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<br />EM 1110-2,1405 <br />31 Aug 59 <br /> <br />28. SAMPLE COMPVTATION. a. A simple form for computation of hypothetical h~'drop:raphs h.,' <br />appli('atioJl of a unit hydro/!I'Hph to pstimatpd J'uillfull l'X('('SS \'u!lws is shown in platl' Xo. 14. In this <br /><'US(', it ""US nssllllH'il thut tIll' SHIlH' unit hydl'ograph ,yollld lIPpI,\' to all unit prriods of tlw (1l'sigll i'\tOl'lll. <br />Rainfall excpss vulut's (column :3) arC' ('xpn's5('d ill illcht's ((t'ptli UVPI" the ~aUI{' Ul'<.'a n'pl'C'sC'lltpd by the <br />unit hydrograph. Att('ntioll is invitt.'d to tll(' foHowing: <br />Time in hours is givell in 6-hour increml:'nts, in ordf'r that sufficient points on the computf'd <br />hydrograph will be obtained to avoid difficulty in sketehinp: the eompleted h~'drograph. Startinp: <br />point (zero time) is arhitrary. <br />The unit hydrographl which l'f'presents runoff in ('.f.s. from a 12-hour lIIlit rainfall excess of <br />1 inch over the entire drainage- area, is tabulated in column 2, giving till' instantaneous dischal'gC" <br />rates at the end of sucl'essive 6-hour increments of timp. The zero-time on th(' unit h~"drograph <br />has no fixed relation to the time scale in column 1. <br />The first 12-hour unit of rainfall excess (0.7 inch) was assumed to oceur at a uniform rate <br />during the interval beginning at hour 12 and ending at hour 24. rnasmuch as the first unit h~'dro' <br />graph ordinate shown represents runoff 6 hours after zpro t.ime on tlw unit h~~drograph, the 0.7 <br />inch rainfall excess value is tabulated horizontally opposite from hour 18 on the time scale (column I) <br />whieh is 6 hours after the rainfall excess began. 'Vitlt this arrangeml'nt, the first runoff value result- <br />ing from the 0.7 inc'h rainfall excpss appt~ars in column 4 horizontally oppositf' from hour 18, and <br />is eqnal to 0,7 X800, or 560 c.!.s. The rcmainder of runoff from tbe 0.7 inch rainfall excess is then <br />determined by multiplying successive values of the unit hydrograph by 0.7 and tahulating in order <br />in column 4. <br />The second, third, and fourth 12-hour mlues of rainfall excess are tahulated in column :3 in <br />proper time relation, being separated suc('essively by 12-hour intrl"vals. Application of the unit <br />hydrograph is carried out in the same manner diseuss{'d above. XotC' t,hat UJ(' vaIut's of rainfall <br />excess are also sho\\"n in thf' headings of {'olumns 4 to 7, for eonvpniellr{' in identification of columns <br />applicable to the respeetive values. Striet adherence to this procedure will be found helpful in <br />avoiding error in synchronizing runoff from sureessive values of rainfall f'x(,pss. <br />The combined runoff from rainfall ex('{'ss values is a summatioll of thr valurs shown in columns <br />4 to 7, horizontallv, and tabulating in column 8. To the values listed in eolumn 8 must be added <br />the estimated base 1I0w to obtain thl' total hypothetical h,nlrograph of runoff shown in column 10. <br />These values may be rounded off to three significant figures. <br />b. As referred to in paragraph 27, in certain ('uses it is advisable to use two or more differrnt unit <br />hydl'Ographs in the computation of h:vpotlwtical hydl'Ographs, in order to simulate the effects of differ, <br />('neE'S in rainfall distribution and hydraulic conditiOl15. III such rases, each of the selected hydrographs <br />would he tabulated in column 2 and identified by A, /1, e, or similar means. The rainfall excess values <br />to which each of the unit hydrographs applied would be identified by a corresponding letter. The detailed <br />computations would then be carried out as eXplaincd above, using appropriate unit hydrographs foJ' <br />t.hf> respective values of rainfall excess. <br /> <br />29. HYPOTHETICAL HYDROGRAPHS FOR DETERMINATION OF SPILLW AY REQUIREMENTS <br />FOR DAMS. a. Inasmuch as the critical volume and concentration of runoff into a reservoir cannot <br />be determined accurately, it is desirable to estimat.e the extent of variations in maximum res('['voir <br />levels that would result f['om various changes in basic assurr.ptions rrgu['ding runoff. The following <br />series of computed hydrographs, derived to reprl'sent runoff from the spillway-design storm rainfall <br />excess under various conditions, is useful in the determination of spillway capacity and height of dam <br />required to providl' safely for the most criticalllood runoff from a patricular hasin. <br />A Prorisional Spillway Design Flood Hydrograph representing runoff from the area abate the dam, <br />site under natural river conditions. This ltydrograph serves as a basis for comparing the design-flood <br />criteria with maximum 1I0ods of ~ecord in natural rivl'r basins. (SCl' Hydrograph D, plate Xo. 16.) <br />A Prorisional Spillway Design Flood Inflow Hydrograph representing ru.noff into a full resafoil'. <br />This hydl'Ograph may be devl'loped in the manner outlined in the discussion of Rl'servoir Inllow Unit <br /> <br />18 <br /> <br />e <br /> <br />. <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />" <br /> <br />e <br />