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RTIMP - effective impervious area, in percent, for urban watersheds or <br />subbasins. <br />For the Clark unit hydrograph, continue with steps 7 through 10. For dimensionless unit <br />hydn~graphs or S-graphs proceed to Step 11. <br />7. Calculate Tc depending on the type of watershed: <br />Rocky Mountain, Great Plains or Colorado Plateau <br />T~ = 2.4 A.' L.zs L.zp S-.z <br />Agricultural <br />Urban <br />8. Calculate E2: <br />T~ = 7.2 A.~ L.zs L.ca S-.2 <br />T~ = 3.2 A ~ L25 L~a S- ~a RTIMP- s6 <br />R= 0.37 T~l.ll L.so A-.s~ <br />9. Enter the values of T~ and R in the UC record for the watershed or each subbasin. <br />10. Determine whether the time-area relation will be developed from an analysis of the <br />watershed or whether a dimensionless synthetic time-area relation will be used. <br />a. If the time-area relation is to be determined by analytic means, proceed with <br />the analysis and input the incremental areas (or percentages of total area) in <br />the UA record. <br />b. If the dimensionless synthetic time-area relations are to be used (Figure 1 and <br />Table 2), <br />i. use the values for Curve A in the UA record if the watershed or subbasin <br />is urban or predominantly urban, <br />ii. use the values for Curve C in the UA record if the watershed or subbasin <br />is undeveloped Rocky Mountains or Great Plains or Colorado Plateau <br />possibly with some interspersed agricultural fields, and <br />iii. use Curve B for all other applications (Curve B is the HEC 1 default <br />relation and the UA record is not needed). <br />19 March 2007 26 <br />