Soil Erosion - Lecture 1 PDF

LECTURE 1 Soil erosion - definition- nature and extent of erosion-types of erosion-splash- sheet-rill-gully and landslides. Soil erosion is the displacement of the upper layer of soil, it is one form of soil degradation. This natural process is caused...

LECTURE 1 Soil erosion - definition- nature and extent of erosion-types of erosion-splash- sheet-rill-gully and landslides. Soil erosion is the displacement of the upper layer of soil, it is one form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans. Definition: “Soil erosion is the wearing away of land surface by the action of such natural agencies as water and wind”. “Detachment and transportation of topsoil particles by wind and/or by water is known as soil erosion”. Soil erosion involves three phenomena Generally, there are two types of soil erosion: 1. Natural or geological soil erosion: This is a normal soil-forming/soil weathering feature of any landscape. Geological erosion takes place steadily but in a long time which developed the present topographic features like valley plains, stream channels etc. 2. Accelerated soil erosion: It occurs due to disturbance in natural equilibrium by the activity of men and animals through land mismanagement, destruction of forests, overgrazing etc. Nature requires on an average, about 1000 years to bulid up 2.5 cm of topsoil but wrong farming methods may take only a few years to erode it from lands of the average slope. Soil erosion has a three-phase phenomena:- * Detachment of soil particles * Transportation of soil particles by wind, water and gravity * and its Deposition. The permissible soil loss limit is defined as the maximum rate of soil erosion which will permit a high level of crop productivity even in soil loss problems, that can be obtained economically and indefinitely. The generally accepted permissible soil loss limit is 11.2 t/ha/yr. According to a recent estimate of ICAR, more than 120 million hectares (m ha) of the land area is under various forms of land degradation in India, out of which 74 m ha is affected by water erosion. It has been estimated that water erosion alone contributes to 5334 million tonnes (mt) of soil loss every year, at a rate of 16.4 t/ha/yr. That means in India the actual soil loss due to water erosion only is greater than the permissible soil loss limit. In India, water and wind cause severe soil losses. Glaciers and gravitational energy cause soil erosion but in negligible quantity. Water erosion (soil erosion by flowing water/rain): Water erosion causes several damages by removing soil gradually. These characteristic soil erosions are:- 1. Splash erosion: It is the first stage of the erosion process. It occurs when raindrops hit bare soil. The explosive impact breaks up soil aggregates so that individual soil particles are 'splashed' onto the soil surface. It is the beginning of soil erosion by water. Fig. 1.1: Splash Erosion - representative diagram Fig. 1.2: A surface after splash erosion A simple example of splash erosion we can identify is the layer of soil particles on the vertical surface of walls of houses, buildings, etc. after rain. Also pebbles or stones over a heap of soil as in Fig 1.2. 2. Sheet erosion: Sheet erosion is the uniform removal of soil in thin layers by the forces of raindrops and overland flow. Raindrops churn the topsoil and along with the runoff, the muddy water moves away from the field. Water moving fairly uniformly with a similar thickness over a surface is called sheet flow and is the cause of sheet erosion. It can be a very effective erosive process because it can cover large areas of sloping land and go unnoticed for quite some time. It is considered as the first stage of erosion along with splash erosion. Fig. 1.3: Sheet erosion - how it begins Fig. 1.4 A field after sheet erosion 3. Rill erosion: It is the detachment of soil particles by surface flow, and it occurs in a series of rills, which are small parallel channels - a few tens of millimetres wide. When the runoff combines, channelization begins and erosion is no longer uniform. Incisions are formed on the ground called rills.  Fig. 1.5: Rill erosion - diagrammatic representation Fig. 1.6 Rills on a slope surface Rill and sheet erosion can be observed besides unprotected tarred roads on slopes and hilly areas. 4. Gully erosion: Unchecked rills results in increased channelization of runoff. Gullies are formed when channelized runoff from vast sloping land is sufficient in volume and velocity to cut deep wide channels. Gullies are the most spectacular symptoms of erosion. If unchecked soil loss will be extreme. Fig. 1.7: Gully erosion - diagrammatic representation Fig. 1.8 A gully formation 5. Ravines: These are of a prolonged process of gully erosion, typically found in the large expanse of deep alluvial soils. They are deep and wide gullies and their formation indicates very advanced stages of gully erosion. The Beehad ravines of Chambal area is a typical example of ravines formed from enormous gullies. Fig. 1.9: Ravines 6. Streambank erosion: Small streams, rivulets, torrents (hill streams) are subjected to stream bank erosion due to obstruction to their flow. Vegetation sprouts when the  streams and obstructs the flow causing cutting of bank or changing of the flow course. Torrents with flashy flows and swift currents, bring boulders or uprooted plants are deposited in the downstream side causing overflowing and stream bank erosion. The erosion of the banks of the Cheruthoni dam spillway canal in the 2018 flood is a typical example. Fig. 1.10: Streambank erosion 7. Landslides: Sliding down of large chunk of soil due to steep slopes is called landslides. Generally, landslides occur in high ranges when slope exceeds 20% and width 6m. Generally, landslides occur since loosened landmass could not hold the excess water which causes the collapse. Fig. 1.11: A landslide in Kerala. (Kavalappara -2019) Wind erosion (soil erosion by wind): Wind erosion is a serious problem in areas where land is bare and devoid of vegetation. It is a natural phenomenon in arid and semi-arid zones. The wind picks up lighter particles, lifts them bodily from the surface and transports them to long distances. The Thar desert is formed mainly by blown-in sand. Generally, wind erosion is very limited in Kerala due to its climate, geographical features and dense vegetation. Wind erosion occurs in three stages: saltation, surface creep and suspension. 1. Saltation: Saltation is the first stage of movement of soil particles in a series of jumps. The soil particles moved by saltation are between 0.1 to 0.5 mm in diameter (fine sand). Sand sized particles rolling along the surface starts this movement. Due to the effect of wind, soil particles jump up vertically into the air and rise to a height  of 30 to 60 cm and fall through the air. Generally, 50 to 75 % of the weight of soil lost by wind erosion is carried in saltation. 2. Surface creep: Rolling of coarse grains, larger than 0.5 to 3 mm in diameter and too heavy to be lifted, by the wind along the surface of the ground is called surface creep. The particles are pushed along the surface of the ground, rolling and not jumping. Generally, 5 to 25 % weight of the soil lost by wind erosion is carried in surface creep. 3. Suspension: Floating of fine dust particles through the air is known as suspension. The particles are smaller than 0.1 mm in diameter. The smaller particles are kicked into the air, they stay in suspension and carried very long distances, even several thousand kilometres. The settle down with rain or after wind velocity is reduced. Generally, 3 to 4 % of the weight of soil lost by wind erosion is carried in suspension. *****