Water Erosion:
It is the removal of soil from the land surface by flowing water. Water erosion is due to the dispersive action and transporting power of water-water as it descends in the rain and leaves the land in the form of runoff.
Rainfall is the chief detaching agent in water erosion.
Water erosion is a complex three-step natural phenomenon which involves :-
- detachment,
- transport, and
- deposition of soil particles.
The process of water erosion begins with discrete raindrops impacting the soil surface and detaching soil particles followed by transport.
Detachment of soil releases fine soil particles which form surface seals. These seals plug the open-ended and water-conducting soil pores, reduce water infiltration, and cause runoff. At the microscale level, a single raindrop initiates the whole process of erosion by weakening and dislodging an aggregate which eventually leads to large-scale soil erosion under intense rainstorms. The three processes of erosion act in sequence
The first two processes involving dispersion and removal of soil define the amount of soil that is eroded, and the last process (deposition) determines the distribution of the eroded material along the landscape. If there were no erosion, there would be no deposition. Thus, detachment and entrainment of soil particles are the primary processes of soil erosion, and, like deposition, occur at any point of soil. When erosion starts from the point of raindrop impact, some of the particles in runoff are deposited at short distances while others are carried over long distances often reaching large bodies of flowing water.
Role of the three main processes of water erosion
Detachment,
Transport ,
Deposition
Soil detachment occurs after the soil adsorbs raindrops and pores are filled with water.
- Raindrops loosen up and break down aggregates.
- Weak aggregates are broken apart first.
- Detached fine particles move easily with surface runoff.
- When dry, detached soil particles form crusts of low permeability.
- Detachment rate decreases with increase in surface vegetative cover.
Transport
- Detached soil particles are transported in runoff.
- Smaller particles (e.g., clay) are more readily removed than larger (e.g., sand) particles.
- The systematic removal of fine particles leaves coarser particles behind.
- The selective removal modifies the textural and structural properties of the original soil.
- Eroded soils often have coarse-textured surface with exposed subsoil horizons.
Deposition
- Transported particles deposit in low landscape positions.
- Most of the eroded soil material is deposited at the downslope end of the fields.
- Runoff sediment transported off-site can reach downstream water bodies and cause pollution.
Types of Water Erosion
Water erosion is caused by water – water that comes in rain and runs off the land as overland flow or streamflow.
At the initial stage, soil particles are detached from aggregates by the impact of falling raindrops or flowing water, which is followed by transport of the detached particles by runoff water.
Runoff water laden with suspended particles also detaches more soil particles in its way across the surface.
Soil erosion is a process of soil loss, particularly from the surface, but sometimes a large mass of soil may be lost, as in landslides and riverbank erosion.
Water erosion can be classified into sheet erosion, internal erosion, and channel erosion.
Channel erosion was further divided into :-
Rill erosion,
Gully erosion and
stream erosion.
However, the following four types of water erosion are generally recognized: s
Splash erosion,
sheet erosion,
rill erosion, and
gully erosion.
- Splash and sheet erosion are sometimes called interrill erosion.
Splash Erosion
At the start of a rain event, falling raindrops beat the soil aggregates, break them, and detach soil particles.
These particles clog the large soil pores and, thus, reduce the infiltration capacity of the soil.
Water cannot enter the soil, and soon a thin film of water covers the ground.
Further, raindrops beat the water and splash the suspended soil particles away.
Soil particles are transported to some distance by the splashing.
The splashed particles can rise as high 60 cm above the ground and move up to 1.5 m from the point of impact.
Processes of splash erosion involve raindrop impact, splash of soil particles, and formation of craters. Actually, splash erosion is the beginning of other types of soil erosion, particularly sheet erosion
Sheet Erosion
When a thin layer of soil is removed by raindrop impact and shallow surface flow from the whole slope, it is called sheet erosion.
It removes the finest fertile topsoil with plenty of nutrients and organic matter.
It is the most dangerous type of soil erosion because it occurs gradually and almost silently leaving little or no signs of soil removal.
