Laserfiche WebLink
21.53 <br />14. Compute the hydregraph rates. Use equation 21.8 and the gc~gp <br />column of the selected hydrogrsph in table ?1.17. 4he computed <br />• rotes are ehovn in col~¢: 3 of table E1.12. <br />The hydrograph is completed with step 14. flow the hydrograph is Further <br />retabulated or plotted for routing through the spillway depends on the <br />routing method to be used. See chapter 17 for routing details. <br />The mass turn for the hydrog=aph can be obtained using the Qt~Q column <br />of the selected hydrograph in table 21.17. Ratios in that colunn are <br />multiplied by the Q of step 4 to give accumulated runoff in inches at the <br />time computed in step 13. For accumalated runoff in acre-feet or another <br />unit, convert Q to the desired unit before making the series of mcA tii,li- <br />cations. <br />In the following example the storm duration is increased because the time <br />of concentration is over six hours. Increasing the duration also reo•=ras <br />increaaing the rainfall amount but if the drainage area is over 10 square <br />miles the increase is partly offset by the decrease in areal rain_~a11. <br />EScample 21.6.--Const~tct a FfI for a class (c) structure with a drain- <br />age area of 23.0 square apes, time of concentration of 10.8 hours, <br />CN of 77, and location at latitude,, lon3itude_. <br />1. Determine the 6-hour design storm rainfall amount, P. For th'_s <br />• structure class the ~ rainfall amount is takec from ES-1020, sheet <br />5 of 5.~ For the given location the m<.p show that P = 25.5 inches. <br />2. Determine the areal rainfall. amount. Use the appropriate cur-:- <br />oa figure 21.2.a r".5-1003-a For th!s location the "Humid and suD- <br />humid climate" curve applies and the ad~us=went factor for the drain- <br />age area of 23.0 agnate miles is 0.93. The ad,]usted rainfall is <br />0.93(25•S) = 23.72 inches. <br />3. Make the duration ad~ustmert of rainfall amount. The duration is <br />made equa_1 to the time of concentration, in this case, 10.8 hours. <br />Enter figure 21.2.c (F.5-1003-c) with the duration of 10.8 boos and <br />find an adjustment factor of 1.18. The ad,Justed rainfall is <br />1.18(23.72) = 27.99 inches. It is rounded to 28.0 inches for the re- <br />mainder of this example. <br />4. Determine the runoff amount Q. Enter figure 10.1 with the rain- <br />fall from step 3 P = 2 .0 inches and at Cf7 = 77 find Q = 24.7 i~chea. <br />5. Determine the hydrog.^aph family. Enter figure 21.3 (ES-1011) with <br />Cti = 77 acd at P = 28.0 inches reed hydrograph family 1. <br />6. Determine the duration of excess rainfall, To. Enter table 2.14 <br />with CN 77 and find that P+, the rainfall prior to the excess rai:- <br />fall, is 0.60 inches. Enter table 21.15 with the ratio P+~? _ <br />0.6028.0 0.0214 and by interpolation read a time ratio o_* O.yjO. <br />Then To = (time ra+.io) x (storm duration) = 0.950(10.8) = 10.2c h;,urs. <br />• <br />