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<br />once on the average during any 10-, 50-, 100-, or 500-year period <br />(recurrence interval) have been selected as having special <br />significance for floodplain management and for flood insurance rates. <br />These events, commonly termed the 10-, 50-, 100-, or 500-year floods, <br />have a 10-, 2-, 1-, and 0.2-percent chance of being equaled or <br />exceeded during any year. Although the recurrence interval represents <br />the long term averal1:e period between floods of a specific magnitude, <br />rare floods could occur at short intervals or even within the same <br />year. The risk of experiencing a rare flood increases when periods <br />greater than one year are considered. For example, the risk of having <br />a flood which equals or exceeds the 100-year flood (l-percent chance <br />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 <br />reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps and flood <br />elevations will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish peak discharge- <br />frequency relationships for each flooding source studied by <br />detailed methods affecting the community. <br /> <br />Following guidelines established by the U.S. Water Resources <br />Council, peak discharges of the 100-year flood were determined by <br />utilizing a flow-frequency analysis of flows recorded at the <br />"Below Rico" (1914-15, 1919-21, and 1952-Present), and "At <br />Dolores" (1896-1903, 1911-12, and 1922-Present) stream gaging <br />stations on the Dolores River, Gage Numbers 09165000 and <br />09166500, respectively. To determine the type of event that <br />would cause the larger, less frequent floods, rain and snowmelt <br />floods were separately statistically analyzed at the Dolores <br />Gage. At the Rico Gage this type of analysis was never <br />determined because of the short period of record. <br /> <br />Based on flows during September 1970, at the Rico and Dolores <br />Gages, Snyder S-Curve and optimization techniques were used to <br />develop unit hydrographs for the Dolores River. Optimization of <br />the unit hydrograph for Lost Canyon Creek was achieved by using <br />data from the October 13, 1941, storm and flood. Loss rates, <br />consistent with previous studies done by the COE, Los Angeles <br />District, and the U.S. Bureau of Reclamation, were calculated by <br />the COE, Sacramento District, and found to be reasonable and <br />accurate (Reference 1). <br /> <br />Peak discharge-drainage area relationships for the Dolores River, <br />the West Dolores River, and Lost Canyon Creek are shown in <br />Table 1. <br /> <br />5 <br />