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This article throws light upon the six main types of soil erosion caused by water. The types are: 1. Splash Erosion 2. Sheet Erosion 3. Channel Erosion 4. Stream Erosion 5. Marine Erosion 6. Landslide or Slip Erosion.
Type # 1. Splash Erosion:
When raindrops strike the ground surface, the soil particles become loose and splashed due to its impact force. Momentary build-up of the pressure gradients towards the edges of the drop disintegrates the soil and shoot some particles out. The falling raindrops at an average speed of 75 cm/sec is capable of creating a force of almost 14 times its own weight.
The amount of damage done by falling raindrops is proportional to their kinetic energy, which ranges from 1,000 to 1,00,000 times the work capacity of surface flow. When raindrops strike bare soil, or thin films of water covering it, they blast it with a magnitude of explosion. These blasts bounce water back into the air carrying with it, in muddy splash, fine particles of earth.
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These splashed particles reach to heights ranging up to more than 10 cm, and horizontally up to 15 cm or more on level surface. On level surfaces, the splashed material tends to scatter uniformly in all the directions, when the rain drops have a vertical drop.
In such cases, the outgoing splash normally balances the incoming splash in a given area. But when rain drops strike sloping land surface (Fig. 2.5), a major portion of the splash moves downhill. Fine sand and silty textures become dis-logged readily.
To produce significant erosion, the splashing must throw soil particles into a place where there is a water stream for transportation.
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The forces that influence splashing are:
(i) The rain drop mass and velocity; and
(ii) The soil characteristics (such as roughness, surface slope and aspect), hydraulic conductivity, moisture content, particle size, elasticity, and associated mass of the surface.
In cohesive soils, smaller particles are difficult to detach. On the other hand, to throw out large aggregates, more energy must be transferred from the rain drop. Coarse grained, dry soil will reduce the momentary build-up of pressure in the rain drop because of surface roughness and faster energy dissipation caused by flow into the soil.
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The saturation of soil will weaken the cohesive forces, reduce seepage and enable stronger splashing. However, a thick enough water layer will protect the soil. Raindrop action can be cumulative, initially breaking aggregates and later throwing them out.
Splash board studies show that the splash is richer in sand, gravel, and aggregates of diam. 0.105 mm than the original soil, while the runoff material contained 95% silt and clay and also aggregates < 0.105 mm than the original soil.
The amount of soil loss in splash erosion depends upon the rain drop characteristics, as given below:
S = k v d i …(2.4)
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where,
S = relative amount of soil splashed in 30 minutes
k = soil constant
v = drop velocity, ft. sec-1
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d = drop diameter, mm
i = rainfall intensity, inches hr-1
Also, erosivity due to raindrop impact can be expressed as:
E = f (I x t x mv2/A)…(2.5)
where,
E = erosivity
t = time
m = drop mass
A = cross-section area of the drop
Type # 2. Sheet Erosion:
As water passes over a soil with gentle and smooth slope, it follows along a sheet of more or less uniform depth. Under such conditions, there occurs relatively uniform removal of soil from all parts of the area having a similar degree of slope.
This moderately uniform removal of surface soil by the action of rainfall and runoff water is known as sheet erosion. More particles will naturally be washed from a bare soil than from those protected by vegetation.
Sheet erosion is the most damaging form of soil erosion by water. Many times it is difficult to recognise that any soil has disappeared but after this process has repeatedly occurred, much of the original surface soil is gone, exposing the sub-soil which is not as good a medium for plant growth as was the surface soil.
Shallow soils suffer greater reduction in productivity than deep soils. Areas where loose shallow top soil overlies a light sub-soil, are most susceptible to sheet erosion.
Sheet erosion is a most serious problem in red soils covering 72 million ha in India. The depth of soils vary from 20 to 100 cm and rainfall intensity is very high though the total annual rainfall may not be high. Similarly, the laterite soils, suffer from severe rill erosion because of fairly high rainfall.
The black soils, occupying nearly 88 million ha, when fallow during the monsoon, are subject to severe sheet erosion hazards. The annual loss of top soil may range from 11 to 43 tonnes/ha along with 10 to 30% of runoff loss.
Type # 3. Channel Erosion:
In contrast to sheet erosion, channel erosion occurs where surface water has concentrated and a large mass of water supplies the energy, both for detaching and transporting the soil. Channel erosion exists as rill erosion, gully erosion and stream erosion.
However, there are no distinct boundaries between rill and gully erosion, and between gully and stream erosion, except that in all the cases the detachment is caused by the energy of flowing water and not by the impact of raindrops.
(a) Rill Erosion:
Rill erosion is the removal of soil by runoff water with the formation of shallow channels that can be smoothed out completely by normal cultivation. Rills develop as a result of concentration of runoff water where the silt-laden runoff water starts flowing along the slopes through small finger-like channels.
The soil eroded from upland areas comes from these small channels, called rills, and from inter-rill areas between them. The primary mechanism for soil detachment and transport from inter-rill areas is the energy resulting from raindrop impact while the primary mechanism for soil detachment and transport for rill erosion is the distributed shear force on the rill channel boundary due to concentrated flow of runoff water.
During a rainfall event flow is quickly concentrated in micro-rills, which in turn flows into larger rills and eventually discharge to an existing channel system. The concentration of flow in rills increases the erosive power of the flow resulting in increased soil detachment from the rill channel boundary.
In general, rill erosion is incipient gully erosion. If rill erosion continues only for a short while, tillage operations may smoothen out the surface completely, so that the resulting soil profile is identical to one that is damaged by sheet erosion.
