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<br />Recorded historical flows show maximum discharges of 1,110 and 307 <br />cubic feet per second (cfs) respectively for the South Arkansas <br />River at Poncha and Poncha Creek at Poncha Streamflow Recording <br />Stations. These flows were recorded on July 5, 1911, and May 27, <br />1912, respectively. Long-term streamflow records from nearby sta- <br />tions indicate that peak flows occurred in a number of years, par- <br />ticularly in 1957. <br /> <br />Longtime residents of Poncha Springs recall that in the spring of <br />1957, both the South Arkansas River and Poncha Creek reached flood <br />stages. Because of a heavy late spring snowfall, followed by sudden <br />unseasonably warm temperatures, a rapid snowmelt occurred, producing <br />flooding conditions that lasted approximately 7 days. <br /> <br />Obstructions to floodflows within the study reach include primarily <br />natural obstructions such as vegetation, fallen trees, and to a <br />lesser extent manmade structures, such as bridges, buildings, and <br />roadways. These obstructions have the tendency of increasing the <br />flood elevations upstream of the obstructions and the velocities <br />downstream of the obstruction. In certain instances, debris accum- <br />ulated against a structure does present a potential threat to the <br />safety of the structure (Reference 1). <br /> <br />2.4 Flood Protection Measures <br /> <br />At the time of this study, there were no known structural or non- <br />structural flood protection measures for the Town of poncha Springs. <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-, 50-, 100-, or 500-year period (recurrence interval) have been selected <br />as having special significance for flood plain management and for flood <br />insurance rates. These events, commonly termed the 10-, 50-, 100-, and <br />500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, <br />of being equaled or exceeded during any year. Although the recurrence <br />interval represents the long-term average period between floods of a <br />specific magnitude, rare floods could occur at short intervals or even <br />within the same year. The risk of experiencing a rare flood increases <br />when periods greater than 1 year are considered. For example, the risk <br />of having a flood which equals or exceeds the 100-year flood (1 percent <br />chance of annual exceedence) in any 50-year period is approximately 40 <br />percent (4 in 10), and, for any 90-year period, the risk increases to <br />approximately 60 percent (6 in 10). The analyses reported herein reflect <br />flooding potentials based On conditions existing in the community at the <br />time of completion of this study. Maps and flood elevations will be <br />amended periodically to reflect future changes. <br /> <br />6 <br />