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<br />EM 1110-2-1405
<br />31 Aug 59
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<br />28. SAMPLE COMPUTATION. a,. A simple form for compulalion of hypothctical h~'dl'O!rraphs h~'
<br />application of a "Ilit hydrogrnph to f"stinUltt"t) ruinCnll ('xc'psg val Ups is :-;ho\\'1I in platt. Xo, 14, In this
<br />rusc', it WN.S asslIOIl'd thnt th(' salll(" Ullil h.Hlrugnlph wuuld npp).'" to all unit ppriods of tilt" cI('si~n storm,
<br />Rainfall ex(O('s..,,; ,"aim's (eolumn ;~) 8,n" (,xPI"t'ss('d ill iu('hps tit-pili 0\'('1" tIlt' SaIUt' ar("ll- l"<,pl"ese-Ilt<.d by the
<br />unit hydrogra,ph, At I('Blioll is invited to the following:
<br />Time in hours is given in 6-hour in('reffil'nts, in order that sufficient points on the ('omputM
<br />hydrograph will he ohtained to avoid difficulty in skell'hin!( thc completed hydrograph. Starting
<br />point (zero time) is arbitrar}'.
<br />The unit hydrograph, which represents runoff in cJ.s. from a ]2-hour unit rainfall excess of
<br />1 inch over the entire drainage area, is tabulated in column 2, giving the illstantaol'Ous discharg('
<br />rates at the end of successive 6-hour increments of tim('-. The zero-time on th(' unit hydrograph
<br />has no fixed relation to the time scale in column ].
<br />The first 12-hour unit of rainfall excess (0.7 inch) was assumed to occur at a uniform rate
<br />during the interval beginning at hour ]2 and ending at hour 24. r nasmuch as the first unit h~'dro-
<br />graph ordinate shown represents runoff 6 hours after zero timp on thp unit hydrograph, flu. 0.7
<br />inch rainfall excess value is tabulated horizontally opposite Irom hour ]8 on thc time scale (column ])
<br />which is 6 hours after thc rainfall exccss began. With this arran!(emcnt, the first runoff value result-
<br />ing from thc 0.7 inch rainfall excess appears in column 4 hori7Amtall.,' opposite from hour 18, and
<br />is equal to 0.7 X800, or 560 c.f.s. Thc remainder of runoff from thc 0.7 inch rainfall excess is then
<br />determined by multiplying successive values of the unit hydro!(raph b.,' 0.7 and tabulatin!( in order
<br />in column 4.
<br />The second, third, and fourth ]2-hour values of rainlall excess are tabnlated in column 3 in
<br />proper time relation, heing separated successively hy ]2-hour intervals, Application of the unit
<br />hydrograph is carried out in thc same manner discussed above, ?\ote t.hat the values of rainfall
<br />excess are also shown in the headings of ('olumns 4 to 7. for ('onvpni('-ncp in identification of columns
<br />applicable to the respective values. Strict adheren,'e to this prof'C(lure will be found helpful in
<br />avoiding error in synchronizing runoff from slt{'cessive values of rainfall ('xc(-'ss.
<br />The combined runoff from rainfall excess values is a summation of tll(" values shown ill columns
<br />4 to 7, horizontally, and tabulating in column 8. To thc valucs lislRd in column 8 must he added
<br />the estimated base flow to obtain the total hypothetical hydro!(raph 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 cast's it is advisable to liSP two or more diffrrt'ut unit
<br />hydrographs in the computation of hypothetical hydrographs, in order to simulate the effC(,ts of differ-
<br />pnces in rainfall distribution and hydraulic conditions. III su('h cases, eaelt of the selected hydrographs
<br />would be tabulated in column 2 and identified by A, n, r, or similar means. The rainfall excess valncs
<br />to which each of the unit hydrographs applied would be identified hy a correspondin!( letter. Thc detailed
<br />computations would then be carried out as explained above, using appropriate unit hydrographs for
<br />the respeetive values of rainfall excess.
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<br />29. HYPOTHETICAL HYDROGRAPHS FOR DETERMINATION OF SPILL WAY REQUIREMENTS
<br />FOR DAMS. a. Ina.'"imuch as the critical volume and concentration of runoff into a reservoir ('an not
<br />be determined accurately, it is desirable to ('Btimate the extent of variations in maximum reservoir
<br />levels that would result from various changes in basic assun,ptions regardin!( runoff. The following
<br />series of computed hydro!(raphs, derived to represent runoff from the spillway-design stol'm rainlall
<br />excess under various conditions, is useful in the determination of spillway capacity and height of dam
<br />required to provide safely for the most critical flood runoff from a patricular basin.
<br />A Prori,,,ional Spillway De"ign Flood Hydrograph representing runoff from the area abore the dam-
<br />site under natural river condition.,;;, This Itydrograph servl'S as a basis for comps-ring the design-flood
<br />('riteria with maximum floods of r:('('ord in natural river basins, (See Hydrograph D, plate Xo. 16.)
<br />A Prorisional Spillway Design Flood Inflow Hydroyraph representing runoff into a full reser/vir.
<br />This hydro!(raph ma~' be developed in the manner outlined in the diseussion 01 Reservoir Inflow Unit
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