Basic Science

Environmental hazards


Environmental hazard is a term used for any situation or state of events which poses a threat to the surrounding environment and adversely affect plants and animals

SOIL EROSION is the washing away of the soil by heavy rain or wind resulting to the formation of gully and landslides and leaving behind silt on which plants can no longer grow.

Soil erosion can also be defined as the removal of topsoil faster than the soil forming processes can replace it, due to natural, animal, and human activity (overgrazing, over cultivation, forest clearing, mechanized farming, etc.). Soil erosion results in land infertility, leads to desertification and devastating flooding.


The causes of soil erosion are as follows:


Soil Erosion

  1. Excessive Rain fall: Due to excessive rain fall, top fertile soil is washed away.
  2. Human activities: Human activities accelerate disappearance of protective cover of natural vegetation and cause soil erosion.
  3. Over grazing: Over grazing leads to the absence of ground-vegetation, causes gradual depletion of soil organisms and soil erosion.
  4. Land use: Humans play a major role in soil erosion through their use and abuse of natural resources, for example deforestation, grazing, arable land use, faulty farming systems, high crop intensity, construction, mining, etc.
  5. Climate: The two most important climatic factors having a direct effect on erosion are precipitation and wind velocity.
  6. Landforms: Slope, gradient, slope length and shape of slope are the important variables of landform that affect erosion processes for all types of soil erosion, e.g., splash, sheet, rill, and gully erosion.
  7. Bush burning


The following are the types of erosion:

  1. WATER EROSION: Water erosion is the removal and carrying away of soil particles by rain, running water, melting ice running rapidly over an exposed soil surface.

2. WIND EROSION: This is the blowing away of the soil particles or topsoil by wind. Wind erosion is common in arid or dry land where there is little or no rainfall


Cross-Section of Soil

Soil forms over many thousands of years from weathered rock fragments and the decaying remains of living organisms. As soil develops, it forms distinct layers, known as horizons. Each horizon has a specific colour, texture, and mineral content, as seen in the vertical cross-section of soil above. The number and type of horizons in a particular soil vary, but in general the uppermost horizon of soil forms the nutrient-rich topsoil. Beneath the topsoil lies the subsoil, which contains minerals that have trickled down from the topsoil. Rock fragments reside below the subsoil, and the horizon forming the foundation of soil consists of unweathered parent rock.


The effects of soil erosion are as follows:

  1. Reduction in soil quality which results from the loss of the nutrient-rich upper layers of the soil
  2. Reduces water-holding capacity of soil.
  3. Displacement of people from their homes
  4. Destruction of farmlands
  5. Collapse of building
  6. Damage of soil surface and roads


The following are measures of controlling soil erosion:

  1. Planting of vegetative cover: Cover crops such as trees, grasses, shrubs can be planted to cover the soil surface. This will reduce the effect of rainfall on the soil. The roots of the vegetative plants help to bind the soil particles together.
  2. Avoidance of indiscriminate bush burning: Bush burning exposes the soil to erosion because the vegetative covers are burnt.
  3. Avoidance of overgrazing: Land should not be overgrazed to prevent water erosion
  4. Mulching: This is the covering of ridges, beds and mounds in the farm with leaves, dry grasses and straw. Mulching reduces the impact of raindrops on the soil and thereby prevents erosion
  5. Crop rotation: This is the growing of certain crops in an order and rotating them every planting season in order to maintain soil fertility and also prevent water erosion
  6. Ridging across slope: Ridges should be constructed across and not along slopes to reduce the speed of run-off.
  7. Application of organic manure: Organic matter prevents loss of water from the soil and makes the soil moist at all time. This makes the particles of the soil heavy for wind to blow away.
  8. Irrigation: Artificial application of water to the soil will make the soil moist, thereby binding the soil particles together and preventing the soil particles from being blown away by wind.
  9. Planting of leguminous crops
  10. Education



Flooding can be defined as an overflowing of water onto land that is normally dry. It also occurs when there is more water on the surface of the land than it can take. This may lead to river overflowing its banks.


