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<br />is a ratio of the peak 110w. A recession constant of 0.65 and a threshold Q ratio 01'0.2 (ratio of <br /> <br />peak flow) werc uscd to rcproduce Ihe recession limb of the observed 1970 1100d hydrograph at <br /> <br />Dolorcs. <br /> <br />6.6 Unit lIydrograph for Historic Flood Reproduction. The user-spccified S-graph method <br /> <br />was used to define the unit hydrogmphs for the subbasins. The HMS model uses an S-graph <br /> <br />imported to it by the model uscr, rather Ihan one of the mcthods included in the HMS package. to <br /> <br />define the unil hydrographs. The uscr-specilied S-graph was the Upper Dolores River S-graph <br /> <br />developed during the 1976 hydrology study from a unit hydrograph and basin physical <br />characteristics for the Dolores River at Dolores gage. This unit hydrograph was derived from the <br /> <br />flood reproductions of thc Septcmbcr 1970 flood cvenls al the Dolores gage. The plot of the S- <br /> <br />graph is shown on P1atc 7 and the related data are tabulaled in Appendix A. Table 3. Unit <br /> <br />hydrograph paramelcrs include the subbasin physical characterislics of: length of the longcst <br /> <br />watercourse (L); length along longest ",:ater coursc mcasured upstream to Ihc center of mass of <br /> <br />the drainage area (Lea); overall slope of the longest watercoursc betwecn hcadwater and <br /> <br />collection point (S); and a cocflicicnt (n), '....hich is a basin roughness coctlicient. Thesc <br /> <br />paramctcrs havc becn corrclalcd with "Lag time," the time required for natumlunit hydrographs <br /> <br />(S-curves) to reach 50% of volume. This correlation, derived from numcrous unit hydrographs <br /> <br />developed from flood events in Colorado. is dcfincd by thc follo,ving cmpirical equation: <br /> <br />lag time = 24 n (L . LC"JSO-5) 0.38 <br /> <br />(Sce Table 7 for dctinitions of Ihe above parameters) <br /> <br />Solution of this equation for various values of -0- is sho"..'n on Plate 8. Unit hydrograph <br /> <br />paramcters for the 10 subbasins wcrc 1istcd in an "Attribulcs ot'Walershed" table generated <br /> <br />19 <br />