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<br />Thc coefflcicnts ustCd iT' the ccmput:ltio;J of the unit
<br />I'.ydrograph, Ct und Cp, have been determined both from
<br />information published in the Urban Storm Drain:lge
<br />Cr i tar i.a ~;anual, anu more recent lnforma tlUll rec;:,i ved
<br />from the Corps of Engineers on studies of recorded
<br />runoff cvents in the Cherry Crcek basiI'>. Our analysis
<br />of thesc coefficicnts for each of tl:.e five basir:s gave
<br />un uverage value for Ct of .32 and .:Ill avcragu ,',,1\.:e
<br />of Cp of .49 for the total basin. Thesl' comp:Hcd to
<br />a Ct of .36 and Cp of .47 computed for the basin under
<br />the Project REUSE study. Thcse differences perh:lps
<br />can be attributcd to the increased developr.,ent in the
<br />basin and the existcnce of more storm sewers and paved
<br />streets to speed tho runoff process as lI'ell as perhaps
<br />a !'lore detailed investigation.
<br />
<br />c.
<br />
<br />nESIGN HAINFALL
<br />
<br />The design rainfall for th(1 study areas "..as taken from
<br />isohyetal maps in the Rainfall Section of the Urban
<br />Storm Drainage Criteria ~lanual. These maps prOVIde
<br />the raInfall depths tor the 2, 10, ~nd 100.year storm
<br />frequencies and I and 6 hour storm curations. The
<br />rainfall data is tabulated in Table Ill. Since the
<br />basin is relatively small, a storm rainfall ty:>ical to
<br />the basi.n "..as used rather than nssumin& a different
<br />dcsign r'linf;111 over (;:J.ch sub.bash:.
<br />
<br />TABLE rII
<br />
<br />Ill'S 1 r:~ RAT 'IF,,!,!.
<br />
<br />StOTD. Frequency
<br />2-year
<br />IO-year
<br />
<br />Basin
<br />1 hour
<br />(in.)
<br />
<br />1 - S
<br />{> hour
<br />Tw. )
<br />
<br />.95
<br />
<br />i.5S
<br />
<br />2.10
<br />
<br />3.30
<br />
<br />i!)'J ,.~.c. r'
<br />
<br />2.55
<br />
<br />, ,-
<br />... ~
<br />
<br />FroI:'. the storm rainfall input, the effl'ctive precipitation
<br />"as COITLputed following the procedure outlincd in rln' Hunoff
<br />Sectien of thel.;anu:il. The effectivc prccipi.::ation is ,l
<br />func:: iO~l (Of the del'ree of illlper'/ieus and PI' "vicu~ ;ne;!.
<br />surfaco tletontioIl, dej1"ossion storage, infi.1cf;1.tion. H"gC'
<br />t:nie:l ir.tercC'T'tlon an,! nol imperviOl's "re:l less pE'H::ent:'l<",
<br />
<br />.9-
<br />
<br />After evaluation of the ~;oils charac tcr is t j c of the
<br />Slaughterhouse Gulch basin, an infiltration rute of
<br />l/2 in ./hr. lias s electod ,IS buill?,: typical of the ba~1n.
<br />Other j.ydr.;;logi.cal factur~ a%',,"od tu he typical of
<br />the IHlsin and used in tr.l.' cc:nputation of the storm
<br />i:ydrcgraphs an.':
<br />
<br />Imporvi.ous depression/detention 5tOJ\'1-gc: 0.10"
<br />
<br />P . 1-'..' .'0" - .40"
<br />erviuus ~epTessiun ~e_en,iun S.orage:
<br />
<br />ImpuTvious lo~s; S~
<br />
<br />D.
<br />
<br />If'iDROCRAPlIS
<br />
<br />As ~entioned above, design storm hydro graphs ~ere
<br />computed at each of the d.,sign points fot the four storm
<br />frcqu.,ncics under consideration and for a projected,
<br />total devOllopment condition. Prior to calcul..ting
<br />these storm hydrogrilphs at each of the design points,
<br />storm hydrcgraphs were calculated for each of the sub-
<br />basins. >:ext, cach reach 1\'as evaluated by deter~.ining
<br />approximat0 flow times thrcHlgh the reach. This ,,;as
<br />done utilizln~: the l" ~ SO' scale IT.J.pping prepared
<br />sp~cifically for the project. The onc existing urea
<br />th"t h'(lS cOl'.~ideroc for p~)nding is the Po\,ers Park
<br />,ktcntion .1H~:J. The flood storage effect of this
<br />faciE ty 1,'as an,i lable frolf' a previous ~ tudy. Naluru 1
<br />l'onciinF als\! will\!ccur along the stnam tell<ling to
<br />attenUJ t~. the flo\!u hydruf(raph as it progresses down.
<br />stream. To COIrputQ th.c peak flow:; at the design poir.ts
<br />in n", b.1~in, it w.1~ r,rr."'~.~a"y tn cnmput,-, ttw storm
<br />bydrorc'.'lph for 0~(.h 0 r thf" h,,~ i,.,~ and then J af. the
<br />llydrogrilph fH,m the upper basin tJHO,lZh the' basin undur
<br />con~id('rilti<.::n ;II'\! add it to the storm hydrograph at the
<br />design puint. At Po'^,ers P:uk, the stOTt:' hydro graph 1,",IS
<br />routc:d through the pond and laggec dOliTIstrc,lm to the
<br />next d"5ign point. The hydrogra.ph i~. then :Idded to
<br />the storm hydrogr!.ph at tha t pu int. Reca~,se of the com-
<br />plexity of this procodur<: and due to the aUhlber of
<br />hydn)gral,Ls ',.;hic1: hav,:, to b<.l ce:lsiue~cd in ae analysis
<br />of this type, th0 tot31 process of computing the storm
<br />~,_,___..__", __n_,., ,..~"._, o_~ ~_Ll,.., ~~ ~'-~ .,~.~
<br />H)~' ....g, ""'" ,...... ~ d , ""0; " ad.... ",",u ,. .w '''~ "u~"
<br />~tn'.1m <1e<;ign roint li~<; )-open et't:1[lHtt'ri ct"; tt' opf'n'tp ()!1
<br />all !)IM ) 130 (;,lInpt:tcr 1,~iJlg \1i:>c :>ton1r.",
<br />
<br />A sumr:ury of the pCJk fLolld flm,s fer c>ac;h frequency "t
<br />the v"rjG~'s dc~igr. l'o'i,~ts i~ "revj,1cd ~n ~1:(' ni~d\;1.rf.('
<br />P:r{l:'~. b; 1 '-',' C\!rv~~;, F' <::J,r" 7., f C-,- bQt 11 t r." m" in.; t '-"'1""
<br />'.II'" Sr)tltl1 ;friiAltnrv of. S1 ~lI>dltprhol\~l' C1:1<.:",.
<br />. ..
<br />
<br />.1l1-
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