Laserfiche WebLink
• Because of the computational requirements of the unit hydrograph procedure, the SCS derived a <br />simplified tabular method as published in Technical Release 55 JR-55). <br />Storage-Indication + Translation Method <br />The basic storage-indication method accounts for only the storage effects of a reach. Other techniques <br />must be used to account for the kinematic effects of long reaches. HydroCAD provides the option of <br />adding a time lag or translation to the normal storage-indication routing. <br />Time Translation <br />The "Stor-Ind+Trans' routing method first performs astorage-indication routing, and then translates the <br />resulting hydrograph by the travel time. A close examination of the resulting hydrograph will reveal <br />that the peak discharge no longer corresponds to a point on the inflow curve, but is translated (delayed) <br />by the prescribed amount. <br />• <br />T _ L <br />` 0.6 <br />where L = Io a x (S + 10'7 <br />1900 x yo.s <br />and S - 1000 _ 10 <br />CN <br />where: <br />Tc=Time of Concentration [hours] <br />L=Lag time [hours] <br />I=Hydraulic length of the watershed [feet] <br />Y=Average land slope [percent] <br />S=Potential maximum retention [inches] <br />CN=Weighted Curve Number <br />Note the use of the average land slope, and not the slope of the hydraulic path. Although some care is <br />required to determine this value, the Curve Number method has the advantage of using a small number <br />• of fairly objective parameters. This provides more consistent results than some other approaches. <br />Whirlwind Mine 07 (rev. April 08) D-34 <br />