(b) Gully Erosion:
As the volume of concentrated water increases and attains more velocity on slopes, it enlarges the rills into gullies. Here rills become so deep that the ground cannot be smoothed out by ordinary tillage tools.
Gully can also originate from any depression such as cart tracks and cattle trails and indicates neglect of land over a long period of time. An advanced stage of gully results into ravine, which are sometimes more than 16 to 33 m deep.
The rate of gully erosion depends primarily on the runoff producing characteristics of the watershed, soil characteristics, alignment, size and shape of the gully and the slope in the channel. Loose, open, well drained sloping soils gully rather easily when water is concentrated on them.
A heavy or compacted soil often checks the rate at which gullying takes place. Gullies are sometimes as deep as 6 to 12 m. Gully erosion is most spectacular in 4 m ha land in India. Four stages of gully development are recognised.
Stage 1: Formation Stage:
In this stage, the rill erosion scour of the top soil in the direction of general slope as the runoff water concentrates. This stage normally proceeds slowly where the top soil is fairly resistant to erosion.
State 2: Development Stage:
In this stage, there occurs upstream movement of the gully head and enlargement of the gully in width and depth. The gully cuts to the C-horizon, and the parent material is also removed rapidly as water flows.
Stage 3: Healing Stage:
In this stage, vegetation starts growing in the gully.
Stage 4: Stabilisation Stage:
In this stage, gully reaches a stable gradient, gully walls attain a stable slope, sufficient vegetation cover develops over the gully surface to anchor the soil and permit development of new top soil.
The evaluation and prediction of gully development is often difficult because the factors are not well defined and field records of the gullying are inadequate. Following relationship, however, has been worked out based on the 20 years’ record.
A = 0.01 Q0.10 At –0.044 L0.8 Ld 0.25 e-0.036I …(2.6)
where, A = change in gully surface area, acres
Q = index of surface runoff, inches
At = level terraced area of the watershed, acre
L = gully length at the beginning, ft.
Ld = gully length from the upper end to watershed divide, ft.
I = deviation from normal precipitation, inches
In general, gullies are classified as below (Fig. 2.9):
(c) Stream Erosion:
Stream erosion is the scouring of soil material from the stream bed and cutting of the stream banks by the force of running water (Fig. 2.10). Stream bank erosion is often increased by the removal of vegetation, overgrazing or tillage near the banks.
Scouring is influenced by the velocity and direction of the flow, depth and width of the stream, soil texture and alignment of the stream. Rivers and streams often meander and change their course by cutting one bank and depositing sand and silt loads on the other. The damage is manifolds during flash floods.
Type # 4. Water Fall Erosion:
Water causing sheet and channel erosion in flowing down over a sloppy land may cut a hole on an unprotected spot (Fig. 2.11). Such a hole often becomes a small waterfall. As the water goes over the face of the fall it tends to undercut at its base, and the top soil falls in. Thus, in time, the waterfall leads to a gully unless the waterfall is controlled in the early stages.
Type # 5. Marine Erosion:
The rivers in spate discharge their silt-laden muddy water into the sea. This usually results in silting up of the harbours and ports which consequently need dredging operations. The tidal waters of sea cause considerable soil erosion along the coast.
The roaring waves rush and dash on the coast swallowing every time bits of coastal lands. In this way, the coastal lands are not only washed off, they are turned into vast strips of Khar lands due to creeping of sea water into the low lying better shore lands.
The marine erosion in India is common on the western coast, extending 1600 km from Cape Camorin to Tapi valley, with certain breaks. The other areas lie on the eastern coast, extending to a distance of 1000 km from Cape Camorin up to deltas of Krishna, Godawari and Mahanadi rivers.
Quilon, Alleppey and Calicut districts of Kerala are most affected by marine erosion (Table 2.2). Nearly 410 km of coast line are threatened by marine erosion.
Type # 6. Landslide or Slip Erosion:
Landslide is the downward and outward movement of slope forming material composed of natural rocks, soil, artificial fills or combination of these materials. Major causes of landslide are excavation or under cutting of the base of an existing slope, gradual disintegration of soil and increase of pore-water pressure in permeable layers.
In hilly areas, along the road side or gullies, the slippage is normally caused by the hydraulic pressure exerted by soil pore-water pressure build up above the impermeable soil or rock strata during heavy rains.
Under such conditions soil failure occurs and a great mass of overlying moist soil on steep land comes down bodily. Here, it is the lubricating action of slowly percolating or even to stationary water and not its kinetic energy that causes erosion.
However, for this movement to occur there must be:
(1) A slope sufficiently steep for soil masses to slide;
(2) A slowly permeable layer in the soil, at some depth below the surface; and
(3) Enough water in the soil mass to fully saturate the layer just above the impermeable layer. The impermeable layer is usually clay or soil with a high clay content, such as B-horizon or rock stratum. Clay shales are frequently the cause for water accumulation and landslides.
Slip erosion is very common on stream and river banks. Streams or rivers especially at flood stage often undercut their banks, particularly on curves, leaving the slope of the cliff-like stream bank steeper than the angle of the rest when wet.
In the early spring or following a long period of heavy rainfall, the perfectly stable bank when dry, becomes unstable on saturation also caused by a combination of factors such as destruction of protective vegetation by excessive utilisation, grazing or fire, unstable geology, erodible rocks, blocking for road making, mining, etc.
Landslides pose an immense threat to highways, villages, agricultural lands, in the outer Himalayas, Peninsular India and elsewhere.
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