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SIM~.A."Y OF FLOOD SITUATION FLOOD DAMAGES resulTing from a recurrence of Kno~n floods would be more extensive bocause of the recant developments on the flood plain. Tha damagos will continue to increase in proportion to the dogrea of urbanization. With the larger Infermediate Regional and Standard Project Floods even more damages and hazards to life would occur becauso of their wider extent, greater depths, and higher velocities. Left-Hand Creek is a mountain stream draining a 72-Square mllo arO(l on the eastern slope of the Rocky Moontains In Boulder County, Colorado. It passes withIn fIve miles of Boulder's northern city limits and extends through the expanding surburbs ot Longmont. Until recently, the flood plain wasdovoted entirely to agriculture. Now, because of expandIng populatIon end 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 the lIrea, and the Close proximIty to population centers will co n- tinue to encourage development of tho flood plain. . . . FLOODED AREAS covered by the Intermediate Regional FlOOd average about 400 teet 'lido and vary from a minimum width of 100 feet to a maximum width of 900 feet. The Standard Project Flood wIdth averagos 900 feet and varies from 150 to 1800 feet. . . . . . . DURATION OF FLOODS Is relatIvely short because of the steep slopes and short length of Left-Rand Creek. After a hoavy rainstorm In the mountains, the flood crest would emerge In about throe hours. It would then take approxImately three more hours for the crest to tra- verse the study roach. THE CAUSE OF FLOODS ;s primarily thunderstorm rainfall during the spring and summer months. The lato spring runoff from the mountain snowpack may produce high flows, and when combIned wIth runoff from rainfllllcanproduce/loodstagas. . . . . . . GREAT FLOODS have historically occurred at an average frequency rate of about once in every 15 years. Little information is evailable to detine and compare tho floods except the recording of three peak dis- charges. The largestrecordlld flood occurred in the foothllls several milesaboveStatoHlghway7,andhadapOilkdischargeof 1,140 cubic feet per second In June 1949. Tho second largest recorded flood occurred In 1938 with a peak diScharge 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 dlscharge of 785 cubic feet per second in August 1951. Two groat potential floods were analYled to show the flood problem at Left-Hand Creek. They are the Intermediete 1l09:0:1;) 1 J..~ the StandJr~ Project F1ood~ on~ arc del i ncd in the Glossary of Terms on 03ge 20. HAZARDOUS CONDITIONS would occur during large floods as a result of rapidly rising streams, high velocities, and deep flows. Velocltlos grs(lter than 3 foet por second com~ined with depths 2 feet or greater ~r" 9..n..r..lly considered halardous, but les.",," vllloclt'es (lnd depths can Oe dangerous. Anv overbank flows On Left-Hand Creek mav be dan- gerous and Should bo avoided or approached with caution. . . . . . . FlOOOCONTROLANQQEFENSEon Left-flandCreek has oot been developed. The need for flood control measures nas not become apparent since tho new develooment o~ the flood plain Is considerebly more recent than the last major flood. Tne ESSA Weather Bureau, Department of Commerce Issues warnings ot possible hl'lavy rains or s.evere tnunderstorms from Its Denver office via v(lrlousmass news media. These warnings and forocasts, whiCh may Include Ilash Ilood statements, are based on ,