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<br />Intermediate Re<Jlonal FIO<.ld to allow for chaoging tiowpatt-erosthat <br />mayacc01lpaoy future <lctivitles in theflooo plain. <br /> <br />Duatlon of flooding from a rainfall event wOJ Id be less th~m II <br />halfdayboJtWOJldbee<tendedbyprolongedralnfallorsnownelt <br />runoff. <br /> <br />Obstructions - Large ~antltles of rOCks and debris are <br />carried along by the swift currents during a flood. Where channel <br />constrictions such as some bridge crossings occur there is always <br />the chance that debris accumulations will severely reduce the chan- <br />nel c"pacityanqraisetlood stages. The increased stress placed <br />on the structures often causes damage or destruction. Water forced <br />toflowaroundabridgestructureoftenerodessectionSOfrO<ldway. <br />8ecause the effect of dobrls is an indeterminate factor, only the <br />physical characteristics of the structures alone w~re considered <br />InprepiJrlng profiles of the Intermediate Regional and Standard <br />Project Floods. The effect of structural obstructions can be seen <br />in tables 5 through 8 by compilring olovationsupstreamand down- <br />streamofstru<;tures. <br /> <br />Velocities of flow - Overban~ flow velocities In the study <br />road. downstre<l1l from Lyons during large potentIal floods wOJld nof <br />be great except ~ear the channel because of the ~Idth of the flood <br />plain and fairly fl~t fiCO(, plain slo?es. I~ Lyons and u~strAam. <br />however, ha~ardous OIerbank flows shoold be expected and consl~ered <br />a aef Inite threat to I ite and property. Sha Ilow flood ing, howevo~ <br />lnnoc.mt look,lng, shOJld b"nvolded. <br /> <br />Rote of rise dnd duration of flooding - Floods caused by <br />~alnsto~ms wou Id oC;Jk within oniy a few hOJrs alter the ral n. <br /> <br />" <br /> <br />22 <br />