Meaning, Causes, Effects and Prevention of Flooding for JSS 1

Environmental Degradation: Flooding

Performance Objectives

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

1. explain the meaning of flooding;
2. state the causes of flooding;
3. identify different ways of preventing flooding;
4. explain the meaning of drainage and outline different types of drainage systems in the environment;
5. discuss effects of flooding on the environment and farmland;
6. carry out activities on flood control, and prevention;
7. embark on field trips, excursions at flood sites in and around environment.

Meaning of Flooding

Flooding is the accumulation of an excessive amount of water over land that is normally dry.

This environmental hazard occurs when water bodies such as rivers, streams, lakes or oceans overflow their natural boundaries, or when heavy rainfall generates more surface runoff than the soil can absorb or drainage systems can carry away.

Causes of Flooding

Flooding can be triggered by natural environmental processes or by human activities that disrupt the natural flow of water. Understanding these causes is essential for developing effective control measures.

The primary natural causes of flooding include:

1. Heavy or Prolonged Rainfall: Intense downpours release large volumes of water in a short period, quickly saturating the soil and leading to rapid water accumulation on the surface.
2. Overflow of Rivers and Streams: During seasons of heavy rain, water bodies can receive more water than their channels can contain, causing water to spill over onto the surrounding floodplains.
3. Flat Topography: Low-lying areas or flat lands have poor natural slopes, which slows down the movement of water and causes it to pool on the surface.

The primary human causes of flooding include:

1. Blocked Drainage Channels: Dumping household refuse, plastic bottles, polythene bags and silt into gutters and canals blocks the free flow of water, forcing rainwater to spill onto roads and into buildings.
2. Deforestation and Vegetation Loss: Cutting down trees and clearing grass reduces the soil’s ability to absorb rainwater, leading to a significant increase in surface runoff.
3. Poor Urban Planning: Constructing buildings, roads and concrete pavements prevents rainwater from sinking into the ground. Building houses directly on natural water paths or floodplains blocks the passage of water, leading to localized flooding.

Effects of Flooding on the Environment and Farmland

Flooding has severe consequences for both human settlements and agricultural areas. The impact can be felt long after the floodwaters have receded.

The major effects of flooding on the environment include:

1. Destruction of Infrastructure: Fast-flowing floodwaters can damage roads, collapse bridges, weaken building foundations and destroy electrical installations.
2. Displacement of People: Families are often forced to abandon their homes when water enters residential areas, leading to temporary homelessness and overcrowding in relief camps.
3. Water Pollution and Disease Outbreaks: Floodwaters mix with sewage, industrial waste and refuse. This contaminates drinking water sources, leading to outbreaks of waterborne diseases such as cholera, dysentery and typhoid.
4. Soil Erosion: Fast-flowing floodwater washes away soil particles, creating gullies and weakening the stability of the land.

The major effects of flooding on farmland include:

1. Destruction of Crops: Excess water suffocates plant roots by cutting off oxygen, leading to the rotting and death of crops such as maize, cassava, yam and rice.
2. Loss of Fertile Topsoil: The nutrient-rich upper layer of the soil is washed away by floodwaters, leaving the farmland less fertile for future planting.
3. Loss of Livestock: Farm animals such as poultry, goats, sheep and cattle can drown in deep floodwaters, leading to severe financial losses for farmers.
4. Siltation of Farmland: Floods can deposit thick layers of sand, mud and debris over agricultural fields, making the land difficult to cultivate.

Meaning and Types of Drainage Systems

Drainage is the natural or artificial removal of surface and sub-surface water from an area.

A proper drainage system ensures that rainwater flows away quickly and safely, preventing water from pooling and causing floods.

