Geological-Related Hazards Induced by Man

Loss of soil due to erosion by water or wind is a natural process. Soil erosion removes the topsoil, which usually contains a varying amount of the organic matter and the finer mineral fractions in soil that provide nutrient supplies for plant growth. Unfortunately, soil erosion can be accelerated by the activity of man. It is difficult, however, to separate natural from man-induced changes in erosion rates. 

Sheet flow may cover up to 50% of the surface of a slope during heavy rainfall but erosion does not take place uniformly across a slope. Linear concentrations of flow may occur within sheet wash. The depth of sheet wash, up to 3 mm, and the velocity of flow are such that both laminar and turbulent flow take place. Erosion due to turbulent flow only occurs where flow is confined. The flow elsewhere is laminar and non-erosive. The velocity of sheet flow ranges between 15 to 300 mm s Velocities of 160 mm S are required to erode soil particles of 0.3 mm diameter, but velocities as low as 20 mm s- will keep these particles in suspension. 

Rills and gullies form when the velocity of flow increases to speeds in excess of 300 mm s- . and flow is turbulent. Whether rills or gullies form depends on soil factors, as well as velocity and depth of water flow. Rills and gullies remove much larger volumes of soil per unit area than sheet flow. Severe soil erosion, associated with the formation of gullies, can give rise to mass movements on the steepened slopes at the sides of these gullies. 

Soil or crop management practices include crop rotation where a different crop is grown on the same area of land in successive years in a four or five year cycle. This avoids exhausting the soil and can improve the texture of the soil. The use of mulches, that is. covering the ground with plant residues, affords protection to the soil against raindrop impact and reduces the effectiveness of overland flow. Reafforestation of slopes, where possible, also helps to control the flow of water on slopes and reduce the impact of rainfall. 

Wind erosion is most effective in arid and semi-arid regions where the ground surface is relatively dry and vegetation is absent or sparse. The problem is most acute in those regions where land-use practices are inappropriate and rainfall is unreliable. This means that the ground surface may be left exposed. Nonetheless, soil erosion by wind is not restricted to dry lands. It also occurs, though on a smaller scale, in humid areas. 

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