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<br />I
<br />,
<br />
<br />data. This data is ~re,ented on Fiuure II-J as rainfall isopluvia 1
<br />1 ines (rainfall "contour 1 ines"). represcnt;i n~ total rainfall depth
<br />froma24hourlony, lOO-year recurrence interval stornl.
<br />Figures IV.l and '','-2 ~reseflt total rainfall depth expected fro,,,
<br />storms of various durations and frequencies. DatafrOlllFiYUf€I';-ZWdS
<br />used to <.levelop the des i gn ra; nfa 11 di s t ri but i on for the o~erd 11
<br />flocdplainstudy. The SCS method of deter,lining runoff was then
<br />applied,ils discussed above in "Metllods".
<br />Figure !V.J the time-intensi ty-frequency rel"t; on for .~oJltrose was
<br />preparedfrOJIIFigureIV_2, and sets forth rainfall intensities for
<br />storms of various durations and frequencies. This family of curves is
<br />used in the Rational Formula, with >lhich the urban drainage analys is
<br />was performed.
<br />The impact of snOlll1lelt was exanlilll;d (and is included) for tile
<br />overall floodplain analysis, Snowmelt ilnpacts peak flow rate, as it
<br />establishes the basestrear.lflow, to whiCh the peak flow is added.
<br />Numerous drainage basins (judged to be reasonably sirnilar to the study
<br />basins) for which snowrrlelt data is availablewereexar.lined (Refer ences
<br />8-17,21, 35). From this data, the IOO-year frequency snowmelt rela-
<br />tion of flow to drainage basin area was defined for the Montrose study
<br />area. This data, in turn, was used to define the snO\~l1elt frequency-
<br />flow relation. ThesnowlIIeltfrequency-flowcurvewasthenstatisti_
<br />cally cOlllbined with the corresponding curve based on rainfall. The
<br />combined flow values as tabulated in Tobl e IV-3 represent lhe i"lp~ct of
<br />botll rai nfa 11 and snoWlnelt (Refl'rell~e 45). The combination of Che
<br />snowrnelt data with the rainfall wasaccOlllplishedaftertheralnfall
<br />generated runoff nydrographs for the vilriou~ sub-basins were combined
<br />with each other (see "Flood Discharge" section, belo..).
<br />O. FlOOd Olscharge
<br />For the overdll floodplain analysis, the runoff hydrogrpahs
<br />(reldtion of stream flow ratc tc til:le) developed by the SCS proce dure
<br />for each of the individUdl subb~sins shown on Fi ~ure J 1-3 were combined
<br />us i n'J th~ Musk i ngum procedure. Th i s process was dCCO"'P 1 i ~hed for the
<br />various sub.basbs comprising each of the thre~ rldjor bagins (Cedar
<br />Cre",k, Montros~ Arroyo, dnd Dry Cedar Creek). The regult WdS COrrlposit~
<br />hydrograph, at each design point, for each design storm. Selected
<br />hydrographs are illustrated on Figures IV-4a. and b. Table IV-2 pre-
<br />sentshydrograph data for the various sub-basins.
<br />Ditch crossing5 in tile Hudy basins, with the exception of the
<br />SoulhCanal,wereall handled in the sar.le loldnner for the purposes of
<br />thehydrographroutingprocedure. The ditches were assllnled to be
<br />flowing full, such Lhat anytributdry runoff would flow acroS5 uniln-
<br />jiedec, re"iainingwithintheSJ"ledrdil1ageba5in. Theditch'sfuncti()rl
<br />aSdstorm.'unoffcunveyancefacility"as i'Jnored,dc')flservativ€pro-
<br />cedur'e.
<br />
<br />The South Canal has tne capability of conveying significant flow
<br />in exces, of its norrMl Operating r'~nse, and also Ms a ~cor'd of safe
<br />operation. Three sub-basins dr~in directly to the South Canal (:1.-3,
<br />~_Il, M-18). An analysis of th€ runoff from these sub-basins indicated
<br />that this flow is intercepted by the South Canal, andconvcycdoutof
<br />the study area. ~unoff of these sub-basi ns, therefore, was assumed not
<br />to combi ne ,lith dr~ i nage reachi ng the study strea,ns. (I f the South
<br />Ca~al were to breach. discharges in the Dry Cedar Creek or Montrose
<br />Arroyowater'course5could be approximatclylDOO cfs highertha nshowo
<br />onTablelV-3,dependingon"herethebreachoccurred.)
<br />Four other sub~bas ins (.'1-9, .~-lD, 14_17, M-19) are provided cross-
<br />lngs under tile South Cana1. The crossing lilllitations for the first
<br />three of these sub-basins results in ponding at the oro5sing during thc
<br />most intense r'unoff periods. A reservoir r'outing procedure was applied
<br />at these locations to account forthc resultant peak flowattenu ation
<br />and elongation of the runoff hydrogr'aph. Significant ponding does not
<br />occur for sub-basin M-19.
<br />
<br />The peak ~drograph discharge represents the flow of concern for
<br />the analysis. Th~ cval uat ion to this ~oint is based only on rai Ilfall.
<br />The im,nct of snol-o'l,elt on the peak flow rate was then dccountedfor, as
<br />di50ussedin the previous section, "PrecipicalionAnalysis". The
<br />i111pact of snowmelt on runoff is significant only for the lowerfre-
<br />quency storms (see Table IV-3). The de5ign flows on which the
<br />floodplain analysis is based includes both the i'npact of rainfall and
<br />snoWlllelt. TablelV-3presents the design flOOd dischdr'ges dt various
<br />points for the study 5tremns. FigureIV-5presentsthestrear:1flowlo
<br />fr'€quency of occurrence relation at tile three stred';lconfluenc es.
<br />Figurc l,-l presents asche;ndtic indication of the lOO-year fr'e quency
<br /><lpsi go flows for the thr'ee streoll'S. "hi Ie Figures IV~6a, b, and c pre-
<br />sent flow qlli"'t it_v profile_ fo.. il11 fO'Jr "Mign ~to>"o.I frequo>"ci..s for
<br />thethrcestudystrea'ns.
<br />E. Ur'banStorlllRunoff
<br />
<br />As discussed previously, rainfall rUl1offrates\lere<leterminedfor
<br />the urbanized area by means of the Rati-onal rorr.;ula (0'" CdxA).
<br />The urban study area WaS divided into ili'proximiltely 70 sllb~bdsins
<br />ranging in size from 2 to215acre5 (see Figllre 1'1.8). The coefficient
<br />(If ~l)nQff, "C", is based O~ fully developed l~~d use w;thin eac~ urb~n
<br />sLlb~ba5 in based on current zoni ng (References 7, 31) (see Hydrologic
<br />LandllseMap,FiyurelV_9). The "C"valuecorrespondstothea:llount of
<br />rdinfallwhich is expected to runoff (dsoppoged to eVaporating,
<br />infiltrating the soil, or lIeing trapped in puddles, etc.). nle "C"
<br />valucs for Vdrious land uses are shown on Table IV-4. Values of "CN"
<br />are also shown for cOlllparison. Thc"C"va1ues for the various sub-
<br />bilsinsaretabularized inTdbleIV-5, asaretheotherp<lrJrrleter5usco
<br />In the Rationill FOrlwla. Table IV-5 also pre5ents the runoff from the
<br />.. il r i 0 US IJ rba n SIJ b- ba sin s for bo l h the in i t i a 1 ( 5 -yea r l des i ';n st 0 r~"
<br />Jlld tile "..jur (lOll-yedr') StorlO.
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