Laserfiche WebLink
<br />e <br /> <br />e, <br /> <br />el <br /> <br />is a ratio of the peak flow. A recession constant of 0.65 and a threshold Q ratio of 0.2 (ratio of <br /> <br /> <br />peak flow) were used to reproduce the recession limb ofthe observed 1970 flood hydro graph at <br /> <br /> <br />Dolores. <br /> <br />6.6 Unit Hydrograph for Historic Flood Reproduction. The user-specified S-graph method <br /> <br />was used to define the unit hydro graphs for the subbasins. The HMS model uses an S-graph <br /> <br />imported to it by the model user, rather than one of the methods included in the HMS package, to <br /> <br /> <br />define the unit hydro graphs. The user-specified S-graph was the Upper Dolores River S-graph <br /> <br /> <br />developed during the 1976 hydrology study from a unit hydro graph and basin physical <br /> <br /> <br />characteristics for the Dolores River at Dolores gage. This unit hydro graph was derived from the <br /> <br /> <br />flood reproductions of the September 1970 flood events at the Dolores gage. The plot ofthe S- <br /> <br /> <br />graph is shown on Plate 7 and the related data are tabulat~d in Appendix A, Table 3. Unit <br /> <br /> <br />hydro graph parameters include the subbasin physical characteristics of: length ofthe longest <br /> <br /> <br />watercourse (L); length along longest water course measured upstream to the center of mass of <br /> <br /> <br />the drainage ar~a (Lea); overall slope of the longest watercourse between headwater and <br /> <br /> <br />collection point (S); and a coefficient (n), which is a basin roughness coefficient. These <br /> <br /> <br />parameters have been correlated with "Lag time," the time required for natural unit hydro graphs <br /> <br /> <br />(S-curves) to reach 50% of volume. This correlation, derived from numerous unit hydro graphs <br /> <br /> <br />developed from flood events in Colorado, is defined by the following empirical equation: <br /> <br />Lag time = 24 n (L * Lea/So.5) 0.38 <br /> <br /> <br />(See Table 7 for definitions ofthe above parameters) <br /> <br /> <br />Solution of this equation for various values of Un" is shown on Plate 8. Unit hydro graph <br /> <br /> <br />parameters for the 10 subbasins were listed in an "Attributes of Watershed" table generated <br /> <br />19 <br />