|
<br />EM 1110-2-1405
<br />31 Aug 59
<br />
<br />e
<br />
<br />major variations in rainfall distribution, pither b:'oT analysis of actual rainfall-runoff records, or b.v use
<br />of synthetic methods.
<br />c. The tE'rm 111lit-rainfall du.rat1on Trfpl's to the duration of runoff-producing rainfall, or rainfall
<br />excess, that results in a unit hydrograph. Tht' unit hydrograph r<'sulting from 8 6-houl' unit-rainfall
<br />duration IS referred to as a o-bour unit h.\"(lrograph. Thi' trrm W-!I, as uSl'd hl'f('in, is thrc lcng'th of limp
<br />from the midpoint of the unit rainfall duration to the peak of the unit hydrograph.
<br />d, 1'h(' uru't-rainfall duration sl'lE'cted for a unit hydrograph should not C'xceed the period during
<br />which the design storm rainfall is assumed to bp approximah'}y uniform in intpl1sity in various portions
<br />of the drainagr arra undpf study, Inasmuch as valley storagr tpnds to eliminate thr effects of minor
<br />variations in rainfall intensity, some\\;ha.t longer unit-rainfall durations than an' otherwise desirable
<br />are suitabk for basins having larg(' vaUfO)' storage capacities. .A 6-hour unit-rainfall duration is suitablC'
<br />and eonvf'nient for, most studies rdating to drainage arE'a.s.largC'r t.han approximatpl.v 100 square milf's.
<br />Only in approximate studies should unit-rainfall durations longC'r than 12 houl's be adopted, in~smuch
<br />as major changes in the areal distribution of rainfall may occur during longer intrrvals. For drainage
<br />areas of less than approximately 100 square miles, vnlues rqual to ahout one half of the lag appear to
<br />be satisfactory.
<br />e, All references made herein to unit hydrograph discharge rates, or ordinates, refer to instan-
<br />taneous disrharge values at the instant designated. In order to aeeurately define the shape of a specifir
<br />unit hydrograph, any convenient series of discharge ordinates ma,y be tabulat.ed. The interval of time-
<br />between the discharge ordinates t.abulated has no relation \",hatever to thf' unit-rainfall duration of tll('
<br />particular unit hydrograph. For examplf', diseharge ordinates separatrd by 6-hour intervals of t.ime
<br />were tabulated in mlumn 2 of plate 1\0. 6 for a, unit hydrograph resulting from a 12-hour unit-rainfall
<br />durat,ion. Discharge ordinates at 12-hour intf'rvals only, might have been usrd to define the same
<br />12-hour unit hydrograph. However, if a 12-hour interval had been used, diseharge values of the finn.!
<br />hypothetiral hydrogrnph (column 10, plate ",0. 16) would have been kllowlI ollly at 12-houl' intervalR,
<br />and it ,,,'ould have bf'en necessary to interpolq,i{' for ini-erm{'diatC' valurs. Considerable care would hav{'
<br />been required ill sketching the final hydro graph through known points in ord!'r to assur!' that the correet
<br />flood volume v,Tas represented. By using 6-hour intervals, a suffieif'nt numb{'r of points wC're ohtain['{l
<br />to permit an accurate plotting of the final hydrograph, although the ordinates at, ] 2-hour intervals wer!'
<br />not changed. It should be ohserved that references to a f2-houl' unit hydrogral'h infers one that results
<br />from a unit rainfall excess of 12 hours' duration, l'f'gardlrss of HI(' interval of time hetween diseharg-r
<br />ordinates used in tahulating the unit hydrograph values.
<br />.f. A unit hydro graph df'rived from actual rainfall-runoff rC'cords for a particular basin repr('senh;
<br />an integration of the many influences that affC'ct runoff undpf the prevailing conditions, Application
<br />of such a unit hydrograph to conditions differing frOID thf' original is a pl'O(,f'SS of (.xtrapolation. TIt{'
<br />validity of the pxtl'apolation should be ehcckrd hy rv('r,v m('ans llvailahk. A study of unit hydrographs
<br />derived for a larg'c numh{,f of basins in which a varipty of vallr,\~ storage' chnrllctC'ristics, basin configura-
<br />tions, topographical features, and IDC't,f'orological ('o!l(litions art' J"('prpsrntrd, off('rs a basis for {'stimating-
<br />the relative effects of predominating influcncf's.
<br />y. Appendix TV prrsents tll{' grnrJ'al appi'Oaeh and assoeiated forms for usp in dptf'l'mination of
<br />unit h!"drobraphs from obsf'rvpd streamflow rN'onls, Division and Distri(,t Engiflpf'rs arf' r('()lH'st('d tn
<br />us(> thcsf' forms in suomitting to HH' (1hid of Eng-iIl(-'('rs all rdiahlr data Oil unit hydrograpb dptp!'minu-
<br />tions that arc assembled ill their rrspp(,tiv(' offic{'s in ('oIllwdioll \-vith IH'pparatioll of proj{'d reports.
<br />h, ThI'f'e gf'nrral In('thods are ordinarily available for drvploping unit. hydr~raphs. J n ddailpd
<br />hydrologieal studi<'s, each of thr following pro('pdu!"ps may be' us('d to advantagp:
<br />B!- ,alla.l!~sis of rainfall-rulloff I'p{'ords for isolate'd 1111l1it storms."
<br />By analysis of rainfall-runoff r('cords for major stonns.
<br />B!~ computation of synthC'ti(' unit hydrograph~ from (1) diI'rd, analogy with basins of ~imilf\r
<br />charad{'risti('s and (2) illdirpf't ullalog,\" ,dth a luq.(p llumlwl' of oth{'f' basins through the'
<br />application of f'mpiri('al I'l'lationships.
<br />
<br />e
<br />
<br />8
<br />
<br />e
<br />
|