Environmental Hazard: Flooding – JSS3 Basic Science Lesson Note (First Term)

A flood is an overflow of water that covers land which is usually dry.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define a flood;
2. name the types of flooding;
3. name the types of drainage pattern;
4. state the causes of flooding;
5. state ways of preventing flooding;
6. state the effects of flooding.

What is flooding?

Flooding is the covering of land by water when a part of the soil or land is submerged.

• Prone areas: some places flood more often because of their soil structure and low position.
• Low-lying land: water from higher ground runs down and collects here.
• Nigerian examples: towns along river banks and cities with blocked gutters often flood in the rainy season.

What are the types of flooding?

There are three types of flooding: flash floods, rapid on-set floods and slow on-set floods.

• Flash floods: these occur within a very short time and are usually the result of heavy rain.
• Rapid on-set floods: these take a longer time to build up, and the flood can last for a day or two.
• Slow on-set floods: these are caused by water bodies overflowing their banks. They develop slowly and can last for days or weeks.

What is a drainage pattern?

A drainage pattern is the arrangement of watercourses or drains that carry off excess water.

• Good drainage: water flows freely and leaves the area.
• Blocked drainage: refuse in a city or town stops water from flowing when rain falls, and flooding follows.

What are the types of drainage pattern?

There are four types of drainage pattern: parallel, rectangular, dendritic and radial.

• Parallel drainage pattern: streams flow side by side in roughly the same direction, usually down a steep slope.
• Rectangular drainage pattern: streams and their branches meet at right angles.
• Dendritic drainage pattern: streams join like the branches of a tree.
• Radial drainage pattern: streams flow outwards in all directions from a high point, such as a hill.

What causes flooding?

Flooding is caused by natural factors and by human activities.

Natural causes

• Heavy downpour: a large amount of rain falls faster than the ground can absorb it.
• Impervious rock: rock deep in the soil does not allow water to pass through.
• Long dry period: hard, dry soil does not absorb water quickly.
• Melting ice: ice melts while the subsoil is still frozen, so the water cannot sink in.
• Natural disasters: tsunamis and hurricanes push ocean water onto land.

Human causes

• Blocked drainages: refuse and industrial waste stop water from flowing.
• Dam overflow: release of excess water from a dam floods the land downstream.
• No gutters: roads without gutters on either side let water collect.
• Building across watercourses: houses and roads block the path of water.

How can flooding be prevented?

Flooding is prevented by giving water a clear path to leave an area and by protecting the land.

• Build canals and drains: government or individuals should construct them, especially where impervious rock lies near the surface.
• Manage rivers: drain rivers properly, especially where dams are built.
• Keep watercourses clear: do not build houses, roads or other structures across canals, drains and watercourses.
• Build walls: erect walls along the banks of rivers, seas and oceans to stop tidal waves from pushing water onshore.
• Plant trees, shrubs and grasses: they break the power of moving water and reduce erosion.
• Avoid dumping refuse: do not throw waste into drains and rivers.
• Educate the public: people should know the danger of floods and how to reduce their occurrence.

What are the effects of flooding?

The effects of flooding include damage to property, loss of life, disease and loss of fertile land.

• Damage to property: houses and other valuable property are destroyed.
• Loss of lives: people and animals can drown.
• Spread of disease: water bodies become contaminated, which spreads diseases such as cholera.
• Infertile soil: flood water removes soil nutrients.
• Less farmland: the land available for cultivation is reduced.

What should people do during a flood?

During a flood, people should move to safety and avoid contact with flood water.

1. Move to higher ground quickly.
2. Switch off electricity at the mains if it is safe to do so.
3. Do not walk or swim through moving flood water.
4. Do not drink flood water. Boil or treat water before drinking.
5. Listen to warnings from the emergency services.

Practice questions

1. Release of excess water in dams can cause: A. erosion B. flooding C. precipitation D. evaporation
2. Where there are no gutters on either side of the road, …………… may occur. A. precipitation B. drainage C. erosion D. flooding
3. Dumping of refuse and industrial wastes in rivers can cause: A. erosion B. flooding C. water movement D. rise in water level
4. Blockage of drainage and gutters can cause: A. flooding B. erosion C. sedimentation D. desertification
5. Which of the following predisposes an area to flood? A. low rainfall B. drought C. low wind D. high rainfall
6. …………… is a good example of a natural disaster. A. fermentation B. rainfall C. flooding D. animal husbandry

Evaluation

1. Define a flood.
2. Name the three types of flooding.
3. Name the four types of drainage pattern.
4. State two effects of flooding.

Assignment

1. List four causes of flooding in your community.
2. State four methods of controlling floods.