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<br />Intermediate Re<Jlonal Flood to allow for chaoging fiowpatterosthat <br />rnavacc01Ipany future <lctivitles in thefiooo plain. <br /> <br />Duatloo of flooding from a rainfall event wOJld be less than II <br />halfdayboJtWOJldbee<tendedbyproloogedralnfallorsnownelt <br />runoff. <br /> <br />Obstructions - Large ~antitles of rOCks and debris are <br />carried along by the swift currents d~ring a flood. Where channel <br />constrictions such as some bridge crossings occur there is always <br />the ch"nce that debris accumulations will severely reduce the chan- <br />nel c"pacityanqraisefiood stages. The increased stress placed <br />OIl the structures often causes damage or destructTon. Water forced <br />toflowaroundabrTdgestructureoftenerooessectionSOfrO<ld"ay. <br />8eeause the effect of debris is an indeterminate factor, only the <br />physical characteristicS of the structures alone were considered <br />InprepiJrlng prof Ties of the Intermediate Regional end Standard <br />Project Floods. The effect of str~ctural obstructions can be seen <br />in tables 5 through 8 by compilring olovationsupstreamand down- <br />streamofstru<;tures. <br /> <br />Y"locities of flow - Overban~ flow velocities In the study <br />roachdownstre<l"llfronLyonsduringlergepotentlal tloodswOJldnot <br />be greet except near the channel because of the width of the flood <br />plain and fairly fl<lt fiCO(, plain slo?es. In Lyons and uostream, <br />however, ha~erdous OIerbank f lows shoo Id be expected and conslqered <br />a definite threat to ITteand property. Shollowflooding, however <br />Tnnoc.mt looking, ShOJld b.. "voidod. <br /> <br />Rate of rise ~nd duration of fluoding - Floods caused by <br />rainstorms ~ou Id pe;Jk "ithin oniy ~ few hcyrs altar the rai n. <br /> <br />" <br /> <br />22 <br />