<br />va!iation of the !ainfall potential sh~n 10. the Atlas fo! the St. Vrain
<br />Creek basin "as simulated by SWMM through the introduction of seve!..l hyeto-
<br />graphs. The time variation of the 6~hour rainfall fo! ~ach hyetograph "as
<br />based On a study of hourly precipitation data rpcotded durcng major storm,
<br />in the South Platte River basin. The rainfall distribution is given en
<br />Table-3belo".
<br />
<br />Button Rock Dam "as then inserted in the calibrated SWMM model of St.
<br />
<br />Period
<br />(lIours)
<br />
<br />[ncremental Rainfall
<br />(Percent)
<br />
<br />VtainCteckattheLyonsgageandrainfall"asappliedtothe211.qu"rpmil",
<br />of mountain area. It ..as found that 6~hour rainfatl over South St. Vrain
<br />Creek and North St. Vrain Creek helo.. Button Rock Dam generated higher dis-
<br />Charges at Lyons than did rainhll Over tile entire mountAin area. Thoi' adopted
<br />frequency CUtV~ dt Lyon~ thercfore docs not reflect any contrib~tion to peak
<br />discharges from the 109 square miles above Button Rod.. The adopted frequency
<br />curve at the mouth ..as modeled by applying 6-hour rainfall to the 924 square
<br />miles of contributing basin arca. This c~rvc ~as developed for existing con-
<br />ditions and "ith Button Rock Dam in place.
<br />
<br />TABLE 3
<br />6-HOliRRAIJ>;FALl,DISTRIB{;TIQN
<br />
<br />c-,
<br />,-,
<br />H
<br />H
<br />H
<br />S~6
<br />
<br />,
<br />,
<br />"
<br />;0
<br />"
<br />,
<br />
<br />r..o difhrent 6-hour Storms "cre used 1n dcrermining the 5., 10-, 50~,
<br />100-, and 5DD~year discharge profiles. The profiles above Boulder Creek "~re
<br />producedbyccntetingastOtmovertheSt.VrainbasinabovelioulderCreek,
<br />excluding the arc a ahovp Button Rock Dam. frorn the mouth of Bould~r Creek to
<br />the mouth of St. Vrain Creek, the disehargf profiles "ere doi'veloped by apply-
<br />1ng rainfall to the entire contributing area of the basin.
<br />
<br />Detention storage value. of 0.15 inch mountab and 0.10 inch ploin,
<br />~ere .electcd by trial and error. Mountain infiltration rates "ere set at
<br />a un1forn: 'ate of 1 bcl> per hour. In ord"r to "alibr,,~e the St. VraiLl model
<br />to the P!an~vill~ gage, it is necessary '0 us~ a pl"ins "rea ;nflitratio!l
<br />rate of O.S inch per hour.
<br />
<br />The resulting pedk flo~s. has~d on th~ ahove described prnc~durc, ate
<br />pre,ented in Iable~4 for the existing basin ~~vetop~~nt conditions, ~nd T~blc-
<br />S for year 2000 Urbani~ation. Th~ diSCharge p.ofiles for existing cOllditioLls
<br />
<br />"to'"e E.>ulJer Cr~ek ..,'" .1.0,",'" ~" <"l\U,~-J.
<br />
<br />Th" hequency analy.is of the ~aged data at Lyons ..a~ usod in th.' cali-
<br />bration of the S>r.1Mmodel of Nonh and South St, Vrain Cre"';:s for oonditio",
<br />in the basin befo,e Button R=k Dam \oIa" c("lStrucred. An "verage a~~ual June
<br />b..,e flo.. of ~OO cubie feet per s~"ond ..as ~dded to obtain th.. :nodd,.d fre-
<br />quency cUrveS at ehe Lyons ga~e. The avera~e annual June base flow of 600
<br />cubic feet per second ..a. added to obtain the modeled discharges at the mouth.
<br />Af'e, calibration of the model for tll., basin. thp modeled tr~qu~ncy~dis-
<br />Charge curves ..ithout Button Rock Dam fit th,' freq"('m'y~di"ch"rg~ Curv~<
<br />uf ga~ed data very ,,~Il.
<br />
<br />C. Comparison Ot the Peak Flo~s for Different D!velopme~' Conditions
<br />
<br />Intonnation rdat.'d to th" peak ilo,," for the IOO~yen flood for tbe
<br />""iating and year 2000 boOS in dpvelop"",nt cnnditions is pre;ented in Tabl~"
<br />4 and 5. From this infonoation. "e C4n "e~ that the p",,'cntage incre.ase
<br />in p~dk flo..s resuHing from tl>e year 2000 developm"ltt conditio,," " minimal.
<br />Su"h incr~"s~" rangin~ from ",me to I.';,:>; ~'il1 l",v" a mini",al i:npact on the
<br />..ater surface elevation, r~sutting from the 100-yea' fr"gu~ncy flood. ,unh."
<br />~he proposed <.I'abag~ improvements should be' ..de qua". to provld,' tlo~ prot<,ct i,m
<br />needed Over the next 20 yean. In SOme c~",'s for tit,. 50~y~a' and 500-yeao iloods,
<br />
<br />~3i-
<br />
<br />-Jo~
<br />
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