All civil engineers know two main types of soil movement, soil creep or the slow flow of soil, and the faster flows, called slides, slides slips or avalanches. The faster movement generany moves layers that are deeper than those moved by soil creep.
But more important than either or these two types (as least in the quantity of soil removed every year) are soil erosion, the removal of soil by flowing water from ground that is usually dry and scour, the removal of soil or rook from the bed or bank of a river or stream. Forest soil under growing trees generany does not erode because the leaves, and prevent its attacking the soil. The main action of dead leaves in delaying ran off is that they hold open the drainage channels into the soil ,In bare soil that is not protected in this way, the soil pores are quickly blocked

土木工程師都知道土壤運(yùn)動(dòng)的兩種形式，即：土壤蠕動(dòng)或土壤的緩慢流動(dòng)；和稱為滑坡，滑移或崩塌的快速流動(dòng)。快速流動(dòng)一般移動(dòng)的土層較深，而土壤蠕動(dòng)所移動(dòng)的土層較淺。
但是筆者兩種形式更為重要的運(yùn)動(dòng)（至少就每年所移動(dòng)的土壤數(shù)量而言，是土壤侵蝕和沖刷。土壤侵蝕是指水流從平常干燥的地面上流過而沖走土壤；沖刷是指溪水或河流的河床或河岸把土壤或巖石沖走。正在生長中的樹木下面的土壤一般不會(huì)被沖刷，因?yàn)闃淙~和枯木斷枝象吸墨紙一樣阻滯雨水并防止雨水侵襲土壤?？萑~抑制地面徑流的組主要作用是使雨水進(jìn)入土壤的排泄渠道保持暢通。在沒有受到這種方式保護(hù)的裸露地面，降雨時(shí)土壤的孔隙由于細(xì)小土粒流入很快