In general, the forms of soil erosion are nomenclature by the agents to cause them. Apart from water and wind as the erosive agents, there are also several other agents to cause the soil erosion but not in pronounced quantity. These agents are the glacial, snow, organic contents, anthropogenic etc. The erosion caused by these agents are referred by their respective names. For example the erosion caused by glacial is termed as glacial erosion.
Form # 1. Glacial Erosion:
This type of soil erosion is predominant in cold regions, where mean temperature is found below 0°C.
The main characteristics of glacial erosion are given as under:
i. Action of a large mass of ice moving very slowly.
ii. The protective measures including encouragement of vegetations have little effect.
iii. Soil is damaged only at the edge of ice.
iv. Major part of the erosion energy is dissipated during erosion of bed rock materials.
v. Most pronounced form of glacial erosion is characterized by furrowing, cutting, ploughing and scouring, is referred as exaration.
The glacial erosion mainly occurs in following forms:
(b) Grinding and rubbing, also referred as glacial abrasion or detersion.
(c) Detraction is the last phase of glacial erosion which is a typical form of disturbance caused by glacial action.
Form # 2. Snow Erosion:
The snow erosion is pronounced in those areas where permanent snow cover is found. The glacial zones are climatically associated with the zones of snow. In contrast to glacial erosion, the snow erosion actively damages the soil, especially in avalanche channels where greater pressure and velocity of snow result erosion rills. In snow erosion the soil particles are eroded by slow and creeping movement of snow towards down slope by rubbing action.
Form # 3. Organic Erosion:
The soil erosion caused by living organism is called organic erosion. This type of soil erosion is fairly common, but being little known. In broad sense, it forms a part of the total destructive geological activities. It is seldom regarded as erosion phenomena.
Organic erosion is divided into two forms as:
(a) Phytogenic erosion; and
(b) Zoogenic erosion.
Phytogenic erosion is the destruction of soil caused by plant’s root system. Sometimes, it is also referred as root erosion. In this type of soil erosion, weathering process replaces the soil loss in the field, caused by outside sources and harvesting of plant materials.
In zoogenic erosion, the soil particles are removed by the animals, particularly when they move from one place to another either in search of foods or excavating for shelters. During excavation of holes/shelters, a considerable amount of soil in loose condition is deposited around the hole which is subsequently carried away by the action of wind or water. This type of soil erosion is commonly observed around dam sites. In these localities, this erosion is likely to get intensified during occurrence of floods.
Form # 4. Anthropogenic Erosion:
This erosion is associated with the activities of human beings which are responsible to cause soil destruction, indirectly. The activities such as destruction of natural vegetations through deforestation, shifting cultivation, cultivation of crops without soil protecting measures, exposing the bare soil, increasing and concentrating the runoff and changing the quality of soil (i.e., decreasing the humus content, impairing soil structure, reducing the level of nutrients, diminishing the fertility etc.) are taken into account as the source to cause soil erosion.
Apart from human based activities the animal activities are also responsible to cause soil erosion, counted under anthropogenic erosion. The grazing of pasture lands by domestic animals is an example of this. Besides grazing, the acceleration of this type of soil erosion is also pronounced due to fire, which diminishes the protective cover of soil.
Form # 5. Underground Erosion:
The precipitation is responsible for surface and underground erosion, both. Surface erosion is clearly visible but underground erosion is not so.
Different forms of underground erosion are given as under:
i. Intra Soil Erosion:
This type of erosion is pronounced in those soils which are composed of large proportion of gravels or stones, where after removal of vegetation the soil is rapidly washed downward into the coarse skelton and is partially transported by the underground flow to the lower parts of the slope or into water courses.
The main features of intra soil erosion are described as under:
i. This type of erosion falls under micro erosion.
ii. In this erosion, the washing of soil particles is carried out by the flow of gravitational water through the pore spaces left between the soil particles. It is a rule that greater the interconnected pore spaces, faster is the flow of water under gravity effect, and more readily the soil particles are washed into by intra soil water flow and underground flow, subsequently.
iii. The formations of soil horizons and various types of soil stratifications are the results of intra soil erosion.
iv. Intra soil erosion is also taking place during leaching action. In leaching action, the dissolved minerals and organic matters are transported in lower strata in solute form which are ultimately transported into the sea water by ground water flow. The saltiness of sea water is probably the result of this phenomena.
v. Amount of soil transported through this erosion depends on the flow rate of water and size of interconnected pore spaces through which conductance of water is carried out. The formations composed of stones or rocks, involve greater loss of soil and other materials like humus etc. by this type of erosion.
vi. In brief, this erosion reveals washing of the fine soil particles by flow of gravitational water through the inter spaces formed between aggregates. The washing of soil particles develops effects on ecological and geomorphological characteristics of land.
