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<br />MAR-28-2000 rUE 10:44 AM
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<br />FAX NO,
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<br />p, 03
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<br />t.:sti1l1l\tian or the lIF.Cl Los$ PanllnetcTS for l{outin~ the Probable Maximum Flood
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<br />ilLSI'Qction uf isohyoLal maps of 40 storm ovents
<br />...,,,,,,10<1 Lhal only four ot'tholn provided a rell~onaLly
<br />II iJ i 101'10 l.overnge over the F.ngh,wood watershed, due
<br />l" the large surface arC!!n of tho watershed, A sm,lll
<br />nnmher of storm l,vonL:; dOl':; IIol provide statistically
<br />i"l'liaLle estimates of tho rainlilUloss pnrn.mllter:;. By
<br />,,,,-,L..,,.t, 20 of the original 10 storms appeared to pro-
<br />,id,. II rcnsoll"bly uniform precipitation over n sub-
<br />1"lsin of Lhe water:;hed, Lhe Dradfonl sub-basin (area
<br />l\If, mi"), 1"'<:n\1sll tho topography and tho soil cover-
<br />;.l~~lJ wurc. CS:-icntially the SOIlU} throughout the Engle.
<br />",,,,,,I wntershed (o~den at ai" 1900), the Bradfortl
<br />,"illm.in was lIsed in this study to estimate the logs
<br />rat". liH' the entire Englewood watershed,
<br />'rho scluctedl!O sLorms occurrod ovcr tho Bradford
<br />",hh.IS;11 dUl,jng' the 2.1-yenr period from 1967 to 1990
<br />I:;,,,, '1'"bk 1). '1'he toLal "mOU11t of rainfall varicd
<br />ild,wl,cn 1.07 and 4,Hli inches, lIourly rainfall data
<br />I'l'(,'lIr..1cd nt the Grcl!nvillu Wntcr Plant W'~l"c ohtained
<br />f1'OUl tho National Ocen nic and Atmospheric "'Iminis-
<br />tral,ion (NOAA). Greenville is nea,' the center ofgnwi-
<br />ty lof the Braclford sllbhilsin. Rainfllll dlltn mcasul'l,,1
<br />at other locations within the Englewood watershed
<br />Wl.1'<1 l1ut considcrmllhm La their relatively low woi~ht
<br />Oil till! Bmdfortl suhlmsin, '\'ho GreQlwille hourly data
<br />"'et'll "xp""s",[ as two-hour rainfall pulse's by adding
<br />twu l'onsC!cutivo ono-hour pulses.
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<br />SIIWnll I(t'l;O lInla for Groenville Creck nellr Brnd-
<br />liml, l'xpressed as bl-hourly river stages, wero provid-
<br />(,(1 lly Lhe Miami Conservancy District, These dnta
<br />WorlJ cnnverted to sLre:unflows using II rating table
<br />""d n silllple ForLran code that interpolates the stage
<br />Iin""rly ll"Lwecn two othllr known v"luc.,
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<br />l:,<t;lIwt;"n ofll"".IloLl" Ullit llydrograph
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<br />Use of ~;qllatiol1 (1) requires volues of the unit
<br />hyd ,'ogrnph for the 13rndford subbasin, The District
<br />l'l"ovidcd It six-hour unit hytlrograph for the sulJhasin,
<br />'1'0 ensure It time interval consistent with the
<br />(:r,.enville }lrcdpiLntlon data, the .ix-hour hydl'o'
<br />l:ra)Jh ol'lllnnles were cOllvorled to two-hllur unit
<br />l,yd"u!:rnph ordinaLos using' the "forward-l1ilcllwarcl S-
<br />11",11\0<1" (TllUl<e, lD78), 'rho rosuHing twu-hour unit
<br />hY'!"'Jgrnph has a pC'll< vnluo of about 3700 cfslillch, a
<br />li".<;-tu'l,,'"k Ill' HI hOllr~, and II bllse time of64 hours.
<br />All J\ltl'rnati\'c method for the derivation of short
<br />,I"meiun L1nit hydrogmphs from \lOit hYllrographs of
<br />1uIII',"" ,111l"tlLiun is pruvidcd in the work by Bouflldcl
<br />( 1 ~J~IH).
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<br />TABLE 1, S,onns Selected ror Ralnfnll 1-0.. Slud~,
<br /> n..inr.U llunoCC ne(!(,.~silln
<br />Storm n.,'lh Du.rlltion ClJon:>fillU,
<br />N\lmb~r Slorm nate (inch) lhour) kld.)')
<br />1 May 1, 1967 1.40 26 20,4
<br />2 May 5, 1967 2,21 40 S2.7
<br />:l December la, 1967 1,07 21 25,2
<br />,) May 22, 10G8 2,96 31 3~.5
<br />r. M.y 25,1968 1.11 11 30,5
<br />6 January 17,1959 1.30 24 24,7
<br />7 Al1!:Uat 10, 196~ 1.75 24 SI,O
<br />II April 1, 1970 1,72 12 -1-1.9
<br />II Fehruary 22,1975 3,14 32 25.1
<br />HI March 2'/,1975 1,20 38 25,6
<br />11 June I, Ill.O :1.08 SG 88.0
<br />]2 API'il 30, 1983 1,28 22 21,5
<br />IS O'~l>cr 1, 1986 1,47 12 32.8
<br />l-l O,...oor 3, 1086 2,77 82 32,8
<br />15 JUM 30, 1!lS7 4,BG 32 27.4
<br />16 May 22, lIl89 1,98 20 30.5
<br />17 May 28,1080 2,G3 24 49,5
<br />18 S.plembor 1, ]080 2.13 10 30.4
<br />19 September l4, 1089 1,1-\ ]0 33,2
<br />20 ~'cbi'o.l"l' H, 1990 2.67 50 39,2
<br /> Av..raJot'G I:
<br /> 33,4 . I,G
<br /> dnYil
<br />Basef10w Separation
<br />
<br />As illustrated in Equation (1), it is neccssnry to
<br />deduct the baseflow from the total streamflow to sol\'e
<br />for the lo~s rate, llnseflow was liSsumed to reCllde at
<br />nn e:<ponential rate:
<br />
<br />B ~ Bo l<;xp ( -1;)
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<br />(2)
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<br />where Ho is th" streamflow at time t = 0 (beginning of
<br />basellow), llnd k is the recession consLant discllSsed
<br />earlier, With this model, baseflow can bo isolated with
<br />three simple steps (Barnes, 1939); (l) plot the logn-
<br />rithm of the lotnl runoff Q(t) against tima on a linear
<br />scale; iriClally, thCl rClcession curve will plot as a
<br />straight line; (2) extend this straight line back to the
<br />time of the inflection point (which is t = 0) on the
<br />recession limb of the hydrut:raph; and (3) COllnllet tloi8
<br />slrnight Une oxten.;'Jll to a point at U,o bOlllnul"lI Ill'
<br />the rising 11mb oCthe hydro graph,
<br />These steps Sejlarate the discharge hydl'ogrnph
<br />inl,o all upper and a lower region. The upper reg-ion 1"
<br />Lhe surface runoff; the lower region is the basoflow,
<br />An iIIusLration oflhi,~ t.I!chnique can be seen in Fi~\iro
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