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<br />flood insurance rates. These events, commonly termed the 10-, 50-, 100-, <br />and SOO-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 basea 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 />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were oarried out to establish the peak discharge- <br />frequency relationships for each flooding source studied in detail <br />affecting the community. <br /> <br />Hydrologic analyses for the detailed-study areas on the Roaring <br />Fork River below its confluence with the Fryingpan River and on <br />the Fryingpan River were oarried out by the COE, Sacramento District <br />(Reference 9), using the methodology detailed below. <br /> <br />The hydrologic techniques used are fully documented in a separate <br />report, Hydrology Report For City of Aspen and Pitkin County (Unin- <br />corporated Areas) (Reference 7). <br /> <br />Data from a large number of stream gaging stations located within <br />the homogenous watershed area similar to pitkin County area were <br />analyzed. These gages are_.located within pitkin County and nearby <br />Summit, Eagle, Garfield, Mesa, Delta, and Gunnison Counties. The <br />streamflow data were separated into snowmelt and rainfall peaks <br />and analyzed separately. <br /> <br />Snowmelt peaks from a total of 40 gages and rainfall peaks from 25 <br />gages were analyzed to obtain individual and regional statistical <br />parameters of mean, standard deviation, and skew for the two types <br />of flow peaks. In numerous instances, the rainfall peaks were of <br />very small magnitude.. Several stream gages did nOt show any sig- <br />nificant peaks caused by rainfall. <br /> <br />Using log-Pearson Type III distribution as described in U.S. Water <br />Resources Council Bulletin l7A (Reference 10), the discharge-fre- <br />quency information was developed separately for the snowmelt peaks <br />and the rainfall peaks for the 10-, 50-, 100-, and SOO-year recur- <br />rence intervals. These two distributions were COmbined to obtain <br />the overall peak discharge-frequency relationship for each stream. <br />For ungaged streams, the values of regional statistical parameters <br /> <br />9 <br />