AEG-101-Class-4 : Forms of Water Erosion

Introductory Soil and Water Conservation Engineering

II Semester 3^rd Feb to 30^th June 2020, 2019-20

Teacher Information

Professor	Email	Phone
Dr. K. C. Shashidhar	shashidhar.kumbar@gmail.com	9448103268

Class-4 Reference Material

Forms of Water Erosion

Soil Erosion

Soil erosion is the detachment and transportation of soil material from one place to another through the action of wind, water in motion or by the beating action of the rain drops.

Erosion of soil by water is caused by its two forms- liquid as flowing water, and solid as the glaciers. The impact of rainfall causes splash erosion. Runoff water causes scraping and transport of soil particles leading first to sheet erosion, followed by rill erosion and lastly to gully erosion. Water waves cause erosion of bank sides of reservoirs, lakes and oceans. The sub-surface runoff causes soil erosion in the form of pipe erosion. The glacial erosion causes heavy landslides. In India, glacial erosions are mainly confined to Himalayan regions.

Causes of Soil Erosion

No single unique cause can be held responsible for soil erosion or assumed as the main cause for this problem. There are many underlying factors responsible for this process, some induced by nature and others by human being. The main causes of soil erosion can be enumerated as:

(1)    Destruction of Natural Protective Cover by

(i) indiscriminate cutting of trees,

(ii) overgrazing of the vegetative cover and

(iii) forest fires.

(2)    Improper Use of the Land

(i) keeping the land barren subjecting it to the action of rain and wind,

(ii) growing of crops that accelerate soil erosion,

(iii) removal of organic matter and plant nutrients by injudicious cropping patterns,

(iv) cultivation along the land slope, and

(v) faulty methods of irrigation.

Types of Soil Erosion

Classification According to Origin: Soil erosion can broadly be categorized into two types i.e. geologic erosion and accelerated erosion.

Geological erosion

• Referred as natural or normal erosion
• It takes place as a result of the action of water, wind, gravity and glaciers.
• Rate is slow
• The soil loss is compensated for by the formation of new soil under natural weathering process
• Considered as a part of the natural soil forming process
• As for as its adverse effects on agricultural lands are concerned geologic erosion is not of much consequence.

ii. Accelerated erosion

• When the vegetation removed and land is put under cultivation the natural equilibrium between soil building and soil removal is disturbed
• The removal of surface soil takes place at a much faster rate than it can be built up by the soil forming process.

Since erosion on cultivated land is of the accelerated type, the term soil erosion or simply erosion used here after will refer to accelerated erosion.

Soil erosion is caused primarily by two agents.

i) Water ii) Wind

Erosion by water

Erosion by water, known as water erosion, is the removal of soil from the lands surface by water in motion.

See Soil and water terminologies for more definitions

Basic processes

Detachment and transport are basic processes occurring on source area while transport and deposition are basic procedures occurring in sink area. Soil particles are detached either by rain drop impact of flowing water.

According to Erosion Agents:  Soil erosion is broadly categorized into different types depending on the agent which triggers the erosion activity. Mentioned below are the four main types of soil erosion.

(1) Water Erosion: Water erosion is seen in many parts of the world. In fact, running water is the most common agent of soil erosion. This includes rivers which erode the river basin, rainwater which erodes various landforms, and the sea waves which erode the coastal areas. Water erodes and transports soil particles from higher altitude and deposits them in low lying areas. Water erosion may further be classified, based on different actions of water responsible for erosion, as : (i) raindrop erosion, (ii) sheet erosion, (iii) rill erosion, (iv) gully erosion, (v) stream bank erosion, and (vi) slip erosion.

(2) Wind Erosion: Wind erosion is most often witnessed in dry areas wherein strong winds brush against various landforms, cutting through them and loosening the soil particles, which are lifted and transported towards the direction in which the wind blows. The best example of wind erosion are sand dunes and mushroom rocks structures, typically found in deserts.

(3) Glacial Erosion: Glacial erosion, also referred to as ice erosion, is common in cold regions at high altitudes. When soil comes in contact with large moving glaciers, it sticks to the base of these glaciers. This is eventually transported with the glaciers, and as they start melting it is deposited in the course of the moving chunks of ice.

(4) Gravitational Erosion: Although gravitational erosion is not as common a phenomenon as water erosion, it can cause huge damage to natural, as well as man-made structures. It is basically the mass movement of soil due to gravitational force. The best examples of this are landslides and slumps. While landslides and slumps happen within seconds, phenomena such as soil creep take a longer period for occurrence.

