Short essay on the Mechanism of Soil Erosion

In nature, the soil forming processes under a particular climato-floral environment is substantially in balance with those of destruction under the normal geological erosion. Soil formation, arising out of the decomposition of parent rock material and proceeding from the surface downwards, is exceedingly slow under natural condition and as many as 300 to 1000 years are required to build a single inch of topsoil.

The normal erosive process is in equilibrium to this extremely slow pace of soil formation under natural vegetative cover. Any interference in this equilibrium by any imprudent act of man leads to an accelerated rate of erosion, endangering the agricultural prospects of the land. Such accelerated or abnormal loss of soil is usually implied by the term ‘soil erosion.’

While the effects of the attack of the soil erosion over the land are felt in three ways, such as the removal of plant food in solution, the removal of fine silt and humus in soil suspension, and finally the removal by scour of the soil surface itself. Erosion in the fields manifests itself in three main types’ sheet erosion, rill erosion and gully erosion.

Sheet erosion is more or less even removal or skimming off of the soil in very thin layers over an entire area of sloping land, consequent upon a heavy shower.

Though often inconspicuous, it is the most insidious type of soil erosion, resulting in the gradual removal of dark coloured humus-charged surface layer and exposure of relatively infertile subsoil strata, accompanied by a gradual fall in the yield. While it is more or less conditioned by the topographic features, climatic conditions and erosion of soil, in general, bare areas with a loose and thin surface layer overlying a heavy sub soil of low permeability, are particularly susceptible to sheet washing. It is a predominating feature on soils of high silt-content, on stiff clays and on all soils deficient in organic matter.

Sheet erosion merges imperceptibly into rill erosion, the ultimate stage of which is the development of gullies, so that rill erosion represents an intermediate stage between sheet erosion and gulling.

In rill erosion the runoff water tends to concentrate in thin rivulets and streamlets, which in turn tend to converge on larger channels and ditches over the whole area, thus often attaining a dendrite pattern of stream-flow.

The effect of this rill erosion over the land is characterised by a crisscross patterned incisions left in the field, which may ultimately develop into a system of well defined gullies. Rill erosion is often very common in areas of intense precipitation and in land of low absorptive capacity and may be virulent on soils ever laying a dense sub soil.

When rill erosion reaches the advanced stage, the increasing volume and velocity of water discharging through the rill channels would cut deep incisions or gullies over the land surface. Such gullies may also originate due to slight depressions on the land, where rainwater normally concentrates.

Once such gulling process has started, the incisions go on expanding in size and volume, more or less influenced by the relative stiffness or resistance of the soil cover and the underlying rock stratum. In many soils and alluvial valleys, where the surface soil and subsoil are friable and erodible, gully erosion tends to develop vertical walls, resulting from the collapse of the banks.

They rapidly develop branching systems and headword erosion is also very noticeable. But where heavy textured subsoil underlies a light surface soil or where geological substratum is resistant to erosion, gullies develop sloping banks and V-shaped cross-profiles. When a stratum of loose and soft rock material underlies a resistant surface layer, gullies will at first take V- shapes, but after the running water has cut through this resistant surface layer, continual under cutting and caving will result in the development of U-shaped valleys.

These gullies may be of enormous size and shape, presenting a serious problem for their control. In regions of exceedingly heavy and plastic clay in humid lands, the tough and adhesive wet clays do not yield to the abrasion of water and after prolonged erosion develop broad and shallow gullies with gently sloping sides.

In every type of gully erosion, the down cutting process proceeds upstream along the main channel and its tributaries and at the so called knick-points miniature waterfalls may develop finally, the under-mining of the subsurface layer at the heads of the trenches, and the gradual recession of these falls up stream contribute to the development and extension of an intricate system of gullies over the entire area.

Often all the types of gully erosion are associated in a single-segment of land, giving rise to an intricate maze of deep ravines, gullies, waterfalls and interlocking gully banks.