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<br />Hvdrolo2ic Re2ion 5 <br /> <br />This hydrolo!;ic region in north-central Iowa (fig. 1) caincides <br />approximately with the northern part of the Des Moines Lobe (Prior, 1976). <br />The magnitude of floods in this region are the smallest per unit area in <br />the State. This i.s due to the flat topagraphy and flood-attenuating effect <br />of abundant bogs, swales, and circular depressions. <br /> <br />FLOODS <br /> <br />Floods at Ga2ed Sites <br /> <br />The magnitude and frequency of floods at gaged sites are determined <br />using flood-frequency curves. These curves relate maximum annual discharge <br />peaks to probability of occurrence or recurrence interval. Probability af <br />occurrence is the chance that a given flood magnitude will be equalled ar <br />exceeded in any year. If there is 1 chance in lO, the probability is .1. <br />Recurrence interval is the reciprocal of the probability of occurrence_ <br />For example, a flood with a probability of occurrence of .01 in any year <br />has a recurrence interval of 100 years. This does nat mean that a 100-year <br />flaod will only occur every 100 years or 100 years from now. The 100-year <br />flaod is a flood of such magnitude that the odds are 1 in 100 (ar 1 percent <br />chance or .01 probability) that it will be exceeded in any year. Several <br />lOO-year floods could occur in a single year. <br /> <br />Flood-frequency curves far gaged sites were prepared using the <br />guidelines for determining flood-flow frequencies published by the <br />Interagency Advisory Committee an Water Data (1982). These guidelines were <br />specifically developed far the analysis of annual flood-peak discharge in <br />drainage basins where streams are nat affected appreciably by regulation <br />and where at least lO years of systematic recards have been collected. <br />Special attention was given to using methods designed to increase the <br />reliability of the frequency curves: the use af histaric flood .data, <br />comparison between flood and storm records, and flood flow analysis at <br />nearby hydrologically similar watersheds. <br /> <br />Flood discharges for selected recurrence <br />from flood-frequency curves for each gaged <br />discharges far all the gaging stations in Iawa <br />1) used for preparing this repart is presented <br /> <br />intervals were determined <br />site. A list of these flood <br />and adjacent States {fiS. <br />in table l. <br /> <br />,~~ <br /> <br />5 <br /> <br />