28 Soil Erosion

J.S. Rawat

epgp books

     Pre-requisites

 

Rocks, Weathering and Mass Wasting

 

Keywords

 

    Sheetwash Erosion, Rill Erosion, Bedload, Suspension, delluvium

 

 

 1.0 INTRODUCTION

 

Soil erosion is the wearing away detachment and transportation of soil from one place and its deposition at another place by moving water and blowing wind or any other cause. Erosion by definition is an exogenic process of removal of soil from the crust of the Earth and then transported to another location either inches centimeter or kilometers away. Studies reveal that due to the exogenic erosional processes, on an average, the earth surface is lowering at the rate of 3 cm per thousand year (Stoddart,1969) having variations from one area to another as depicted by the distribution map of erosion rates in the world (Fig.1). Asia carry some 80% of the world’s erosion to the oceans each year. In the low land areas the rate of erosion varies between 2.2 cm to 7.2 cm per thousand year while in mountains it varies between 20.6 cm to 91.5 cm per thousand year (Fig.2). The maximum rate of erosion, i.e., 91.5 cm per thousand year stands in the youngest mountain chain of the world,viz., the Himalaya (Schume, 1963; Corbell, 1964). Many factors influence the rate of erosion in a particular area which contribute the world wide variations in erosion. Map (Fig.1) showing the pattern of world erosion suggests that the highest annual yield of above 240 tonnes/km2 occur in south-east Asia where the influence of man is especially notable. The next highest in mountains and in south-eastern parts of the United State of America, followed by the rates in the tropics which are higher than those of temperate latitudes, in turn higher than those of the Artic ( Gregory and Walling, 1073).

Fig.1:Spatial variations in distribution of erosion in the world (after Strakhov,

 

1967).

 

Fig.2: Average rates of erosion in mountains, continents and low lands (Gregory and Walling, 1979).

    2.0 TYPES AND MEASUREMENT OF SOIL EROSION

 

Genetically, the soil erosion processes are divisible in to two major groups. These are water born and wind born erosion. The water born erosion is further divisible in two groups, i.e., pluvial (rainwater born) process and fluvial (stream water born) processes.

 

2.1 Water Erosion

 

The erosion caused by rainwater, i.e., rainsplash and sheetwash erosion are the outcome of pluvial processes, while erosion caused by channels water, i.e., rill erosion and gully erosion, suspended, dissolved and bed load flow are the out come of fluvial processes. The progressive removal of sediment and mineral material from valley sides and channel floor is known as channel erosion. A brief account of these different types of erosions caused by pluvial and fluvial processes is given in the following paragraphs.

 

2.1.1 Rainsplash Erosion– Direct force of rain drop causes splashing in which soil particles are detached, lifted and then dropped into new position. The rainwater from the sky hits the soil causing a disbursement of soil upon impact and creating a crater where the rain has hit (Fig.3). This is micro form of erosion in which soil particles are transported at a distance of millimeter to few centimeters. For measurements of rainsplash erosion high speed photography may be used to demonstrate the explosive effect of rain drop impact (Ellision, 1950) but for more quantitative measurements of soil splash can be done by using trays of soil or small field plots or troughs to catch the splash material. A rainfall simulator is also used to measure the rainsplesh erosion (Osborne, 1953, McQueen, 1963).

 

Fig. 3: Rainsplash erosion by raindrops (Source:https://www. google.co.in/search?q=Rainsplash+erosion+photo)

2.1.2 Sheetwash Erosion– The rainsplash soil particles are component of sheetwash erosion. During the course of overland flow, water flows on land surface in sheet form which is coupled with soil particles. This transportation of soil particles in sheet form by the overland flow is known as sheetwash erosion (Fig.4). To measure sheetwash erosion , Gerlach trough is installed at different slopes (Fig.5) . During rain, transported sheetwash material from the plot is caught by the Gerlech trough. By weighing this material, rate and volume of sheetwash erosion is estimated. Study of rainsplash and sheetwash erosion help in understanding the impact of anthropogenic activities. i.e., particularly deforestation and grazing.