Erosion may be classified as

1. Raindrop erosion
2. Sheet erosion
3. Rill erosion
4. Gully erosion
5. Stream channel erosion
6. Mass erosion / movement

Raindrop Erosion

Raindrop erosion is soil detachment and transport resulting from the impact of water drops directly on soil particles or on thin water surfaces. The impact of raindrops breaks the soil crust and splashes the soil particles away. The mass of each raindrop is directly proportional to its kinetic energy. Wischmeier and Smith (1978) gave the following equation for the calculation of K.E.

Where,

                  E = kinetic energy of raindrop (MJ/ha-mm),

                   I = rainfall intensity (mm/hr).

Raindrop impact (splash). (Source: http//utexas.edu)

Erosivity of Rainfall: The potential ability of rain to cause erosion is called erosivity of rainfall. Erosivity is expressed by EI₃₀ index. Using rainfall and soil loss information from experimental plots Weischmeier et al. (1958) concluded that best estimator of soil loss is a compound parameter, the product of kinetic energy of the storm and the maximum intensity of storm during a continuous period of 30 minute.

The EI₃₀ index was developed under American condition and it is reported not to be very accurate under tropical and subtropical conditions. An alternate method was developed to describe the erosivity of rainfall. This is based on concept that low intensities of rainfall cause no erosion and there is a threshold value of intensity at which rain starts to become erosive. Experimental evidence confirmed this concept and it was observed that rainfall intensities less than 25 mm/h have not resulted in significant erosion. The index for erosivity as KE > 25, indicating that the total kinetic energy of the rain falling at intensities greater than 25 mm/h is considered as erosive.

Sheet erosion

• The removal of a thin relatively uniform layer of soil particles by the action of rainfall and runoff.
• Extremely harmful
• Usually so slow that the farmer is not conscious of its existence
• Common on lands having a gentle uniform slope
• Results in the uniform skimming off the cream of the top soil with every hard rain
• To the eye the field appears to be the same as before
• Shallow top soil overlies a tight sub soil are most susceptible to sheet erosion
• Movement of soil by rain drop splash is the primary cause of sheet erosion
• Sheet erosion has damaged millions of hectares of slopping land throughout the India.

Rill erosion

• Rill erosion is the removal of soil by running water with the formation of shallow channels that can be smoothed out completely by normal cultivation
• There are no sharp lines of demarcation where sheet erosion and rill erosion begins but rill erosion is more readily apparent than sheet erosion
• Rills developed when there is a concentration of runoff water which if neglected grow into large gullies
• More serious in soils having a loose shallow top soil
• Transition stage between sheet erosion and gullying

Gully erosion

• Removal of soil by running water with the formation of channels that cannot be smoothed out completely by moved cultivation
• Advance stage of rill erosion
• Any concentration of surface runoff is a potential source of gullying
• Cattle paths, cart tracks, dead furrows, tillage furrows or other small depression down a slope favor concentration of flow.
• Unattended rill get depended and widened every year and begin to attain the form of gullies
• Every year increases
• Within in a few years’ time an entire landscape may be filled with a network of gullies
• Spectacular than other type of erosion

Stream channel erosion

• Erosion caused by stream flow
• Closely resembles rill erosion
• Intensive channel erosion areas are on the outside of lands where flow shear stresses are high.

Mass movement

• Mass movement of soil
• Although mass studies have been widely studied by geologists, geomorphologists and engineers, it is generally neglected in the context of soil erosion.
• Landslides, land slips, soil and mudflows are various forms of mass movement.

Agents of Soil Erosion

Soil erosion is the detachment of soil from its original location and transportation to a new location. Mainly water is responsible for this erosion although in many locations wind, glaciers are also the agents causing soil erosion. Water in the form of rain, flood and runoff badly affects the soil. Soil is in fact a composite of sand, silt and clay. When the rain falls along the mountains and bare soil, the water detaches the soil particles, and takes away the silt and clay particles along with the flowing water. Similarly, when wind blows in the form of storms, its speed becomes too high to lift off the entire soil upper layer and causes soil erosion.

Other factors responsible for soil erosion are human and animal activities. Vegetation is the natural cover of soil. When the animals continuously graze in the pastures, the vegetation is removed due to their walking and grazing. Bare lands left behind are easily affected by soil erosion. Activities of human like forest cutting, increased agriculture, and clearing of land for different purposes are the other agents that cause erosion of the soil.

Soil erosion agents, processes and effects. (Sources: Das, 2000)

Factor affecting erosion by Water

E = f (CTVS)

Erosion is directly a function of rainfall and runoff. So, factors affecting runoff will have the same effect on erosion also.

