Erosion

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The relief of our planet consists of convexes (mountains), flat surfaces (valleys) and concaves (lakes). These forms of relief are created from a series of νatural phenomena, which are divided in two categories:

  • Τhese that produce the heterogeneity of relief (movement of tectonic plates, orogenesis, etc) and 
  • These that tend to smoothen the produces convexes (erosion, soil fluction, landslide, etc).

Erosion is the progressive decomposition of soil sediments in time influenced by environmental factors, such as wind, water, ice or even the living organisms (bioerosion), due to chemical reactions.

Erosion development

Figure 1: Perspective development of erosion
Hypothetical non-eroded surface (left by continental glaciations or sea-level drop)

Broad uplands, stream valleys narrow and few

Increased of networking in hydrographic network

Broad valleys, incised uplands

Broad valleys, isolated uplands


Water erosion (Figure 1) functions as an “emery cloth” that continuously and uninterruptedly erodes the surface of the earth’s crust from the upper parts of the relief (mountainous areas of the watershed) to the low altitude regions. The produced sediments are drifted from the surface waters that flow and end in lower regions with more gentle slopes (plateaus, hydrographic network, rivers, sea) (Figure 2).


Figure 2: Longitudinal development of erosion

Erosion and vegetation
Erosion, as it is already mentioned, erodes the soil surface. The soil does not have self-protection mechanisms. However, in order to be protected, it creates the proper conditions (moisture retention, nutrients’ availability) for vegetation appearance (grass, bushes, trees) in its surface. Vegetation assists soil in many ways:

  1. The existence of root system contributes to better soil “containment”. The more extensive the root system, the more soil is held around it. Moreover, it causes better soil ventilation.
  2. Higher and larger plants (bushes, trees) cover, also, bigger soil part in a more permanent basis, reducing the rain falling speed on it. The smaller ones (grass, brushwood that usually coexist with the first ones), reduce the runoff speed.
  3. The existence of plants (and their life circle in nature) contributes, first, in the porosity increase (and hence to the greater infiltration capacity) and second in the enrichment with organic matter, which, in turn, contributes in the enovation of vegetation, the greater soil consistency and erosion decrease.

So, the natural system soil-vegetation can be self-regulated, with its harmonious function, for its protection.

Figure 3: Soil and vegetation


Many times, however, this self-regulation cannot function due to the existence of external factors such as:

  1. The deforestation logging,
  2. The predatory grazing,
  3. The incendiarms

which are violent human interventions on the system “soil-vegetation”. In addition, any king of human intervention cannot be stopped but it can be done systematically and reasonably with ultimate goal the sustainable management of natural resources.

Factors that affect erosion
Erosion, from purely natural phenomenon of the development of the relief up to the accelerated erosion with its disastrous consequences for humans and natural invironment in general, is affected from factors both in intensity and extent such as vegetation (mentioned before), the soil slope, the bedrock, the soil consistency, the recipitation (rain, hail, snow) and the land uses.

  • Soil relief slope. The larger it is, the larger the runoff water speed (as long as it is not infiltrated to the deeper soil layers and, finally, to the aquifer) and, hence, larger the sediment magnitude that can carry and transfer to the lower elevation regions.
  • The kind of bedrock. It affects the precipitation impact so that various slopes
    are created respectively.


Figure 4: Affection of bedrock in slope formation


As it can be seen in Figure 4, case A (more resistant layer for cliff creation, sandstone or limestone in dry climate) creates very steep slopes while case B less resistant layer for slope formation, typically shale) creates gentle slopes. Among these two cases, there are intermediate ones with respective slopes
formation with the result of a multi-formed relief.

  • The soil consistency. The smaller the specific weight and the “cohesion forces” of soil substances (depends on the kind of bedrock), the easier the carry and transport of sediment due to surface runoff and vice versa.
  • The precipitation intensity. Mediterranean territories (such as Greece) accept in some seasons (autumn and winter) rapid rainstorms, in mountainous areas mostly, where the event rapidness grows. The bigger the precipitation intensity, the larger the water magnitude that does not have time to be penetrated (infiltrated) underground and the surface runoff.
  • Land uses. They affect the soil erosion rate significantly by reacting to the formation of the runoff coefficient of each region:
    i. Urban areas: They have the bigger runoff coefficient due to lack of green cities so that the precipitation magnitude that is running off in a high percentage.
    ii. Agricultural areas: They have a medium runoff coefficient between the urban and pasture-forest areas. The runoff coefficient depends on the crop species and tillage practice.
    iii. Pasture-forest areas: They have the smaller runoff coefficient, among the others. The number of “layers” and the species mixture rate limit he water runoff.

Soil mitigation measures from erosion:
Indicative measures are:

  1.   Rational design of “landuse”, in order to reduce the runoff coefficient and for vegetation maintainance and protection.
  2. Rational design of crops in areas assigned for farming. Preference in tree crops where the erosion risk is greater, avoidance of agricultural practices which contribute to the impairment of soil organic matter (intensive farming, excessive use of fertilizers and plant protection products) and practices encouragement that enrich the organic matter (crop rotation, fallow, etc.). 
  3. Use farming practices that contribute to the reduction (in microclimate) of the relief slopes: plowing along the contours, and “roof” and sill creation etc.
  4. Rational management planning of forest resources (ban of deforestation logging, predatory grazing, arson prevention measures) to prevent the soil stripping and desertification.
  5. Grazing management to maintain the pastures in perpetuity for soil protection.
  6. Tech works to keep the terrain (walls, terraces, belts, branch batches, log batches, micro-dams, etc.).
  7. Torrent management for the limitation of abduction of sediments from the highlands to the lowlands and the flood occurrence. The mitigation measures, in medium- and long-term period, operated non-erosively for the entire catchment.