The main types of drainage systems in the environment are:

1. Natural Drainage Systems: These are water pathways formed by nature without human intervention. Examples include rivers, streams, creeks and natural valleys that channel rainwater into larger water bodies.
2. Open Drainage Systems: These are man-made, uncovered channels or gutters constructed along roadsides and residential areas. They collect surface runoff and direct it to larger channels. While easy to construct, they can easily become blocked by refuse if not properly maintained.
3. Closed or Underground Drainage Systems: These consist of covered pipes, culverts or concrete tunnels built beneath the ground. They collect water through street inlets and carry it safely underground. This type of system keeps the environment cleaner and prevents pests from breeding, but it is more expensive to build and maintain.

Ways of Preventing and Controlling Flooding

Preventing and controlling floods requires a combination of proper waste management, environmental conservation and engineering solutions.

The key steps needed for flood control and prevention include:

1. Proper Waste Disposal: Refuse should be disposed of in designated waste bins rather than being dumped into gutters, canals or water channels. This ensures that drainage systems remain clear.
2. Regular Clearing of Drains: Communities and local authorities should regularly clean gutters and drainage channels to remove silt, sand and debris that block water flow.
3. Afforestation and Reforestation: Planting trees and maintaining grass cover helps absorb rainwater into the soil, reducing the volume and speed of surface runoff.
4. Construction of Standard Drainage Channels: Governments should construct wide, deep and well-connected concrete drainage systems that can handle large volumes of water during heavy rains.
5. Enforcing Town Planning Laws: Authorities must ensure that buildings are not erected on natural floodplains, wetlands or water paths. Proper setbacks from rivers and streams must be maintained.
6. Construction of Dams and Levees: Dams can be built to store excess water from rivers, while levees (raised banks) can be constructed along river edges to prevent water from spilling onto adjacent land.

Demonstrating Flood Control and Prevention

Practical demonstrations help illustrate how different soil covers and drainage structures affect water flow and soil erosion. The following activity demonstrates the role of vegetation and drainage in preventing flooding.

Activity: Soil Cover and Runoff Demonstration

The required materials are:

1. Three empty plastic bottles (2-litre size) cut horizontally to create open trays
2. Dry soil
3. Grass turf or living plants with roots intact
4. Dry leaves or mulch
5. A watering can or plastic cup with small holes in the bottom
6. Three clear plastic cups to collect runoff water
7. Water

The procedure involves the following steps:

1. Fill all three plastic bottle trays with the same amount of dry soil.
2. Leave the soil in the first tray bare (representing cleared land or deforested areas).
3. Cover the soil in the second tray with a layer of dry leaves or mulch (representing covered soil).
4. Plant the grass turf firmly in the third tray (representing forested or vegetated land).
5. Prop up the closed ends of the trays slightly to create a gentle slope, placing a clear plastic cup under the open spout of each tray to catch the runoff.
6. Pour an equal amount of water slowly over each tray using the watering can to simulate rainfall.
7. Observe the speed, volume and clarity of the water that drains into each cup.

Observations from this activity show that:

1. The cup under the bare soil fills up quickly with muddy water, demonstrating high runoff and soil erosion.
2. The cup under the mulched soil fills up more slowly with slightly cleaner water.
3. The cup under the vegetated soil collects the least amount of water, and the water is the cleanest, demonstrating that plant roots hold the soil and absorb water, thereby preventing flooding and erosion.

Guidelines for Field Trips and Excursions to Flood Sites

Visiting a local flood site or a drainage facility provides real-world observation of environmental degradation and flood control measures.

Important guidelines for a safe and educational field trip include:

1. Safety First: Students must never visit a flood site during active flooding or heavy rainfall. Visits should only take place when the weather is clear and under the strict supervision of teachers and guides.
2. Observation of Causes: Students should look for visible causes of flooding at the site, such as blocked gutters, buildings constructed close to water channels, or lack of drainage systems.
3. Assessment of Effects: Students should observe and document the damage caused by the flood, such as eroded roads, damaged buildings, ruined crops or displaced soil.
4. Recording Findings: Students should take notes, draw simple sketches, or take photographs of the drainage systems and flood impacts for classroom discussion.
5. Interviewing Community Members: Where safe and appropriate, students can ask local residents about how often the area floods, the height the water reaches, and how the community manages the situation.