Climate: Climatic factors affecting erosion are precipitation, temperature, wind, humidity, and solar radiation. Temperature and wind are the most evident through their effects on evaporation and transpiration. However, wind also changes the raindrop velocities and the angle of impact. Humidity and solar radiation are somewhat less directly involved in that they are associated with temperature and the rate of soil water depletion.

Rainfall intensity

• It is a chief detaching agent in water erosion
• If intensity is high then erosion will be serious

Duration of rainfall

If duration is more quantity of runoff is more so erosion will also be made.

Distribution

Higher the distribution coefficient higher will be the erosion.

Direction

If the direction and land slope are opposite them we can see the most striking difference Temp. & wind.

Watershed characteristics

• Geologic features
• Size and shape
• Topography
• Drainage pattern

Soil: Physical properties of soil affect the infiltration capacity and the extent to which particles can be detached and transported. In general, soil detachability increases as the size of the soil particles or aggregates increase, and soil transportability increases with a decrease in the particle or aggregate size. That is, clay particles are more difficult to detach than sand, but clay is more easily transported. The properties that influence erosion include soil structure, texture, organic matter, water content, clay mineralogy, and density or compactness, as well as chemical and biological characteristics of the soil.

Sandy soil

Average rain – no problem of erosion.

High intensity – More serious of less finding material i.e. fine soil particle

Clay soil: Ordinary rain – more runoff in moderate & steep slopes but high water holding capacity. Silt loam, loamy and fire sandy loam are the more desirable soils from the point of view of minimising soil erosion.

Size and shape effect like runoff.

Topography: Topographic features that influence erosion are degree of slope, shape and length of slope, and size and shape of the watershed. On steep slopes, runoff water is more erosive, and can more easily transport detached sediment down the slope. On longer slopes, an increased accumulation of overland flow tends to increase rill erosion. Concave slopes, with lower slopes at the foot of the hill, are less erosive than convex slopes.

If Slope increases by one-time velocity will increase by two times- When the velocity is doubled the erosive capacity is doubled the erosive capacity as represented by the kinetic energy of the flowing water is increased about 4 times. The amount materials of a given size that can be carried is increased about 32 times of the size of particles that can be transported by pushing or rolling in increased about 64 times. There is a limit after the limit this relation will not very much.

Soil loss by erosion is proportional to the length of slope to the power 0.5 (E & VI).

Vegetation/Biological Factors of Soil Erosion: Biological factors that influence the soil erosion are the activities like faulty cultivation practices, overgrazing by animals etc. These factors may be broadly classified into following three groups:(i) Energy factors, (ii) Resistance factors, and (iii) protection factors.

(i) Energy Factors: They include such factors which influence the potential ability of rainfall, runoff and wind to cause erosion. This ability is termed as erosivity. The other factors which directly reduce the power of erosive agents are reduction in length/degree of slope through the construction of terraces and bunds in case of water eroded areas and creation of wind breaks or shelter belts in case of wind eroded areas.

(ii) Resistance Factors: They are also called erodibility factors which depend upon the mechanical and chemical properties of the soil. Those factors which enhance the infiltration of water into the soil reduce runoff and decrease erodibility, while any activity that pulverizes the soil increases erodibility. Thus, cultivation may decrease the erodibility of clay soils but increases that of sandy soil.

(iii) Protection Factors: This primarily focuses on the factors related to plant cover. Plant cover protects the soil from erosion by intercepting the rainfall and reducing the velocity of runoff and wind. Degree of protection provided by different plant covers varies considerably. Therefore, it is essential to know the rate of soil erosion under different land uses, degrees of length and slope, and vegetative covers so that appropriate land use can be selected for each piece of land to control the rate of soil erosion. The quantity of soil moved past a point is called soil loss. It is usually expressed in unit of mass or volume per unit time per unit area.

Mechanics of Soil Erosion

Soil erosion is initiated by detachment of soil particles due to action of rain. The detached particles are transported by erosion agents from one place to another and finally get settled at some place leading to soil erosion process.

Process of water erosion by the impact of raindrops.

(Source: www.landfood.ubc.ca)

Mechanics of soil erosion due to water and wind is discussed below.

Mechanics of Water Erosion

There are three steps for accelerated erosion by water:

i)   Detachment or loosening of soil particles caused by flowing water, freezing and thawing of the top soil, and/or the impact of falling raindrops,

ii)  Transportation of soil particles by floating, rolling, dragging, and/or splashing and

iii) Deposition of transported particles at some places of lower elevation.

Rain enhances the translocation of soil through the process of splashing as shown in Fig.2.2. Individual raindrops detach soil aggregates and redeposit them as particles. The dispersed particles may then plug soil pores, reducing water intake (infiltration). Once the soil dries, these particles develop into a crust at the soil surface and runoff is further increased.