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<br />small rocks), but the ratio of debris volume removed to basin area or to <br /> <br />water runoff volume is probably quite small compared to the lower basin <br /> <br />area processes. <br /> <br />The lower basins produce the majority of th,~ solid debris that eventually <br /> <br />becomes mobilized as flows and is transported to the residential areas. <br /> <br />Slopes in the area are steep (:t5' to 40') and an, near 1 y deva id 0 f stab i 1 i zing <br /> <br />vegetation, particularly on south-facing slopes. Rainfall on these slopes <br /> <br />quickly saturates, weakens, and removes soil, boulders, and other unconsolidated <br /> <br />material and results in intense and rapid erosion, primarily through landslides <br /> <br />and debris avalanches. The rain water i. incorporated into the debris <br /> <br />and, in contrast to the situation on the upper basin, little additional <br /> <br />free water is available as runoff. The cascading ,rock and soil falls into <br /> <br />the main channels imnediately below the slide and avalanche areas at elevations <br /> <br />of roughly 6100 to 6500 ft (1860 to 1980 m) in all three basins. Figure <br />3 shows a typical debris-avalanch~/landslidE' chute that terminates in the <br /> <br />main channel. <br /> <br />Debris at some l,~cations in the channels may accumuJate <br /> <br />to depths exceeding 10 ft (3 m) but because of its initially high strength, <br />probably does not continue to flow dcwn the channel immedi.ately. Therefore, <br /> <br />it forms a temporary dam in the channel ..hich stops the water flowing into <br /> <br />the channel from above. <br /> <br />Debris flows in these basins begin at elevations of 6100 to 6500 ft <br /> <br />(1860 to 1980 m) in the mair, channels as flood water from the upper basins <br /> <br />- 15 - <br />