River a base flow error of :I: 5 percent results in a model <br />inconsistency of:l: I foot. If the lack of definition of the <br />loop is added 10 the base error, sizable inconsistency can . <br />be explained. Slope stations are gaging stations which <br /> <br />5-15 <br />