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<br />CHAPTER III. DEBRIS'-FLOW DYNAMICS <br /> <br />A. Formation and Mobilization of the Debris Flows <br /> <br />As discussed in Chapter II, all thrE~e basins contain sufficient debris <br /> <br />for future events of magnitudes similar to the last one. <br /> <br />The formation <br /> <br />and movement of the flows require certain combi.nations of geologic and <br /> <br />hydrologic conditions. As a result of detailed field study of each of <br /> <br />the three drainage basins, it is thought that the following sequences of <br /> <br />events produced the recent flows. <br /> <br />The basins consist of two distinct parts--an upper area of moderate <br /> <br />(80 to 200) surface slopes Ilnd light to moderat.ely heavy vegetation; and <br /> <br />a lower, steep area (250 to 400) 'with much unconsolidated debris and minimal <br /> <br />vegetative cover (Figure 1). During intense rain over the entire basin, <br /> <br />these two areas respond and contribute differently to the overall basin- <br /> <br />erosion process. <br /> <br />The upper basin areas cont:ribute primarily flood waters with some <br /> <br />entrained soil, rock, and organic debris. During intense rain, sheet runoff <br /> <br />of water is common on the moderate slopes resulting in large discharges <br /> <br />of water and fine material into the central channel. In these upper basin <br /> <br />areas (above 6600 ft or 2010 m), the process is dominated by impressive <br /> <br />water runoff volumes while landslides and other solid'-mass movements are <br /> <br />uncommon or very localized in nature. The deep, high velocity water in <br /> <br />the upper channels efficiently erodes and transports material (including <br /> <br />- 14 - <br />