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POTENTIAL FLOQJ$ cel!sproducingralnfallofSt",n<.lardPr<Jjectm<lgnitude. TheSt"ndard Project F1QQd on Left-Hand Creek would ha~e a discharge of 12,300 cubic feet per second. This section discusses the Intermediate Regional and the Stand<lrd Project Floods and theirpotontial affects on the viii ley. The Intermediate Regional Flood represents a flQQd with an average frequency of occurrence of about once In a hundred years and is Tncre<lsinglybeingacceptedbythepubllcasamlnimumll",ltfor application of local regulations /.lnd as a standard for flood protection. The Standard Project Flood is of greater magnitude and represents a reasonabTeupper limit of expected floo<.ling. However, itlsrmtthe largest possible flood that might Q<:cur. It is emphasized here that floods which are smilller than the I nto,-..,ed iate Regional Flood a~ much more likely to occur. Although these lesser floods would inundate smallerarcas, they would also cause damage, hardship, and inconven- Ience. Greater Floods Floods even larger than the Standard Project Floopossible, but the combination of factors necessary to produce such floods "00 1<.1 be extreme I y severe and rare. Such ~ loods are not fu,rther discussed In th,s report ~lthOlJgh they should not be overlOOked where It Is desirable to entirely avoid the riSk of floodl~g. Sta~dard Project Floods The Sta~d~rd Project Flood is presented in this report as a practical upperllmitofflO<Xling. This Is an extremely rare ~ton" "uJ fr"<1u..nci,,~ .;:a"not b" d~~i9ned with an'! r"~~on~biG dayroe of accuracy, but It Is oot an impossible storm. For ox,~mp Ie, the 1965 floods in the Denver "rea resulted from storms with individual HazerdS of Greet Floods The effects of the Intermediate Regional and Standard Projoct Floods on Left-Hand Creek were analyzed to determine the flood potentla I. The analysis was made using li:5 brldg6 "nd ~alley cross sections whiCh were surveyed In 1967. The water surf"ce profiles were determined using backwater computations employing the Standard Step method. The bridge head losses were computed by using data presented In "HydraulIcs of Bridge WaterwllYS" published by the U.S. Department of Commerce and by using standard orifice equations. Weir and sub- merged weir equations were used In conjunction with the above whenever water overtopped any portion ot the roadways. Soveral ot the small brldg6S were considered Incapable of withstanding these greater floods and were eliminated from the computatIons. The effect of debris clog- glng the bridge openings Is to raise the upstream wat6r surface; however, It was noteonsidered slnee jtwas rmt known If or how the debris would accumulate. The watar surface profiles for the respective floods, along with certain physical features o~ the stream, are presented on plates 6 <om.! 7. Th.. prof 1 I..~ !oIIOW tn.. J..pth~ of flood log; nth.. valley to relate the floods to structures or other fe"turesalong t he stream. Intermediate Regional Flood The Intermediate Regional Flood was computed fr~ a st~tlstiea I ana Iys Is of avai lable d"ta. Discherge records from the fwo gag i ng stations on Lett-H~nd Creek were ce<nblned ~nd analyzed as one reGard. These results "ere compared with data from other stremr.s in the region "Mre more complete records are available. The Inter- mediate Regional Flood waS determln9d to ha~e a discharge of 4,250 cubic fe9tper second. rhlsdlschJrgeexceedSilnyoftherecordod floods on Left-Hand Creek. " '"