Erosion Endangers Shore Home

Natural forces such as wind, rain, and temperature cycles continually erode the land. This Chesapeake Bay home was built on a cliff that has undergone severe erosion. Many people battle erosion by planting grass and trees to form a strong root system or laying down netting to keep rock and soil from washing away.

Causes of Floods

The causes of floods are:

  1. Deforestation: When large areas of forests near the rivers are cleared, the land may be used for settlement, roads and farmland. Less vegetation protects the soil, the soil is quickly lost to rivers and sea. This raises the river bed, so the river overflows its banks easily, and then flooding occurs
  2. Poor water management: When the dams are poorly constructed or maintained, they can easily collapse and these results in flooding.

3. Population pressure: Because of large amounts of people, more food, wood, etc are needed for consumption; thereby leading to over-cultivation of lands which le ad to erosion and increases the risk of flooding.


Glacial Deposition

Glaciers, such as this icy formation in Switzerland, deposit materials as they grow and shrink. Glacial deposition is a process that includes the deposits laid down by the glaciers and the landforms that these deposits form. Glaciers transport till, materials such as rocks, sand, and clay. Till collects to form features such as terminal and lateral moraines.

  1. Amount of rainfall: The amount and intensity of rainfall determine the amount of run-off on the land and this will determine the amount of water available for flooding to occur.
  2. Closeness to sea or ocean:When the level of water rises above the bank of the sea or river, flooding occurs in areas close to the sea or river
  3. Social habit of the people:In areas where people tend to build structures to block drainage system, there may be blockage of water channels and free flow of water is hindered which may lead to flooding.
  4. Type and condition of the soil:


The effects or consequences of flooding on communities are as follows:

  1. Loss of human life
  2. Damage to property
  3. Deterioration of health conditions owing to waterborne diseases.
  4. Communication links and infrastructure such as power plants, roads and bridges are damaged and disrupted, some economic activities may come to a standstill
  5. People are forced to leave their homes and normal life is disrupted
  6. Disruption of industry can lead to loss of livelihood.
  7. Damage to infrastructure also causes long-term impacts, such as disruptions to supplies of clean water, wastewater treatment, electricity, transport, communication, education and health care.
  8. Reduction in purchasing power and loss of land value in the floodplains can leave communities economically vulnerable.
  9. Floods can also traumatise victims and their families for long periods of time.


Flooding on farmland has the following effects:

  1. Widespread damage to crops and fencing
  2. Loss of livestock
  3. Waterlogged soils
  4. Delay in harvesting are further intensified by transport problems due to flooded roads and damaged infrastructure.
  5. Food prices increase due to shortage in supply.


The following are the ways to prevent flooding:

  1. Construction of dams to take excess water that may lead to flooding
  2. Construction of reservoir that will hold excess water which can be used for other purposes
  3. Clearing of gutters of waste that can prevent free flow of water
  4. Prevention of construction of structures along river and sea banks
  5. Having adequate and proper town planning which will bring about construction of adequate drainage system
  6. Education


Drainage is the process by which water or liquid waste is emptied from an area.


The types of drainage patterns are :

  1. SURFACE DRAINAGE SYSTEM: This type of drainage system is found in rural areas and it is not designed by qualified engineers. The drainage is exposed and water can easily overflow the drainage and flood the area.
  2. SUB-SURFACE DRAINAGE SYSTEM: The sub-surface system is covered with concrete while water flows underground, thereby preventing overflow of the boundaries. It is designed by qualified engineers. This type of drainage is usually found in cities such as Abuja, Lagos, e.t.c.


  1. Explain flooding
  2. List five causes of flooding
  3. Explain four effects of flooding
  4. Mention and explain four activities of man that promote flooding
  5. Explain how flooding can be prevented or controlled
  6. Explain two types of drainage system
  7. Define erosion
  8. List four causes of soil erosion
  9. State five ways of controlling soil erosion
  10. Mention five effects of erosion

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