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<br />The hazard from these debris tlows is difficult to predict because <br />it is difficult to classify d~bris flows in terms of recurrence <br />interval, since historical data on amounts and depths of flow are <br />nonexistent; and because debris flows commonly change channel location <br />on the fan between events or within a single event. <br /> <br />The debris-flow season coinci4es with the season of maximum rainfall, <br />July through October. Most o~ this rain falls during intense cloud- <br />burst storms. Typically, twoior three debris flows occur in this <br />area each year; however, none:were recorded between 1976 and 1980. <br /> <br />Damage from debris flows has been limited to the periodic closing <br />of State Highway 162. With increased pressure for development in <br />this locale, the potential for damage and loss of life will increase. <br /> <br />2.4 Flood Protection Measures <br /> <br />There are no existing flood-cQntrol structures on the South Arkansas <br />River, poncha Creek, Chalk Cr~ek, or Cottonwood Creek. <br /> <br />Chaffee County has adopted zoning regulations for the management <br />of flood hazard areas (Refere~ce 4). <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in!detail in the community, standard <br />hydrologic and hydraulic study meth6ds 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 sOO-year period (recurrence interval) have been se- <br />lected as having special significance for flood plain management and for <br />flood insurance rates. These event~, commonly termed the 10-, 50-, 100-, <br />and sOO-year floods, have a 10, 2, l, and 0.2 percent chance, respectively, <br />of being equaled or exceeded duringiany 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 e*ceeds 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-~ear period, the risk increases to <br />approximately 60 percent (6 in 10).] The analyses reported herein reflect <br />flooding potentials based.on condit~ons existing in the community at the <br />time of completion of this study. Maps and flood elevations will be <br />amended periodically to reflect fut~re changes. <br /> <br />. <br /> <br />6 <br />