Laserfiche WebLink
<br /> <br />e <br /> <br />~ <br /> <br />" <br /> <br />e <br /> <br />,. <br /> <br />: <br /> <br />e <br /> <br />o cC\.lrrence , that they range in lIIllgIlitude t1'Olll13,OOO c.t.s. tcJ/ 3l,700 c.t.s., <br />and that two peaks (there could be IIlOre) occur during some lO-year periods <br />and one or none in others. In the design ot highway drainage structures <br />we are not interested in what mip;llt take place in the next 1000 years or <br />in averages derived for such long periods but rather in what might be <br />expected. to happen in the next 10 or 50 years. For example, we might like <br />to know what chances were ot a peak equal to Q125' or 28,000 c.f.s., <br />occurring during the next 10 years. If the chances were high we would <br />probably design our drainage structure for such a peak. If the chances <br />were small, however, then a design tor such a large peak would probably <br />have been uneconomical. <br /> <br />It follows from the above discussion that, if we are to arrive at <br />an economical <iesi!9l for our s'tream crossing, we will require informatio'l <br />regarding stream flow in addition to that which can be gleaned from the <br />minimum peaks for average recurrence intervals. <br /> <br />Let us turn again to our synthetic lOOO-year streamflow record. <br />This time we will count, not the number of peaks that are equal to or <br />greater than some specified minimum., but the number of time intervals <br />that contain a't least one such peaJ<. If we now compare this number <br />with the tota.l number of the time intervals in 1000 years, we will <br />have a measure of the frequency with "Jhich we may expect such an <br />interval to contain one or IAOre peaks equal to or grea~er than the <br />ndnlmwc. For example. i1' we were to count the 10-year periods that <br />contained. one or more {leaks equal to or greater than Q1U' we would <br />finu the number to be 67. Since there are 100 ten-year periods in <br />1000 years, we would conclude that 67 out of every lOO ten year periods <br />coulli. be expected to contain at least one peal< equal to or greater than <br />Q10 or that the probability of this event oCC\.lrring in any one 10-year <br />periocl vas 0.67. Likewise, if we were to count the lO-year periods that. <br />contained one or JIlOre peaks equal to or greater than Q125' we lIOuld <br />find the number to be eight. In this case we would conclude that eight <br />out of every 100 ten-year periods could be expected to contain at least <br />one peak equal to or greater than Q125 or that the probability of this <br />event occurring in any one 10-ye.;.r period was 0.08. <br /> <br />The above procedure was followed to l1litermine probab1.li ties for <br />combinations of 10, 20, and :>O-year periods with values of Q10' Q20, <br />Q5U and ~125' The results are tabuJ.a.ted in table 2. <br /> <br />Y Since this represents but one of the alJDost infinite possibilities, <br />the range in magnitude could be considerably greater. <br /> <br />3-5 <br />