Unit 1. Lands Degradation Problems in Nepal 1.1 Introduction to land degradation and its consequences 1.2 Water Erosion Erosion is a process of detaching soil particles from the land surface of one place and their transportation and deposition to another place. Three Processes of Erosion : 1. Detachment Process depends upon type of soil, OM, moisture, nature of detaching agents (energy). 2. Transportation Process depends upon size, density and shape of detached materials and velocity of the transporting agent. 3. Deposition Soil that is eroded from the original location is always deposited somewhere else. This may be close to its place of origin position, it may be the longest distance down to the sea or at any point between the place of origin to the sea. Process depends upon soil particles and velocity of the agent. Example: -- Coarse sand particles in eroded soil move the shortest distance and deposit first. -- Fine sand and silt deposit next as run-off water slows down. -- Some very fine silts settle out only in standing water. -- Very fine clay and colloidal humus will not settle out even in standing water but stay suspended in the water indefinitely 1.2.1 Types of water erosion Geological (Natural/ normal) Geological ( Natural/ normal ) erosion are caused by : action of water, geology, wind, temperature, gravity, glaciers, earth quakes. Examples are : Naturally wearing away of hills and mountains: sculptured hills/ mountains, canyons/gorge , stream channels, deltas etc. Man-made ( Accelerated ) Man-made ( Accelerated erosion are caused by : human or anthropogenic activities. Change in land use, destruction of natural cover and soil conditions are main elements responsible for accelerated erosion. Agents responsible for Soil Erosion are : Water, Wind and Gavity

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