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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />hydrologist with a reasonable unit hydrograph shape for a given hydrologic <br />region and land development practices. <br />Equations 4-1 through 4-6 are used to define the peak discharge and its <br />location for the unit hydrograph. The widths of the unit hydrograph at 50 and <br />75 percent of the peak can be estimated using Figure 4-4. Note that the unit <br />hydrograph width at 50 and 75 percent of the peak are given in hours. The two <br />equations shown on Figure 4-4 mathematically describe the two lines on the <br />figure. <br />In addition to knowing the location of the unit hydrograph peak and its <br />width at two points on its ordinate, it also helps to know how to distribute <br />the two widths around the peak. A study of many unit hydrographs generated <br />using recorded rainfall and runoff events indicates that, as a general rule, <br />0.35 of the width at 50 percent of peak is to the left of the peak and 0.65 of <br />the width is to the right of the peak. At 75 percent of the peak, 0.45 of the <br />width is left of the peak and 0.55 of the width is to the right of the peak. <br />However, on some hydrographs this rule needs to be modified. Whenever the <br />above rule results in the hydrograph at 50 percent of peak being to the left <br />of the peak by more than 0.6 Tp (Tp = the distance from zero to the peak of <br />the unit hydrograph); the x-coordinate at 50 percent of peak should be placed <br />at 0.6 Tp and at 75 percent of the peak it should be placed at 0.424 Tp' <br />Figure 4-5 shows how a typical unit hydrograph may be shaped to best <br />approximate the trends found in the rainfall/runoff data. <br /> <br />4.11 Calculating a Hydrograph <br />Once the unit hydrograph has been determined (4.10) and the effective <br />precipitation from a design storm determined (2.4), the design storm <br />hydrograph can be calculated. The time units of the unit hydrograph should be <br />the same as the time units of the excess precipitation. <br />Set up a table such as Table 4-1, putting time intervals in the first <br />column and unit hydrograph ordinates in the second column. Place the design <br />effective precipitation values as determined in Column 11 of Table 2.5 across <br />the top, and then multiply the first excess precipitation value (0.04 in <br />example) times all the unit hydrograph ordinates in Column 2 and put answers <br />in the third column. Next multiply the second excess precipitation value <br />(0.09) times the unit hydrograph ordinates lagged one time unit as shown in <br />Column 4. <br /> <br />5-1-84 <br /> <br />. <br /> <br />. <br /> <br />. <br />