When rainfall intensity exceeds the absorption capacity of the soil, then puddles are formed in the field, which are then joined by water rivulets; and when these rivulets attain a certain speed of 25 cm/s, then according to Hjulstrom (1935) they take on their own energy. The erosion generated from such energy is confined and concentrated in the flow lines on the steepest slopes, and no longer spread over the whole surface.
Linear erosion is therefore an indication of well-organized runoff, picking up speed and acquiring a kinetic energy capable of cutting the soil and carrying away larger and larger particles – not only the clay and silt as in selective sheet erosion, but also gravels or pebbles and larger blocks once gullying begins.
Forms of Linear Erosion:
Once runoff starts, the light weight particles are carried away, particularly the organic matters, crop residues, as well as fine particles of clay, loam and sand. “Flood debris” can be seen on the ground, often made up of long, fibrous, organic matter, and also ribbons of sand, which are very frequent in wadis. Linear erosion appears when sheet flow becomes more organized, digging deeper and deeper into the ground.
In results there may be the formation of grooves or rills, in which grooves are small channels with only a few centimeters deep, and rills when they are over 10cm deep which can be eliminated through tillage operations. Gullying is when the channels are between 10 and 20 cm deep but as much as several metres wide, and real gullies are when the channels are at least 50 cm deep and more specifically, when they Can cannot be eliminated through cropping techniques.
The gully category can be subdivided into different classes. First, there is small gullies, where the bed is still overgrown with leafy and especially shrubby growth, and that can be quickly stopped by biological methods. On the other hand, in large gullies, which can stretch for several kilometers, the central channel contains rocky boulders as evidence of large-scale sediment transport and certain flashiness.
Since, the bottoms of such gullies are mobile, therefore, biological methods alone are not capable of stabilizing them, and expensive mechanical methods can be suitably used. The cause of linear erosion is due to runoff energy, which depends on runoff volume and its squared speed.
ii. Tunnel Erosion:
The terms tunneling erosion, tunnel-gully erosion, rodents erosion, soil piping, piping erosion, piping etc. are used to express the same meaning as the tunnel erosion. It is being a specific form of underground erosion, in which washing of soil particles through sub surface corridors or passage is carried out by underground water flow. The in-washed soil particles get accumulate over impervious layer.
The special features of tunnel erosion are described as under:
i. In tunnel erosion, the corridors of underground formation are enlarged, i.e., there is created a large size hollow below ground surface and thus weakened the ceiling, as a result the overlying layer gets collapsed and thus developed a big size channel or gully over the land surface. The ultimate stage of tunnel erosion is also considered as gully erosion.
ii. This type of erosion is pronounced in the regions of semi-arid climate, and where soil and geological substrata are deeply fissured.
iii. In the soils with fissures below sub-surface, when rain water enters into these fissures, it washes the formation materials; and after sometimes there is created visible vertical openings of varying dimensions. In extreme condition, the vertical openings are linked with the aquifer, which allow the water to flow over inclined underground layers.
Form # 6. River Erosion:
This erosion occurs particularly in those rivers in which permanent water flow takes place, usually with varying rates. This erosion is likely to be more effective in the water courses of smaller catchment area and those having less favorable conditions for draining the discharge. In such situations, the uppermost stream branches also resemble the gully and so constitute a transition between the rivers and gully.
The main features of river erosion are given as below:
i. In this erosion, as per prevailing direction of influence, a clear-cut distinction is made between vertical and bottom erosion, which deepens the river bed profile. The vertical erosion refers to the removal of soil mass from vertical sides of the river cross-section, whereas bottom erosion indicates the erosion from river bed portion. The bottom erosion is more pronounced due to scouring phenomena, while vertical erosion is resulted due to scouring, bank sliding, under mining and over-fall actions.
ii. By some points the gully and river erosion are very similar, but in river erosion the area of the water course gets change only to a small extent, and damage of soil is only when course of river meanders. In case of gully erosion, the area covered by gullies is very large that may considerably deteriorate the agricultural land.
Scale of Soil Erosion:
It measures the extent of erosion.
According to scale, the erosion may be classified as under:
(a) Micro erosion – erosion in small scale.
(b) Meso erosion – erosion in middle scale.
