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<br />MAR-28-2000 rUE 10:47 AM
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<br />Comparison of the Desig1LCNSTL ~!ue With
<br />Other Studies' ", . ': ,',.. ,',,',
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<br />, The design CNSTL .value found in this sludy is
<br />dose to CNSTL values obtained from hydrologically
<br />similar watersheds, Weiss and Ishii ,(1987) illve~ti!:at-
<br />ed 616 storms over 98 gaged rural basins ill. Illinois
<br />. , and computed a loss rate CNSTL' of about 0,07 inchl
<br />hour. Likewise, MelchiIlg et 0.1:' (1987) studied 32
<br />runolT llVents from one watershed in llIinoi~ over the
<br />period 1965 through:'1983'and computed a loss rnte of
<br />0,08 inchlhour. Apart frolp these recsnt studies in IlIi-
<br />nol., no othal' .y.hlm.tic il\vutilraUan9 of rainfall
<br />losses in the Midwest region could be ,found.
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<br />
<br />PMF ROUTING'
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<br />'rhe runolTrouling schemo in the Englewood water-
<br />shed used by the Miami'Conservaney District and
<br />adopled by the ODNR study was used here. It con-
<br />Aiat, nf combining tho innow~ from the four subbasins
<br />and then routing the resulting hydrogruph through a
<br />eoneeptutll reservoir upslream of th~ d'\m. The con-
<br />coptual reservoir represents the storas:e of wllter
<br />upstream of the daJll. In essenec. it represents tbe:
<br />effect of the EnRlowood daJll in retarding the: lluLlIow
<br />from the wa~rshed, In this section, the routing of tho
<br />PMi" was,con<lucted nt'six-hour intervals to anow
<br />compnrison with the ODNR study.
<br />Using the dosign parameters (STRTV=l,O inch;
<br />CNS'fL=O.l inch/hour,.where for convenienco 0,1
<br />ineh/hour was selected instead of 0,095 inch/hour) nod
<br />the PMP used i1\ tho ODNR study (Fi(:oro 7,,), the
<br />PM~' at the outlet,of the Enlllewood watcnhod is
<br />shown in Figuro 7b, Tho PMF obtained uy tho ODNlt
<br />study is also shown. The peak PMF obtained in this
<br />study was about 75 percent of lhe peak PMF outaine(1
<br />froul OONR's study,:However, the time-la-peak wns
<br />the salDe in both cases (t ,= 3,25 days). The apIlarent
<br />linear variatiOn of the hydrographs with time (t > 6
<br />days) was due to the linear relation between the out-
<br />t10w !lnd the storage in tho conceptual reservoir
<br />motlel, which WM decreasing very, slowly.
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<br />,Sensitillity of PMF ~~ 1-o~s,fG"Gmliters
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<br />'l'he effect of varietion in STRTL;', and/or CNST!, 011
<br />the PMF was nssessed by changing one parameter at
<br />l\ time by 60 percent' from the de~igll value while fix-
<br />ing the other parameter: ',,', : :'",'1'"
<br />Io'igure 8n shows that,the parameter STR'l'I. had
<br />o9sentially no effect' o;n the P:~1P:; the hydrograph
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<br />JAWRA
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<br />FAX NO.
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<br />IlDurnu..l
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<br />resullillg frolll S'I'RTL = 0.5 inch i~ ident.ical [') I.h~
<br />design hydroll'taph, Setting STitT!. at 1.r, io\che~
<br />caused II redueliol\ in lho pook PMF 1>)1 "" b' ~r.o .r.,
<br />which Is nbout 0,5 percent of t.he peak. 'l'his i" in con-
<br />trast to the effect of S'l'RTL on the outflow f""1ll the
<br />Brailford subbasin, which cnn be nsses_cd c....\11 the
<br />varhltion of Ule objective funcUon ill Table. 2 (nile cnn
<br />slipulate an approximalely linear relalion \,,'lw~en
<br />the objective functioll and lhe hydrogrnph l>o~nu"e of
<br />the square root in !!:quation 4), Table 2 show~ that r.,r
<br />a fixed CNSTL value. the varintion of the ohjf'ltive
<br />function was between one and three fl)l,l~ when
<br />S'l'RTL wa~ varied by OJ':; inch. A Jlhy"icnl n~)I1:11latinn
<br />for tho h,o,,,,.lllvl,y of I,he PMTo' to 8'1'1l1'L III 1,:i\"'1\ liS
<br />follows: the STRTI, pnramoter call be view.,,[ us "
<br />storago term {inches of r>\infnllloss Inultip\kd l>y the
<br />surface a.roa of the. wntl>rshedl. When routio!: Ihrotlilh
<br />the Englewood watcrshed, the res~.rvoir (It. 1,1", dam
<br />providetl a largo storall" volun1(' that masl(c<l I ho "lor-
<br />"ge effects of S'rRTL, However, lhis was notl.hc cnse
<br />for tho Ur!ltlford ~\lbbn.in, where th" .uhunoill .tor-
<br />age was t1iclated only by change,~ in 5TH,'!'! ,.
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<br />}o'lgu~ 7. liMP P,venL. (A} Tncrcn'lc-ntnJ Six-Ill:llll' lbill(,IlIl'l\hes
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<br />a.t the Outlet cCtho F,np;lcwoOt! WllUfSlu"l
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<br />210
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<br />JounNAl OF THE AMERICAN WATER n.SOUR~"". r," " C",',,,,
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