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<br /> <br />e <br /> <br />. <br /> <br />", <br /> <br />e <br /> <br />.~ <br /> <br />.. <br /> <br />e <br /> <br />HYDROIOOY OF A HIGHWAY STREAM CROSSING <br /> <br />ArticJ.e No. ~ <br />by W. D. Potter, Hydraul1c EIIg1neer <br />Division of Hydraulic Research <br /> <br />Introd11ction <br /> <br />The following discussion may be considered as the first of a <br />series of artic~es designed to give the highway engineer a better <br />understandi!1g of those characteristics of stn:am f~ow that may affect <br />the design of a highway stream crossing. In "t.his series no attempt <br />will be made to present a scientific treatise on streemf~ow hydro~ogy <br />but rather to explain in simple terms only that portion necessary to <br />the effective use and evaluation of the various analyses of peak rates <br />of runoff that may be ava1lab~e and to the exercise of sound engineering <br />Judgmer,t in cases where no such analyses have been made. <br /> <br />This first. articJ.e deals with generaJ. characteristics of stream <br />flcw and presents a deoonstration of the a~aning of Gverage recurrence <br />interval and probability as applied to a high_y crossing. <br /> <br />GeneraJ. Characteristics of Stream.f~ow <br /> <br />The flow in a stream consists of that portion of precipitation <br />that reaches the stream by the process of either running off the sur- <br />face of the soi~, as sun'ace :NIloff, or by flowing through the soi~, <br />as groundwater. The resuJ.tant stream now is, therefore, referred to <br />as r\lI'.off and the rate of such now as the rate of runoff. <br /> <br />If we were to examine an hour by hour record. of streamflow <br />(figure ~) we would see that the rate of runoff is se~dom, if ever, <br />constant. It may rise rapidly to a peak as a result of some rain stom <br />and fall more slowly in the interval between storms. It would be <br />noted that, not only does the rate of :NIloff vary from minute to minute, <br />but also that the magnitude of the instantaneous peak rates vary from <br />stream rise 1;1) stream rise. If our graph of runoff could now be extended <br />to cover severaJ. years of streamflow history, it would also be apparent <br />that the pattern of stream rises var:l.es from year to year, not only as <br />to the magnitude of the peak rates, but also as to the sequence and <br />time ot' year of their occurrence. <br /> <br />The above discussion should make it evident that the peak rate of <br />r\lnoff ~.o be used in the design of a highway bridge or culvert cannot be <br />determined by any one measurement of the rate of runoff at the proposed <br />crossing. Suppose such a measurement had beE:ll made and the peak rate <br /> <br />3-1 <br />