Sheet erosion involves the removal of a more or less uniform layer of soil over the whole slope of the land.
Soil particles are detached primarily by raindrops and secondarily by frost, hooves of farm animals, tillage, and mechanical action of farm machines.
Detached particles are transported by runoff water as overland flow. Sheet erosion is more uniform and gradual, as the surface becomes smoother. However, water may still accumulate even on the smoothest slope.
The intensity of accumulation of runoff water depends on the height of the water stream, the coarseness of the surface, vegetation, or crop distribution.
Sheet erosion removes deeper layer of soil gradually, if allowed to proceed unhindered and the subsoil is exposed over a large area. The subsoil is usually of different texture and color and is more compacted. However, slopes are often not so uniform over the whole area, and water accumulates in tiny channels, so that the surface is crisscrossed by discontinuous rillets. It is then known as the interrill erosion.
In addition to soil particles, sheet erosion removes
(i) organic residues accumulated on the surface soil and
(2) Soluble and easily dissoluble matter, matter made soluble by weak acids in rain water.
Sheet erosion represents microerosion, that is, the eroding and washing of the soil to produce small- scale forms which may encompass raindrop erosion, laminar erosion, rillet erosion, and layer erosion. The first phase of sheet erosion, specific with regard to form, is soil removal by raindrop action – raindrop erosion. In raindrop erosion, the surface is acted upon selectively so that small holes, micropyramids, and other forms occur, raindrop erosion thus becoming a part of pedestal erosion, pinnacle erosion, etc.
The second subtype in sheet erosion is laminar erosion. It occurs in any flow of water on an inclined soil surface where the kinetic energy of the water is small and only the finest soil particles are consequently washed away in a strongly selective manner. By virtue of the accumulation of sheet runoff water, rill erosion develops, causing small rills with the dimensions of a few centimeters diameter in cross section, and with a depth not exceeding that of the arable layer.
The rillets that develop in rows and furrows, with the effect of increasing their dimensions and conspicuousness, are removed during cultivation. In this form of erosion, soil and particles displaced by water may be intensively separated and sorted. In layer erosion, the soil is washed away neither in laminae nor in rillets, but in a layer up to several meters wide and 10–25 cm deep from a tilled surface, that is, in apparent strips from which the topsoil has been entirely removed.
Rill Erosion
When rainfall exceeds the rate of infiltration, water accumulates on the surface, and if the land is sloping, it moves along the slope. On gently sloping lands, with standing crops or in fields that have been recently tilled, moving water concentrates along tiny channels called rills.
Rills are less than 30 cm deep.
The cutting action of flowing water detaches soil particles, and runoff water carries them away. The amount of soil loss may be high, but the small channels do not usually interfere with tillage implements. The rills may be leveled by normal tillage operations.
Rill erosion is often the initial stage of gully erosion.
Rill erosion is largely caused as a result of large amounts of material that are released and transported for variable distances in concentrated areas.
On the other hand, the flow of water over the surface has a smaller effect on soil detachment, but a larger transportation effect. Yet flowing water, especially on tilled land, can become the agent of transport of particles loosened mechanically, chemically, or by means other than the water flow itself, and therefore it is a phenomenon of great importance from the point of view of total soil losses.
Water concentrates in places over the field due to reduction in infiltration, increase in precipitation, and surface roughness of the land. Water concentrates along tillage lines, rows of crops, impedance by exposed roots, around clods, etc. and from shallow and narrow channels known as rills. As this gathering of water proceeds, the total amount of water remaining the depth of the water increases, together with the velocity, kinetic energy, and detaching as well as carrying capacity of the water. At high precipitation intensities, there is greater clogging of pores, and the proportion of precipitation water making up the surface flow and the numbers of particles separated from the soil by raindrops both increase.
Greater predominance of rill erosion may be found on steeper slopes with impermeable soil material consisting of younger sediments which are susceptible to erosion. As is seen usually, rill erosion prevails and affects the whole length of the slope, which means that precipitation water, as soon as it reaches the soil, flows away through the dense network of rills, virtually cutting the slope into thin plates.
