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POTENTIAL fLOCOS celts prod~clng rainfall of Standard Project magnitude. The Standard Project Flood on Left-Hand Creak WQ~ Id have a dtscharge of 12,300 cubic feet per sacond. This sectIon discusses the Inter~e~iate Regio~1 and the Standard Project Floods and thei r pote~t;a I affects on the vi'lley. The Intermediate Regional FlOOd represents a flood with an average !rcq~ency of occurrence of about once in a hundred years and is in<:reasl ng I y be; ng accepted by the p~bl ic as a minimum I imi t for application of local r"9ulationSllndasastandardtorfioodorot<;>ction. The Standard Project Flood is of greater magnitude and represents a reasonable upper limitofexpoctedflooding. However, it iSnotth.. lergestpossiblefloodthiltmightoccur. It is emphasIzed here that floods which are smaller than the Intermediate Regional Flood are muCh more likely to occ~r. A Ithough these lesser floods wou Id inundate smaller areas, they would also cause damage, hardshIp, and inconven- ience. Greater Floods Floods even larger than the Standard Project Flood are possible, but the combination of factors necessary to produce such flOOds would be extremely severe and rare. Such floods are not f~rther discussed In this report although they should not be overlOOked where it is desirable to entirely avoid the rIsk of flooding. Standard Project Floods The Standard Project Flood is present~ in this report JS a practical upper limitofflOOding. This IS an extremoly rare storm and froquencies cannot be assigned with any reasonable degree of accuracy, but it is not an impossibl" "torm. Foroxilmpie, the 1965 floodS In the Denver areil resulted from storms wIth individual Hazards of Great FloodS The effects of the Intermediate Regional and Standard Project Floods on Left-Hand Creek were analyzed to determine the flood potential. The analysis was made using 113 bridge and v~l.l.\:lLS.r9_~s sections whrch were s~rveyed In 19~7. The water surfaca profiles were determined using backwater computations employing the Standard Stop method. The bridge head losses were comp~ted by ustng data presented in "Hydra~ I ics of Bridge 'o'/aterways" publiShed by the U.S. Department of Commerce and by ~slng standard orifice equations. Wolr and sub. merged weir equations were ~sed in conjunction with the above whenever water overtopped any porttonof the roadways. Several of the small bridges were consIdered Incapable of wlthstondin9 those greater flOOdS and were eliminated from the computations. The effect of debris clog- ging the bridge Openings is to raise the upstream water surface; however, it was not considered since it was oot known if or now the debris would accumulate. The water surface profiles tor the rSSpective floodS, alongwithcertatnphyslClll teaturesofthestream,arepresentedon plates 6 and 7. The profiles show the depths ot flOOding tnthe valley to relate the f loads to structures or other feat~res alo"9 the stream. Intermediate Regional FlOOd The Intermediate REl9lonal Flood was computed trom a statistical analysis of available data. Disc:harge rec:ords from the two gaging statlonson Lett-HandCreekwE;reCQr1lbineddnd arralyled as one record. These reS'J Its were <:<;<"pClred with data f~"" other ~treN"5 in the region wherernore complete records areava;labl&. The Inter- mediate RegIonal flOOd waS dotermined to 1ave a discharge of 4,250 cubic feet per second. This discharge exceeds any of the recorded floods on Left-Hand Creek. " "