Erosion, Flooding and Sediment of Water Resources Development
This process acting with other natural agents such as wind and ice leads to erosion. The question now is what is erosion? What are the agents of erosion, and how can the problem of erosion be controlled? Answers to these questions will be the focus of this article.
Definition of Erosion
Erosion is the wearing away and removal of soil and rock fragments at the surface of the earth by wind, water, ice, or other natural agents. It is part of the process known as gradation, which besides the wearing away of rocks to form sediments also involves the transportation and eventual deposition of the sediments.
Working together, the processes of erosion and deposition have done much to shape the earth’s surface (Encyclopedia Americana, 1990).
The New Encyclopedia Britannica (2007) defines erosion as the removal of surface material from the Earth’s crust, primarily soil and rock debris, and the transportation of the eroded materials by natural agencies from the point of removal.
Erosion will often occur after rock has been disintegrated or altered through weathering. Weathered rock material will be removed from its original site and transported away by a natural agent.
With both processes often operating simultaneously, the best way to distinguish erosion from weathering is by observing the transportation of materials. Erosion is the part of a cycle of geological events that may be complemented by processes that originate beneath the earth’s surface. For example, mountain ranges are first raised by great crustal movements and are then slowly worn away by the agents of erosion.
Agents of Erosion
Rocks are continually being worn away and land areas gradually lowered as agents of erosion attack the minerals of which the rocks are made. The fragments thus produced are carried elsewhere by wind, water, ice or other transporting agents.
Most of the sediments eventually are deposited in the ocean, where they settle at the bottom to probably become the sedimentary rocks of the geological future. There are five principal agents of erosion- running water, wind, groundwater, glacier and marine.
Streams have been recognized to cause more erosion than all other geological agents combined. The erosion starts when rain begins to runoff the earth’s surface. The runoff at first is sometimes in the form of sheets of moving water, but such sheets usually are soon channeled into streams and rivers.
The ability of a stream to erode is governed by several factors. Thus an increase in the volume of water and in the velocity of stream flow greatly increases the rate of erosion.
Streams running down steep slopes in areas that lack vegetation are especially effective erosional forces. Stream volume and velocity also determine the stream’s sediment load at a given time.
Transported rock fragments become cutting tools that widen and deepen a channel by rubbing against the channel bed. The fragments also are eroded by knocking against each other, and some rocks are slowly dissolved by the stream water as well.
In addition, turbulent water may remove materials from the stream channel by a “plucking” action caused by hydraulic pressure, or by cavitations, an erosive process caused by the sudden collapse of vapour bubbles against the channel wall.
Wind erosion is more likely to occur in dry areas where soil is loose and unprotected by vegetation. Winds of high velocity in these areas carry a load of rock fragments that cause wind abrasion.
Evidence of such abrasion is seen on rock fragments with facets that have been cut and polished by the sandblasting action of windblown sediments, and in rocks undercut to form table rocks or pedestals.
Read ALso : Soil Erosion: Meaning, Types and Factors of Soil Erosion
A form of erosion that occurs when loose rock particles are blown away by the wind is called deflation. Features produced by deflation include blowouts, and desert pavement.
Erosion by groundwater takes place just beneath the earth’s surface. Carbon dioxide from the air and decaying organic matter from the soil may combine with groundwater to form carbonic acid. Groundwater containing carbonic acid and circulating through rocks commonly removes soluble minerals and carries them away in solution.
Glaciation is a powerful erosional agent. As a glacier advances, the moving ice quarries pieces of material from the bedrock over which it passes. The fragments become embedded in the ice and serve as abrasive tools that further scratch and gouge the bedrock.
The ocean acts as an erosional agent primarily through waves and the currents produced by waves. Most ocean waves are generated by wind currents that agitate the water’s surface.
Large waves may pound the shore with tremendous force, producing a hydraulic- lifting action that dislodges large masses of coastal rock. The blocks fall to the ocean bottom, where they undergo more erosion along with other wave-battered rock debris that has accumulated there.
The sand and rock fragments grind against each other and scour the shore. In areas where the coast is formed of soluble rocks, the ocean may also erode the rocks by dissolving their minerals.