Occurrence of splash erosion or sheet erosion is not seen, but there was some erosion of these forms too. Similar phenomena occur on steep slopes, even on impermeable loamy clay material. On impermeable or still heavier and more resistant material, rill erosion forms ridges which are separated by sharply cut rillets and gullies.
The rillets are occasionally so narrow that they resemble cracks, and it could be termed crack erosion. On steep slopes composed of material of varying resistance, vertical openings are formed, and these soon develop into tunnel erosion or hollow erosion, separating the washed forms into isolated pipes, etc.
Where the material is more homogeneous and the incline less steep, rilling prevails. If the material is more coarse grained and less resistant, flowing water carries the soil along rapidly and creates triangular or trough-shaped forms with respect to the cross section of the channel. In such cases, the lengths of the rills are greater, but the interrill lamellae are thinner, and the edges sharper.
The more coarsely grained and more permeable the material, the less pronounced is the channeling, until finally the rills are widely shaped, and resemble more the form of moderately undulating depressions, even on very steep parts of the eroded slope. On permeable, coarse-grained, non-resistant fluvioglacial deposits, on the other hand, shallow, rapidly growing rills develop with an immense production of silt. Here, the action of fl owing water is the predominant force. These forms are transitional toward gully erosion.
Rill erosion usually begins,to appear in the lower part of the slope. This is true especially when the source of the water is thawing snow or precipitation of low intensity. As soon as the intensity of the rainfall increases, the intensity and velocity of surface runoff both increase also, and consequently the proportion of the total erosion due to rills becomes greater, depending on the permeability of the soil.
Gully Erosion
Gullies are large channels deeper than 30 cm. Gullies develop when large quantities of water accumulate and run through a single channel with high speed in relatively steep slopes .
Gullies may also develop by the gradual deepening of rills.
There are two types of gullies:
- ephemeral and
- permanent.
Ephemeral gullies form shallow channels that can be readily corrected by routine tillage operations.
On the other hand, permanent gullies are very large and cannot be smoothed by regular tillage. Gullies of various size and form develop by the gradual deepening of rills.
A number of forms may be distinguished in gully erosion. The first form includes gully with a depth between 30 cm and 2–3 m.
In this form, typical wash prevails with a marked backward or retrograde erosion and vertical or depth erosion.
Gullies have larger dimensions and their development is more complicated.
Besides retrograde and vertical erosion, lateral erosion also appears here, together with accessory landslide, soil flow, and other phenomena.
Gullies may grow into gorges and canyons in high altitudes and very steep slopes.
Gullies may be flat, narrow, broad, and round. Flat forms occur mostly on shallow soil or in connection with a specific lithic structure of the slope.
this form, characterized by a broad V-section, lateral erosion prevails over vertical erosion. Narrow, acute forms are created with a narrow V-section, the breadth of the gully usually being equal to its depth or smaller.
Broad gullies have a wide bottom and are U shaped. Here, lateral erosion prevails over depth erosion. Active gullies maintain steep or even perpendicular sides. It happens frequently that recent forms replace older forms so that their origin and age cannot be assessed from superficial observation. The main feature of gully erosion is the volume and velocity of water at the lowest level. The energy of flowing water increases its cutting and smashing power and often results in bank erosion.
Gully Erosion
Riverbank/Stream Bank Erosion
Stream/riverbank erosion occurs due to bank scour and mass failure. The direct removal of bank materials by the physical action of flowing water is called bank scour. It is often dominant in smaller streams and the upper reaches of larger streams and rivers. Mass failure occurs when large chunks of bank material become unstable and topple into the stream or river.
Riverbank erosion can be accelerated by lowering streambed, inundation of bank soils followed by rapid drops in water flow, saturation of banks from off-stream sources, removal of protective vegetation from stream banks, poor drainage, readily erodible material within the bank profile, wave action generated by boats, excessive sand and gravel extraction, and intense rainfall.
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