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S~~RY OF FLOOD SITUATION FLOOD ~~~CES resulting from a recurrence of known floods would be more extensive ~ecause of the recent developments on the flood plaIn. The damages will continue to Increase in proportion to the degree of urbanization. Wltl>tl>e larger Intermediate Regional and Standard Project Floods even more damages and hazards to life would occur because of theTr wider extent, greater depths, and higher velocities. Left-Hand Creek Is a mountain stream draining a 72-square mIle area On the eastern siope of the Rocky Mountains in Boulder County,Colorado. It passes withIn flvemiles of Boulder'SrlQrthern city limits and extends through the expanding surburbs of Longmont. UntIl recently, the flood plain was devoted entIrely to agriculture. Now, because of expanding population and industrIal- Ization, urban development has begun at both ends and the middle of the study reach. The rugged beauty of the stream, the accessibility to tho area, and the close proxImity to population centers will con- tInue to encourage development of the flood plain. . , , FLOOOED AREAS covered by the IntennedlateReglonal Floodaver<lgeabout 400 feet wldo and vary from a mInImum width of 100 feettoa maximum width of 900 teet. The Standard Project Flood width <lverages 900 feet afid v~rles from 150 to 1800 feet. . , , , , . DURATION OF FLOODS is relatIvely short because of the steep slopes and short length of Left-Han4 Creek. After mounta ins, the flood crest wo~ Id emerge In atlout three hours. It would then take <lpproximately three more hours for the crest to tra- verSe the study reach. THE CAUSE OF FLOODS is primarily thunderstorm rainfall during the spring and SU"""'lr months. The late spring runoff from the mountain snowpack may produce high flows, and when ccmbl~d with runoff from ralntall can produce flood shlges. . , , , . . GREAT FLOODS ha'le historically occurred at an average frequency ra te of <lbO<.lt OI1ce I n every 15 years. Little Information i s avalla~le to define and compare the floods except the recording of three pe<lk dls- ch"rg"s. Th" I~ry"st record,,<.J f luoJ occurred in thl< foothl lis s"v"ra I mllesilboveStateHI<:jhway7,andhadapailkdischargoot 1,140 cubIc teet per sec.ond In June 1949. The second largest recorded flood occurred In 1938 wIth a peak dlschergo of 812 cubic feet per second at U.S. HIghway 287 near Longmont. The third largest recorded flood also occurred In the foothills with a dlsclmrge ot 785 cubic feet per second In August 19SI. Two great potontial floods wore aMlyzod to show the flood problem at LGtt-Hilnd Creek. They are tM Intermodlate ReqlQMI and the St<lndard Project Floods and are defined in the Glossary of Terms on page 20. HAZARDOUS CONDITIONS would occur during large floods as a result ot rapidly risIng streams, hlghv'llocities, and deep tlows. Yeloclties greater t~n 3 feet per second combined with depths 2 feet or greater <Ire generally considered hazardous, but lesser velocitIes i1fld depths c~n be dangerous. Any ov~rbank flows on left-Hand Creo~ may be da~- gerous and Should be avold€d or approaChed with caution. . , . FLOOD CONTROL AND DEFENSE on Left-~nd Creek has not been devolopod. The neod for flood control measures has not become aDparent since the new development on the the last major flood. flood plaIn Is considerably more recen~ than The ESSA Weatner Bureau, Oepartment of Commerce , , . is~uos ~~rn!ngs of possible hoavy rai~s or sovere th"ndcrstcr~s fr~ Its Denver offIce 'Ila v<lrious m<lSS news medi<l. These warnings and forocasts, whiCh may Includo flash flood statements. are based On 2