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<br /> <br />Debris Fan <br /> <br />A Ot:8r{IS FAI'" is a sloping, wedge-shaped deposit of loose <br />rOCK, earth and vegetative debris near or at the junction of <br />a sma I I er stream vi I th a I arger stream va I ley, or where the <br />gradient of a stream abruptly decreases. It is created by <br />debris flows--the downstream/downslope propulsion of rocks, <br />vegetat i va matter, junk and other mater i a I in a watery, muddy <br />slurry. The stream which deposited the fan normally <br />traverses or runs along one of the edges of the fan. <br /> <br />1 <br /> <br /> <br />~ <br /> <br />l <br /> <br />. <br /> <br /> <br />~ <br /> <br />"'~I': <br />~.'-: o"'~ <br /> <br />" <br /> <br />... <br /> <br />. <br /> <br /> <br />. <br /> <br />3 <br /> <br />4 <br /> <br />Debris/mud flow fans frequently offer scenic bui Iding sites in mountain valleys. <br />This case' Ilustrates the tollowlng sequence. 1) A time In the past, shortly <br />after a debris f low that removed many of the trees and left the channel on one <br />side of the fan. 2) Some years later the sol I and vegetation have recovered and <br />the fan shows little evidence ot past debris tlow events. 3) Man develops the <br />fan area with housing. 4) An intense cloudburst causes a debris flow which <br />destroys structures and vegetation and moves the channel to a new location untl I <br />the next event. <br /> <br />16 <br />