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<br /> <br />Characteristics <br /> <br />The cho!ll"'acterlstlcs of o!I mount"ln torrent or flash flood <br />dl tter trom the m"lnstrOo!lm IOlf-gradlent flooding described in <br />the chapter on floodln~. The 9Ktremely rapid rise of _ater. <br />Its very high velocity as It rushes dOlfn mountainsides ~nd <br />across open lIreas and the 8lttremely hl;)h percento!lge of <br />sediment and debris carried In the .ater mo!lkes flash floods <br />especIally destructive dnd dangerous. ~here the gradient of <br />the stream Is steep, tremendous erosive po_ars o!Iet on the <br />strOo?lm cho!lnnel and bdnllS, transporting and destroying .!Ilmost <br />everything In the oIay. This material then Is deposited <br />do.nstream "here the gradient decreases. (See debris to!ln.) <br />Flash flooding of smaller basins can occur either .Ith major <br />mainstream flOOding or as small, Isolated events. <br /> <br />There Is seldom much time to "arn people or evacuate the <br />areas threatened by mountain torrents. 3ecause ~any streams <br />are periodically dry or contain only Intermittent flo"s, <br />there Is a danger of underestimating the potential hazard. <br />""dny channels which contain a small, peaceful stream can <br />become raging torrents .Ithln minutes after a cloudburst. <br /> <br />Consequences <br /> <br />The tremendous destructive power of the mountain torrent and <br />fldsh flood process can destroy essentially all "orks of man <br />_Ithln the flood path. Erosion can undercut buildings that <br />are above the flood, caus I n9 them to fa II I nto the torrent. <br /> <br />Oam failures can also cause flash floods. They ore often <br />catastrophic In loss of life and property because of <br />development below them. The levels of flood waters often <br />8..ceed any that .."uld occur naturally had the dam not <br />e..lsted. <br /> <br />Aggravating Circumstances <br /> <br />E..cesslve lo~glng, over~razlng and forest fires reduce the <br />land's capacity to absorb water and slow down runoff. <br /> <br />10 <br /> <br />Mitigation <br /> <br /> <br />If property damage and loss of life are to be avoided. the <br />only economically reasonable and physically safe mitigation <br />Is complete avoIdance of areas subject to mountain torrent <br />and flash flood processes. <br /> <br />Monitoring of dams can glv8 warning of Impending fal lure so <br />repa I rs can be made or peop I e evacuated. I" some cases <br />Inundation maps prepared In advance can precisely detail the <br />route and ddpth of a flood In case of dam fal lure. <br /> <br /> <br />- ....~ <br /> <br />~ .... .. ~""\.i. ., <br />, ....~ <br /> ""- <br />. <br />..-~::-... - . -. <br />.:It,....... <br /> <br />.... <br />, <br /> <br />~ <br /> <br />.- <br /> <br />ThIs house .as totally destroyed by the 1~76 dig Thompson <br />flood. Note debrIs and sediment pi led around the trees and <br />the house. <br />