(c) Macro erosion – erosion in big scale.
i. Surface Erosion:
Soil erosion which occurs from the top of soil surface is called surface erosion. The water erosion such as raindrop and sheet erosion are the examples of surface erosion. The surface erosion caused by precipitation, implies the destruction of soil by rain drops (hail) and surface runoff, both.
ii. Laminar Erosion:
It is a form of sheet erosion in which a thin layer of soil is removed by surface flow. It occurs by any flow of water on a small inclined soil surface where kinetic energy of flowing water is small and only the finest soil particles are washed away in a strong selective manner. The laminar erosion is also referred as selective erosion but it is not a common term.
iii. Layer Erosion:
It refers to a distinct type of erosion which normally takes place on tilled land. In this type of erosion, the soil is neither washed away in laminae nor in rillets or rills, but in a layer form of several meters width and 10 to 25 cm depth (i.e. in the form of apparent strip from which the top soil has been entirely removed.) This type of soil erosion is also liable to occur in arid regions, where top soil layer is easily erodible.
iv. Furrow Erosion:
It is the washing away of soil particles from furrow section. Normally, in ploughed fields, a network of furrow system is formed and when either rainfall takes place or artificial water is supplied to these fields, then excess water accumulates in the furrows and tends to move along slope inclination, as a result few amount of soil particles are scoured from the furrow section which are carried away to some other places by the water flow.
v. Polymorphic Erosion:
The polymorphic erosion includes all those forms of soil destructions which are the main modelling factor of bad-lands. Due to this reason sometimes, this erosion is also named as bad land erosion. In geological terms, the bad-lands are referred by those terrain lands, which are composed of soft and easily erodible rocks in semi-arid regions which are washed by deep rills during torrential flow. The soils are entirely destroyed and slope becomes densely furrowed by the system of gullies, ridges etc.
Classification of Soil Erosion Based on Erosion Intensity:
It is expressed in different units. For example in sheet and wind erosion, it is usually expressed in terms of soil loss or soil removal in the unit of m3/ha or tons/ha. For small erosion intensities it is expressed as kg/ha. Likewise, for long-term erosion the erosion intensity is presented in form of average annual value.
In case of gully erosion the intensity is indicated by the length or density of gullies, i.e., length per sq. km. Also, in case of gully erosion, it is measured in terms of annual length increment of gullies or in terms of proportion of active gullies as a fraction of gully activity (i.e., total length of the gully).
The erosion intensity is also expressed in terms of erosion height, which is defined as the depth of soil removed by erosion in one year or any convenient time. The unit of this is mm per annum. According to the time period in which erosion is expressed as erosion height, may be presented in terms of annual change, average annual change for specific number of years, total change in n-years etc.
The classification of sheet and gully erosion based on erosion intensity is given in Table 3.12 and 3.13, respectively.
The intensity of soil erosion, basically measures the degree of soil erosion taken place from a location. The degree of soil erosion may be within tolerable limit and may also not be.
As per erosion damage, the erosion can also be classified as:
(a) Harmless erosion; and
(b) Harmful erosion.
The harmless erosion is defined as the erosion in which rate of soil removal is less than the rate of soil formation. Sometimes, harmless erosion is also referred as normal erosion, because in this erosion there is generated some soil layer over the ground surface, by which the maintenance of nutrients etc. in the soil through process of pedogenesis is carried out.
In harmful erosion the soil removal from soil surface is predominant, i.e. rate of soil removal is greater than the rate of soil formation. Since loss of soil is more than the soil formed, therefore this erosion brings about creation of undulate profiles over the land surface.
The erosion, in which rate of soil destruction is equal to the rate of soil formation is termed as compensating erosion. The measures for erosion control is applied to reduce the damage of soil to the level of compensating erosion or below this level. The formation of soil is concerned, it generally includes the origin and development of soil mantle under the influence of soil forming factors.
Mostly the rate of soil formation depends on the properties of substratum. On quantitative aspect, the hardness and state of weathering of substratum are important, because they determine the rate of soil formation of weathered materials over un-weathered bed rocks. The soil formation is also assumed in the situation of sediment deposition over the stream bed or on the land surface in a thick layer form.
The weathering phenomena is also affected by vegetation. From field studies, it has been observed that the greatest weathering takes place at the situation of higher vegetative growth, such as forest which accelerates weathering process and ultimately the soil formation, too. The soil formation on a particular place can be marked by the changes made in the soil profile.
Form # 7. Permissible Erosion:
Smith and stamey (1964) defined the permissible erosion as that erosion, which involves the soil formation on one hand and conservation of soil fertility at the same time on the other hand. It is also termed as tolerance erosion. From soil conservation point of view the compensating soil erosion is more or less is permissible soil erosion.
In harmful erosion the rate of soil removal is always greater than soil formation. In other words, the soil removal or soil loss is predominant. The predominance may vary from low to very high. Accordingly, the degree of soil erosion or loss can be expressed as weak, medium, serious, severe and catastrophic erosion, may be considered as classification of harmful erosion.
These are described as under:
(i) Weak Erosion:
Erosion causing annual soil loss from 0.05 to 0.5 mm depth or 0.5 to 5 m3/ha is counted as weak erosion. The erosion, which involves the soil loss 0.05 mm or 0.5 m3/ha is referred as harmless erosion.
(ii) Medium Erosion:
That erosion, which causes annual soil loss ranging from 0.5 to 1.5 mm of soil depth or 5 to 15 m3/ha/year is assumed as medium erosion. At this level of soil erosion, approximately same quantity of nutrient is removed as taken by the plants for their development.