 

Fig. 4: Sheetwash erosion by overland flow

(Source-https://www.google.co.in/search?q=sheetwash+erosion+photo)

Fig. 5: Gerlech trough (Below) installed at different slopes to measure sheetwash erosion (after Gregory, K. J. and Walling D.E.,1979).

 

2.1.3 Rill Erosion- Where hillslopes are steep and runoff from rain storm is extremely high, sheetwash erosion progresses into a more intense actively, that of rill erosion (Fig.6) or rilling. All the ephemeral and seasonal channel on steep hills of watershed starts rilling during high intensity rainfall and cause severe soil erosion and floods in the lowland areas of watershed. The amount of rill erosion can be measured in two ways. Firstly, by measuring material transported by rills at their outlets and secondly, attempt may be made to document the development of the rill system in both section and plan by periodic surveying and to calculate the amount of material removed (Tackfield, 1964).

2.1.4 Gully Erosion- Rills on the delluvium (i.e., mass wasting and regolith deposits) and alluvium (i.e., river terraces and fans) soon begin to integrate into still larger channels, termed as gullies (Fig.7).Gully erosion is an advanced stage of rill erosion Gullies are mainly confined on the gentle and level surfaces of watershed on its valley region. Measurement of development of gullies may be done by doing periodic monitoring of erosion pins or can also be documented both section and plan by periodic survey.

 

Fig. 7: Gully  erosion.

(Source-https://www.google.co.in/search?q=gully+photo)

.

 

2.1.5 Suspended load- Suspended load includes all those material which flows in the forms of suspension (Fig.8) with channel water such as clay, slit and some times fine sand. Watershed under natural conditions carry suspended sediment only during rainy period but channels of anthropogenically and technogenically disturbed watersheds may carry suspended load throughout the year.

Fig.8: River with suspended material (left) and without suspended material (right).

 

2.1.6 Dissolved load -Dissolved load includes chemical elements and compounds that are carried in solution with channel water. Usually the most abundant catations in the total dissolved solids are Ca++, Mg++, Na++, and K+ and dominant anions are usually Hco-3, So4 and No3-. Apart from these heavy metals phosphates and phenols are not uncommon. The major sources of dissolved load in a stream are notably precipitation, chemical weathering and erosion, atmospheric fallout, mineral springs and anthropogenic and technogenic activities of man. Measurement of sediment and dissolved loads require appropriate sampling technique. For suspended and dissolved load measurements series of water samples (Fig.9 left) are collected from a river or a water gauging station (Fig.9 right B) and by using gravimetric method concentration of suspended and dissolved loads are determined. If frequent measurements of suspended and dissolved load discharge are made at a stream gauging station having weir in order to define the variations through time, estimates of sediment yield can be computed on individual rainstorm, daily (during the rainy season) and weekly or fortnightly basis on dry period. At gauging site having no weir, the channel cross section is divided in different sub-sections of homogeneous flow and water samples are collected at 0.6 depth for suspended and dissolved loads by sediment samplers .

 

2.1.7 Bedload- Bed load includes the channel transportation material which move along the channel floor by rolling or sliding and on occasional low leaps. Bed load includes gravel, pebbles, cobble, block and boulders. The bed load samples are collected from the stream bed by using bedload sampler by constructing bedload trough (Fig.9 right B) across the channel behind the weir at the gauging station. In large rivers, basket sampler (Fig.10) is used to measure bedload. On larger rivers a lifting system to basket samplers from river bed is also required.

 

Fig.9: Water samples from different depth of river for determination of concentration of suspended and dissolved load in water (left after Gregory, K. J. and Walling D.E.,1979); and a hydrological station at a small stream (A) with bedload trough (B) (right).

Fig.10: A bedload sampler basket used to collect bedload samples from large river

(Source-Gregory, K. J. and Walling D.E.,1979).

