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<br />Cone Ius ions <br /> <br />The curves developed in this paper are not intended to add to state <br /> <br />of the art for refined flood plain mapping. The purposes are rather <br /> <br />to develop a quick and rough procedure for making rough estimates and <br /> <br />for checking the reasonableness of reported results. Perhaps the great- <br /> <br />est contribution is a means for checking results by observing where they <br /> <br />plot on the curves and deciding whether deviations are explainable in <br /> <br />terms of known departures from average conditions. Another important <br /> <br />contribution is in providing those who have a study providing magnitudes <br /> <br />or stages for small floods on small basins (a frequent situation for <br /> <br />drainage engineers working on small urban basins) a means of extrapolat- <br /> <br />ing s~ages for rarer floods. <br /> <br />The discharge and stage index slope methods provide a quick, easy <br /> <br />and reasonably accurate approximation of the IOO-year flood plain for <br /> <br />flood plain management purposes until more detailed studies are made or <br /> <br />where they are not justified. If stages of _the 10 and 25-year floods can <br /> <br />be approximated, Fig. 5 can be used along with an estimated regional <br /> <br />skew coefficient to determine flood stages of rarer floods. If dis- <br /> <br />charges are determined for the 10 and 25-year floods. Fig. 2 can be <br /> <br />used to determine discharges for rarer floods, and the discharges can <br /> <br />then be converted to stage. For very rough approximation. SIS values <br /> <br />can be estimated from Figs. 6-1) rather than local data. <br /> <br />The "stage index slope" method should not be used in situations <br /> <br />where channel improvement or flood plain development influences flood <br /> <br />heights significantly. Neither should the method be used in situations <br /> <br />where the discharge-stage relationship is affected by debris jams, tidal <br /> <br />backwater effects, sedimentation. and flood plain crosS sections of un- <br /> <br />usual shape. An example of an unusual flood plain shape would be a <br /> <br />river with flat flood plains extending out to steep canyon walls. The <br /> <br />28 <br /> <br />- <br /> <br />"discharge index slope" method should be used in these cases to estimate <br /> <br />discharges that are then converted to stages using cross sections that <br /> <br />represent prevailing conditions. <br /> <br />In evaluating flood heights for a particular site, one should be <br /> <br />particularly alert to flow conditions substantially at variance with <br /> <br />normal flow. If, for example. a constrictive road crossing is downstream <br /> <br />of the site. backwater from the constriction may be the controlling fac- <br /> <br />tor. and backwater computations should be performed through the bridge and <br /> <br />upstream to the site. <br /> <br />29 <br />