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<br />Storms over the upper Arkansas River basin are either low-intensity, <br />long-duration rainfall over a large area, or high-intensity, short- <br />duration rainfall over a small area. The former are more common <br />during the autumn, winter, and spring. Thunderstorms most often <br />occur during July and August. <br /> <br />Floods on the Arkansas River are either spring floods or summer <br />floods. Spring floods are caused by snowmelt and are often aug- <br />mented by storm runoff. Spring floods are characterized by com- <br />paratively moderate rates of flow of long duration with large vol- <br />umes of runoff. <br /> <br />Summer floods are caused by storm runoff and are characterized by <br />high peak rates of discharge with relatively smaller volumes of <br />runoff. <br /> <br />2.4 Flood Protection Measures <br /> <br />Levees have been constructed along the Arkansas River by local <br />interests. These levees are ineffective in preventing large floods. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study_ Flood events of a magnitude which <br />are expected to be equaled or exceeded once on the average during any <br />10-, SO-r 100-r Or SOO-year period (recurrence interval) have been <br />selected as having special significance for flood plain management and <br />for flood insurance rates. These events, commonly termed the 10-, 50-, <br />100-, and SOO-year floods, have a lOr 2r 1, and 0.2 percent chance, res- <br />pectivelYr of being equaled or exceeded during any year_ Although the <br />recurrence interval represents the long term average periOd between floods <br />of a specific magnitude, rare floods could occur at short intervals or <br />even within the same yeara The risk of experiencing a rare flood in- <br />creases when periods greater than 1 year are considered a For example, <br />the risk of having a flood which equals or exceeds the 100-year flood (1 <br />percent chance of annual exceedence) in any 50-year period is approxi- <br />mately 40 percent (4 in 10) r and, for any gO-year period, the risk <br />increases to approximately 60 percent (6 in 10). The analyses reported <br />herein reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps and flood ele- <br />vations will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak dis- <br />charge-frequency relationships for each flooding source studied in <br />detail affecting the community. <br /> <br />6 <br />