    2.2 Wind Erosion

 

Wind erosion is a natural process. It is is a common cause of land degradation in the arid and semi arid regions of the world where average annual ranfall is between 100 mm to 150 mm only. The lighter texture soil in the low rainfall regions are the most susceptible to wind erosion. Wind erosion is one of the processes leading to desertification. Significant wind erosion occurs when strong winds blow over light -textured soils that have been heavily grazed during periods of drought. Wind erosion is also a natural process.

 

In the arid and semi-arid regions, wind is the main agent of erosion which develops arid landscape constituted of different types of erosional (i.e., incelbergs etc) and depositional landforms (i.e., sandunes etc.). The main factor in wind erosion is the velocity of moving air. Because of the roughness imparted by soil, stones, vegetation and other obstacles, windspeeds are lowest near the ground surface. From the surface at which the wind velocity is zero windspeed increases exponentially. The processes of wind erosion are divisible in three different types. These are- wind erosion, surface creep, saltation and suspension (Fig.11).

 

Fig.11: Three types of erosion by wind, i.e., suspension, saltation and creep

(source-Department of Environment and Resource Management, Quensland Government,www.derm.qld.gov.au) .

 

2.2.1 Suspension – Suspension describes the movement of fine particles, usually less than 0.1mm in diameter, high in the air and over long distances. Which can be moved into the air forming dust storms when taken further upwards by turbulence. These particles include very fine grains of sand, clay particles and organic matter. However, not all dust ejected from the surface is carried in the air indefinitely. Larger dust particles (0.05 to 0.1 mm) may be dropped within a couple of kilometres of the erosion site. Particles of the order of 0.01 mm may travel hundreds of kilometres and 0.001 mm sized particles may travel thousands of kilometres. Through this process, Australian soil has been carried to New Zealand and beyond. Fine dust may remain in suspension in the air until it is washed out by rainfall.

 

2.2.2 Saltation – Saltation is the process of grain movement in a series of jumps. It occurs among middle-sized soil particles that range from 0.05 mm to 0.5 mm in diameter. Such particles are light enough to be lifted off the surface, but are too large to become suspended. These particles move through a series of low bounces over the surface, causing abrasion on the soil surface and attrition (the breaking of particles into smaller particles).

 

2.2.3 Surface creep –Surface creep is the rolling of coarse grains along the ground surface. Larger particles ranging from 0.5 mm to 2 mm in diameter, are rolled across the soil surface. This causes them to collide with, and dislodge, other particles. Surface creep wind erosion results in these larger particles moving only a few metres.

 

3.0 FACTORS INFLUENCING EROSION

 

The important factors controlling soil erosion system are rainfall intensity and wind velocity also known as erosivity; physical property of soils and rocks known as erodibility; topography and slope; vegetation; and anthropogenic activities. A brief accounts of these influencing factors of soil erosion is presented in the following paragraphs.

 

3.1 Rainfall

 

The soil erosion is closely related to rainfall partly through detaching power of raindrop striking the soil surface and partly through contribution of rain to runoff (Morgan, 1979). Studies reveal that the average soil loss per rain event increases with the intensity of the storm (Table-1). The low intensity rain enter the soil where it strikes and some will slowly runoff, but the high intensity rain, there is not enough time for the water to soak and infiltrate through the soil and it runsoff causing soil erosion. The runoff that causes soil erosion, therefore, depends upon intensity, duration, amount and frequency of rainfall.

 

3.2 Soils Properties

 

Properties of soils both physical and chemical play significant role in determining the erodibility of any region because these defines the resistance of the soil to both detachment and transport. Thus, the erodibility is influenced by soil texture, structure, organic matter and chemical composition. Soil detachability increases as the size of the particle increases but soil transportability increases with the decrease in particle size. The fine textured soil clay particles are more difficult to detach than sand, but are easily transported even in level slopes because of very small particle size. During low intensity rain, there is less erosion in sandy soil because the rain water is absorbed readily due to high permeability in sandy soil. Similarly, more organic manure in the soil improves granular structure and water holding capacity, hence, as organic matter decreases, the erodibility of soil increases. The fine textured and alkaline soils are more erodible.

