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<br />These four conditions are all necessary and are related to the geology <br /> <br />of the area. However, they are not sufficient by themselves to produce <br /> <br />the debris flow. A sufficient intensity and duration of precipitation <br /> <br />is obviously also necessar~r although unrelated to basin characteristics <br /> <br />and geology. The precipitation event helps determine the probability of <br /> <br />debris flows and will be discussed in Chapter IV. <br /> <br />C. Relative Susceptibilities of Ba!ins to Future Debris Flows <br />One of the objectives of this study 1S to evaluate qualitatively <br /> <br />the susceptibilities of the basins to future flows of the types observed <br /> <br />in July 1977. For instance, if all or most of the material was removed <br /> <br />in the last event, then a basin would not: be able to produce debris flows <br /> <br />in the future. In contrast, if mass wasting in the basins associated with <br /> <br />the last event undermined certain debris areas, they might be even more <br /> <br />likely to contribute to future flow. Therefore, to assess drainage-basin <br /> <br />susceptibility it is necessary to relate various hydrologic and geologic <br /> <br />characteristics of the basins and to compare the amount and position of <br /> <br />the remaining debris to t:he debris removed in this storm. <br /> <br />Each of the basins can be described .n term. of its total area and <br /> <br />the area of metastable debl'is. <br /> <br />The area of metastable debris is always <br /> <br />less than the total basin area (Table 1). <br /> <br />- 9 - <br />