|
<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"). represcn'.;; 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 develop 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 tile urt>an drainage analys is
<br />was performed.
<br />The impact ofsnOlll1lelt was exanlilllod (and isinc1uded) 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 sncwrrlelt data is availablewereexar.lined (Refer ences
<br />8-17,21, 35). From this data, the 100-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 cOlllt>ined with the corresponding curve based on rainfall. The
<br />combined flow values as tabulated in T<lbl e IV-3 represent lhe i"lp~ct of
<br />botll rai nfa 11 and snoWlnelt (Refl'rell~e 45). The combination of ehe
<br />snowrnelt data with the rainfall was accolllplished after the rainfall
<br />generated runoff hydrographs for the vilrious sub-basins were combined
<br />with each other (see "Flood Discharge" section, belo..).
<br />O. FlOOd Olscharye
<br />For the overall flood~lain analysis, the runoff hydrogrpahs
<br />(relation of stream flow r<lte tc ti':\e) developed by the SCS proce dure
<br />for each of the individual subbasins shown on Fi ~ure J 1-3 were combined
<br />usi n'J th~ Musk ingum procedure. This process was acco"'Pl ished for the
<br />various sub.basbs comprising each of the three najor baSins (Cedar
<br />Cre",k, Montrose Arroyo, and Dry Ced<lr Creek). The 1"esu1t was composit~
<br />hydrographs at each design point, for each design storm. Selected
<br />hydroyra~hs are ill ustrated on Figures IV-4<l and b. Table IV-2 pre-
<br />sentshydrograph d<ltaforthe various sub-basins.
<br />Ditch crossings in tile study basins, with the exception of the
<br />SouthCanal,wereall handled in the sar.le l-l<tnner for the purposes of
<br />thehydrographroutingprocedure. The ditches were assllnled to be
<br />flowing full, such Lhat anytributdry runoff would flow dcross uniln-
<br />veded, re"iainingwithintheSd"ledrail1agebasin. Theditch'sfuncti()rl
<br />dS<lstorm.'unoffcunveyancefaci1itY"ds i'Jnored,ac,)nservativ€pro-
<br />cedur'e.
<br />
<br />The South Canal has the c<lpability of conveying significant flo"
<br />in excess of its norrr.dl operating ranse, and also Ms a ~cord of safe
<br />operation. Three sub-basins drain directly to the South Can<ll (:1.-3,
<br />~_Il, M-18). An analysis of the runoff from these sub-basins indicated
<br />that this flow is intercepted by the South Caf1aI, andconveycdoutof
<br />the study area. ~uncff of these sub-basi ns, therefore, was assumed not
<br />to combi ne ,lith dr~ i nage re<lchi n9 the study stre<llns. (I f the South
<br />Canal were to breach, disch<lryes in the Dry Cedar Creek or Montrose
<br />Arroyo watercourses could be approximately 1000 cfs highertha nshown
<br />onTablelV-3,dependingon"herethebreachoccurred.)
<br />Four other sub~b<ls ins (.'1-9, .~-lO, 11_17, M-19) are provided cross-
<br />lngs under the South Canal. The crossing limitations for the first
<br />three of these sub-b<lsins results in ponding at the crcssing during the
<br />most intense r'unoff periods. A reservoir routing procedure was applied
<br />at these locations to account for the resultant peak flowattenu <ltion
<br />and elongation of the runoff hydrograph. Significant ponding does not
<br />occur for sub-basin M-19.
<br />
<br />The peak hydrograph discharge represents the flow of concern for
<br />the analysis. The eval uat ion to this ~oint is b<lsed only on rai nfal1.
<br />The im,nct of snol-o'llelt on the peak flow rate was then accounted for, as
<br />discussed in the previous section, "PrecipieationA.nalysis". The
<br />i111pactofsnowmeltonruncff is signific<lnt only for the lowerfre-
<br />quency storms (see Table IV-3). The design flows on which the
<br />floodplain analysis is based includes both the ilnpact of rainfall and
<br />snoWlllelt. TablelV-3presents the design f100d discharges at various
<br />points for the study stredlns. FigureIV-5presentsthestre<lr:1flowto
<br />frequency of occurrence reldtion at tile three strea,;]confluenc es.
<br />Figure l,-7 presents asche,natic indication of the lOO-year fr equency
<br />dl'si gn flows for the three streoll'S, "hi Ie Figures IV~6a, b, and c pre-
<br />sent flow qlli"'t it_v profile_ for all fOIJr "Mign ~tQr", frequ,,"ci..s fOr
<br />thetl1reestudystreillnS.
<br />E. UrbanStormRunoff
<br />
<br />As discussed previously, rainfall rUlloffrates\lere<leterminedfor
<br />the urbanized area byme<lns of the Rati-on<ll Forr;;u]a (Q"'CxlxA).
<br />The urban study area was divided intCdi'proxim<ltely 70 sub.bdSi ns
<br />ranging in size from 2 to 215 acres (seeFigllrel'J.8). The coefficient
<br />(If rl)noff, "C", is bdsed on fully developed l~~d use w;thin eac~ urb~n
<br />sLlb.bJ5in based on currellt zoning (References 7,31) (see Hydrologic
<br />L.lndllseMap,FiyurelV_9). The "C"valuecorrespondstothea:1l0unt of
<br />rJinfallwhich is expected to runoff (as opposed tQevaporating,
<br />infiltrating the soil, or being trdpped in puddles, etc.). nle "C"
<br />values for variOus land uses dre shown on Table IV-4. Values of "CN"
<br />JreJlsosho"nforcOlllparison. The "C"vd1lJes for the various sub-
<br />basi ns are tabulari zed in Table IV-5, as are the oth~r p<lrJrrleters use<l
<br />I n the Rclt ion;]l FOrlwl a. Ta~le 1'1-5 also presents the runoff from the
<br />.. a r i "US IJ rba n SlJ b- b<ls ins for bo t h the in i t i a 1 ( 5 -yea r l des i ';n st 0 r~l,
<br />Jlld tile OIl<ljur (lOll-yedr) stor!O.
<br />
<br />-19-
<br />
<br />-20-
<br />
|