 

3.3 Topography and Slope

 

Topography with special reference to slope of land also play very significant role in determining erodibility of any region. Under same rain fall intensity and even under same soil properties, the erodibility may vary from place to place due to different topography specifically different slope conditions. Slope variation changes direction of rainsplash particles and also increases the length of their transportation. On flat surface raindrops splash soil particles randomly on all directions but on sloping surface more soil is splashed downslope to a larger distance, compare to upslope. Slope accelerates soil erosion as it increases the velocity of running water. Even a small difference in slope can make large difference in soil erosion. Hydrologic studies reveal that a four time increase in slope doubles the velocity of running water. This double velocity accelerates erosive power four times and the carrying capacity by 32 times. Thus, erosion would normally be expected to increase with increases in slope steepness and slope length as a result of increases in velocity as well as volume of surface runoff.

 

3.4 Vegetation Cover

    The ground cover, its nature and extent have direct relationship in soil erosion. The presence of vegetation retards erosion upto some limit. Trees, shrubs and grasses are more effective in providing cover to control soil erosion. The canopies of vegetation intercept the erosive beating action of falling raindrops retards the amount and velocity of surface runoff, permits more waterflow in to the soil for infiltration and creates more groundwater storage capacity in soils and rocks.In fact, it is the lack of ground cover that creates erosion permitting conditions.

 

The effectiveness of vegetation cover in controlling soil erosion depends upon the height, density and continuity of the canopy above the surface and the root density and depth of plants below the surface. The height of the plant canopy is important because water drops falling from 7m may attain over 90 percent of their terminal velocity. Further, raindrops intercepted by the plant canopy may coalesce on the leaves to form larger drops which are more erosive for rainsplash activity. Ground cover intercepts the rain; dissipates the energy of running water and winds; imparts roughness to the flow and thereby reduces its velocity, consequently erosion is largely controlled. Below the earth surface, the main effect of the plant root network is in opening up the soil, thereby enabling water to penetrate and increasing infiltration capacity (Morgam,1979).

 

Thus, vegetation cover is most effective in reducing erosion because of their canopy. For adequate erosion protection it is recommended based on various studies that at least 70 per cent of the ground surface must be covered (Fournier,1972; Elwell and Stocking, 1976).

 

3.5 Anthropogenic Activities

 

For the shake development, now man himself has become an active agent of erosion through its anthropogenic (i.e., deforestation, grazing, agriculture) and technogenic (i.e., various engineering works such as road and building construction) activities. Studies from Himalayan region reveal that due to anthropogenic and technogenic activities the rate of soil erosion has been accelerate considerably. A Himalayan study reveals that removal of forest cover and agricultural activities have greatly accelerated the rate of soil erosion (Rawat and Rawat, 1993). This study estimates rate of erosion as 0.09 mm/year on deforested land and 0.18mm/year on agricultural land, by contrast natural erosion rate vary between 0.02mm/year on oak forest and 0.04mm/year on pine forest. The two decade ago the erosion rate of Himalaya was estimated 0.91mm/year (Corbel, 1964; Schume,1963). More recen1,tly in the Central Himalaya, it has been estimated as 1.7mm/year (Valdiya and Bartarya, 1989). There are several factors that may contribute to this acceleration which includes deforestation, poorly managed agriculture, forest fire, over grazing, and substandard constructions of roads and buildings on ill suited sites. However, the actual impact of these human activities remains poorly understood and unquantified (Haigh,1989). Geormorphic study (Rawat et el.,2000) conducted in Uttarakhand Himalaya in India suggests that technogenic (urban) and anthropogenic (deforestation and agriculture) activies increase the intensity of erosion by a factor 2 to 47.

 

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