Erosion can be controlled through various measures, which include reforestation, contour ploughing, strip cropping, crop rotation, control grazing and deforestation, avoidance of over cultivation amongst other measures.
Reforestation is the restocking of existing forests and woodlands which have been depleted. Trees can be used as windbreakers or shelterbelt – a plantation usually made up of one or more rows of trees or shrubs planted in such a manner as to provide shelter from the wind and to protect soil from erosion. They are commonly planted around the edges of fields or farms.
Contour ploughing or contour farming is the farming practice of ploughing across a slope following its elevation. The rows formed slow water runoff during rainstorms to prevent soil erosion and allows the water time to settle into the soil.
Strip cropping is a method of farming used when a slope is long and steep or when other types of farming may not prevent soil erosion. Strip cropping helps to stop soil erosion by creating natural dams for water, helping to preserve the strength of the soil.
Certain layers of plants will absorb minerals and water from the soil more effectively than others. When water reaches the weaker soil that lacks the minerals needed to make it stronger, it normally washes it away.
However, when strips of soil are strong enough to slow down water from moving through them, the weaker soil can’t wash away like it normally would, thereby preventing erosion.
The carrying capacity of land for cattle varies with the rainfall and the fertility of the soil. However, when the carrying capacity is exceeded, the land is overgrazed.
Overgrazing reduces the diversity of plant species, lead to reduction in the growth of vegetation and dominance of plant species that are relatively undesirable to the cattle, increases the loss of soil by erosion as the plant cover is reduced and results to damage from the cattle trampling on the land.
For example, paths made as the cattle travel to the same water hole or stream develop into gullies, which erode rapidly in the rain. To control erosion caused by overgrazing, proper care should be taken to ensure that the carrying capacity of the land is not exceeded.
Another practice that encourages erosion is deforestation – harvesting trees for commercial and other uses and burning forest to convert lands to agricultural purposes. This practice exposes the land to the active agents of erosion.
Deforestation has to be properly managed by ensuring that trees are replanted to replace fell ones, and the forest has to be given time to regenerate. This management method will help in achieving sustainability of the forest, and consequently reduce erosion.
Over cultivation of farmlands is another negative agricultural practice that promotes erosion. Over cultivation exposes the land for erosion.
For example, when a particular farmland is tilled year in year out, the soil structure is affected, and the loose soil can easily be eroded away by rain or wind.
The stoppage of over cultivation will help to maintain the soil structure, thereby enhancing its ability to withstand erosion.
Another effective way of preventing soil erosion is the practice of crop rotation. This practice can greatly affect the amount of soil lost from erosion by water.
In areas that are highly susceptible to erosion, farm management practices such as zero and reduced tillage can be supplemented with specific crop rotation methods to reduce raindrop impact, sediment detachment, sediment transport, surface runoff, and soil loss.
Protection against soil loss is maximized with rotation methods that leave the greatest mass of crop stubble (plant residue left after harvest) on top of the soil.
Stubble cover in contact with the soil minimizes erosion from water by reducing overland flow velocity, stream power, and thus the ability of the water to detach and transport sediment.
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Crop rotation helps to prevent the disruption and detachment of soil aggregates that cause macrospores to block, infiltration to decline, and runoff to increase. This significantly improves the resilience of soils when subjected to periods of erosion and stress.
In summary, even though most people see erosion as a negative process, it also has its benefits to man though they are usually not so striking. Erosion usually helps is breaking down bedrocks to form soil.
However, erosion unless kept in check, also destroys soils and removes them from the land. Urban development aggravates some erosional problems, as when housing projects are built on unstable soils that are subject to landslides.
In addition, commercial operations such as strip mining, quarrying, and poor agricultural practices take their toll on precious soil. Consequently, soil-conservation practices are becoming increasingly important as population grows and land areas are abused on a wider scale.
Erosion is the wearing away and removal of soil and rock fragments at the surface of the earth by wind, water, ice or other natural agents.
There are five principal agents of erosion, which include running water, wind, groundwater, glacier and marine.
Some measures of erosion control include reforestation, contour ploughing, strip cropping, crop rotation, control glazing and deforestation, and stoppage of over cultivation.
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