(iii) Serious Erosion:
It causes great danger to the soil because top or upper fertile soil layer is removed in this erosion, as result the crop yield gets significantly reduced; and thus the human beings of the concerned area are severely affected. The range of soil loss in this erosion varies from 1.5 to 5 mm of soil or 15 to 50 m3/ha/year. In this erosion the rate of nutrient loss is several times greater than the rate of nutrient uptake by the crops.
(iv) Severe Erosion:
It is extreme soil erosion, in which there is severe danger to the soil. The range of annual soil loss varies from 5 to 20 mm depth. It destroys the soil relatively in shorter time. This type of erosion creates disastrous consequence to the soil, because in heavy storms the entire top soil as well as deeper layers are severely eroded.
(v) Catastrophic Erosion:
In this erosion, the erosion intensity is much greater, as result there is greater consequence than the severe erosion. The average rate of soil removal is found more than 2000 m3/ha/year. In this erosion the entire top soil is demolished.
Form # 8. Coastal Erosion:
Coastal erosion may be defined as the wearing away of land or removal of beach or dune sediments by wave action, tidal currents or wave currents. The storms, wind or fast moving motor craft are the main sources to form waves and cause coastal erosion. The study of coastal erosion and sediment redistribution is called coastal morphodynamics. On rocky coasts, the erosion takes place in the areas where coastline contains rock layers or fracture zones with different degree of resistances to erosion.
The softer rock layers are eroded at faster rate than the harder rocks, which is normally around the constructions such as tunnels, bridges, columns and pillars. In sedimentary coasts, the erosion creates a greater danger to the human beings. The California coast is an example of sedimentary coastal erosion, which contains soft cliffs of sedimentary rock with heavy population, is regularly under incidence of housing damage as cliffs erosion. Similarly, the Holderness coastline of England is the fastest eroding coastline in Europe because of soft clay cliffs and strong sea waves.
The coastal erosion is also called shoreline erosion, occurs both on the exposed and sheltered coasts, primarily through the action of currents and waves, but the change in sea level due to tides also play great role to cause shoreline erosion. In sea coast the cliff face is generally under erosion problem, which is mainly caused by sea waves.
The ability of sea waves to cause erosion from the cliff faces depends on a host of factors, are outlined as under:
i. The hardness or erodibility of the rocks at the cliff base – It determines the rock strength and the presence of fissures, fractures etc. and presence of sand, silt or clay particles.
ii. The rate of removal of cliff fall debris from the foreshore – It depends on the wave energy striking the beach. The wave energy must be at critical level to remove the materials from the debris lobe.
iii. Presence/absence of beach at cliff base – The presence of beaches plays very significant role to protect the cliff from marine erosion. The beaches dissipate the wave energy on foreshore; and thus, protect the cliff against erosion.
iv. Stability of foreshore – It is the key factor for affecting the rate of cliff recession. The beach should be sufficiently lowered for better result; however, if it is not then the foreshore should be widen. At this condition the beach becomes more capable to dissipate the wave energy.
v. Supply of beach material in coastal cell from up-drift – There should be supply of up-drift materials onto the foreshore below the cliff to make the beach stable.
The wave action is one of the very prominent source to cause coastal erosion.
It is described as below:
The followings are the main types of wave actions responsible to cause coastal erosion:
These are described as under:
1. Hydraulic Action:
It takes place, when waves are striking the cliff face. Due to this action the air gets compressed in the cracks on the cliff face, as result there is development of tremendous pressure on the surrounding rock formations. Sometimes, this pressure gets rise to such an extent, that there is explosion.
In explosion the stones/rock pieces come out with massive force. At the same time the cracks at cliff face are enlarged. In extreme situation the cracks take the shape of cave. The removed rock pieces from cliff face fall to the sea bottom, which activates another wave actions (Attrition) and Corrosion (Abrasion).
It is the process, in which the rocks get grind together by the sea-water. This action makes the rocks smooth and reduces their size, also. In sea water, these small pieces of rocks are in sliding form. In sliding action they also collide with others. This action again makes the rock pieces in reduced size, smooth and round in shape. During colliding of rock pieces with others, several rock pieces also get collide with the base of the cliff face, as result the base of cliff get break down, which leads to cause next action corrosion.
3. Corrosion (Abrasion):
It occurs when waves get break on the cliff face; and slowly eroding it. As the sea pounds the cliff faces, it also uses the screen from other wave actions to batter and break off pieces of rock from higher up the cliff face which can be used for the same wave action and to Attrition.
4. Corrosion or Solution:
This is a rocking type action in the sea environment. This action takes place, when pH of sea is less than 7.0. In this condition, the rocks on cliff face are corroded. In corrosion action the limestone materials are significantly removed, because of high pH of limestone.
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