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<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 tit <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equalled 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 Inanagement and for flood <br />insurance premium rates. These events, commonly termed the 10-, 50-, <br />100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, <br />respectively, of being equalled or exceeded during any year. Although ~ <br />the recurrence interval represents the long terlm average period between <br />floods of a specific magnitude, rare floods could occur at short intervals <br />or even within the same year. The risk of experiencing a rare flood <br />increases when periods greater than 1 year are considered. For example, <br />the risk of having a flood which equals or exceeds the 100-year flood <br />(1 percent chance of annual occurrence) in any 50-year period is approxi- <br />mately 40 percent (4 in 10), and, for any 90-year period, the risk in- <br />creases to approximately 60 percent (6 in 10). The analyses reported <br />here reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps and flood eleva- <br />tions 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 discharge- <br />frequency relationships for floods of the selected recurrence <br />intervals for each flooding source studied in detail affecting <br />the community. ~ <br /> <br />For Fountain Creek, flood discharges were computed by the U.S. <br />Army Corps of Engineers in the 1974 Flood Plain Information report <br />(Reference 3). A log-Pearson Type III analysis of streamflow <br />records at the Pueblo gage from 1941 to 1965 and 1935 was used <br />to determine flood frequency (Reference 50). Other gages on Fountain <br />Creek were not used due to short length of record (Colorado springs <br />and Security) and inaccurate representation of the flood potential <br />of the stream (Fountain). <br /> <br />The Standard project Floods (SPFs) for Fountain Creek at Manitou <br />Springs, Colorado Springs, the El Paso-pueblo County boundary, <br />and pueblo were developed by the U.S. Arn~ Corps of Engineers <br />(Reference 3) using the June 1921 storm. The Standard project <br />Flood is defined as the flood that may be expected from the most <br />severe combination of meteorological and hydrological conditions <br />that is considered reasonably characteristic of the geographical <br />area in which the drainage basin is located, excluding extremely <br />rare combinations. Drainage areas above an elevation of 9,000 <br />feet were not included in the SPF computations for Fountain Creek <br />because they do not make a significant contribution to storms <br />of the SPF magnitude. A discharge-drainage area curve was developed <br />and used to compute SPF discharges for Fountain Creek at locations <br />in Manitou Springs. <br /> <br />e <br /> <br />8 <br />