MEV 002: Block-1 (Unit 03) – Floods and landslides

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 UNIT 3: FLOODS AND LANDSLIDES

3.0 Introduction

Floods and landslides are among the most frequent and devastating natural disasters worldwide. These hazards affect millions every year, causing significant human, economic, and environmental losses. Both are primarily triggered by natural factors such as heavy rainfall, rapid snowmelt, and geological instability, but they are increasingly worsened by anthropogenic influences like deforestation, urbanization, and climate change. Understanding the causes, types, and impacts of floods and landslides is vital for effective disaster preparedness and risk reduction.

3.1 Objectives

After completing this unit, learners will be able to:

  • Explain the nature, causes, and types of floods.
  • Describe the environmental and socio-economic effects of flooding.
  • Understand drought as a related hydrological hazard.
  • Identify causes and effects of landslides and mass movements.
  • Examine the role of natural and human-induced factors in worsening these hazards.

3.2 Floods

Floods occur when water overflows onto normally dry land. They may be caused by excessive rainfall, river overflow, dam failure, or coastal storm surges. While some flooding is a natural part of the water cycle and even beneficial to agriculture, severe floods cause immense destruction.

3.2.1 Overview of Floods

Floods can happen suddenly or gradually, and can be local (affecting a small area) or widespread (impacting entire regions or countries). Flood-prone areas are typically low-lying regions, river basins, and coastal zones.

3.2.2 Causes of Floods

  • Natural Causes:
    • Intense or prolonged rainfall
    • Rapid snowmelt or glacier melt
    • Riverbank erosion
    • Cyclones, hurricanes, and tsunamis
    • Earthquakes triggering dam or embankment failure
  • Human-Induced Causes:
    • Deforestation and removal of vegetation
    • Unplanned urban development
    • Poor drainage systems
    • Encroachment of river floodplains
    • Climate change leading to erratic rainfall patterns

3.2.3 Types of Floods

  • Riverine Floods: Overflow of rivers due to heavy rainfall.
  • Flash Floods: Sudden floods with high velocity, often caused by intense rainfall in a short time.
  • Urban Floods: Occur in cities due to poor drainage, blocked sewers, or excessive concrete surfaces.
  • Coastal Floods: Caused by storm surges or tsunamis.
  • Pluvial Floods: Caused by extreme rainfall without any water body overflowing.

3.3 Environmental Effects of Flooding

Floods affect the environment in various ways, classified into primary, secondary, and tertiary effects.

3.3.1 Primary Effects

  • Destruction of homes and infrastructure
  • Loss of life and livestock
  • Erosion of fertile soil
  • Disruption of transportation and communication systems

3.3.2 Secondary Effects

  • Outbreak of waterborne diseases (cholera, dysentery)
  • Contamination of drinking water
  • Displacement and refugee crises
  • Crop loss and food shortages

3.3.3 Tertiary Effects

  • Long-term economic decline in affected regions
  • Psychological trauma among survivors
  • Decrease in property values
  • Changes in river courses and loss of biodiversity

3.4 Droughts

Droughts are prolonged periods of insufficient rainfall that lead to water scarcity. Though not as dramatic as floods, droughts cause long-lasting and widespread hardship.

3.4.1 Drought Consequences

  • Agricultural failure and food insecurity
  • Depletion of groundwater resources
  • Desertification and land degradation
  • Migration and social unrest

3.4.2 Stages of Drought

  • Meteorological Drought: Below-normal precipitation.
  • Agricultural Drought: Soil moisture deficit affects crops.
  • Hydrological Drought: Low surface and groundwater levels.
  • Socioeconomic Drought: Water scarcity affects society and economy.

3.5 Landslides

Landslides are the downward movement of soil, rocks, and debris under the influence of gravity. They are often triggered by heavy rainfall, earthquakes, deforestation, or slope instability.

3.5.1 Causes of Landslides

  • Natural Causes:
    • Intense or prolonged rainfall
    • Earthquakes
    • Volcanic activity
    • Weathering and erosion
  • Human-Induced Causes:
    • Deforestation and mining
    • Road and infrastructure development in hilly areas
    • Poor slope management
    • Overgrazing

3.5.2 Failure of Slope

Slopes fail when the driving force (gravity and water pressure) exceeds the resisting force (soil cohesion and friction). Water infiltration reduces soil strength and increases the chances of landslides.

3.5.3 Factors That Affect Mass Movement

  • Type of soil and rock
  • Degree of slope
  • Amount of water infiltration
  • Vegetation cover
  • Seismic activity

3.5.4 Effects of Landslides

  • Destruction of property and infrastructure
  • Loss of life and livestock
  • Blocking of rivers, leading to flash floods
  • Isolation of remote communities
  • Loss of arable land

3.6 Let Us Sum Up

Floods and landslides are major hydro-meteorological hazards that can have devastating impacts on both human life and the environment. While natural factors contribute significantly, human activities often intensify the frequency and severity of these disasters. Effective management requires improved forecasting, sustainable land-use planning, and strong disaster preparedness systems. Understanding the underlying causes and consequences is crucial for designing mitigation and adaptation strategies.

3.7 Keywords

·         Flood-Overflow of water onto land that is usually dry.

·         Flash Flood-A sudden, intense flood occurring within hours of heavy rain.

·         Drought-A prolonged period of below-average precipitation leading to water scarcity.

·         Landslide-Downward movement of rock, soil, and debris on a slope under gravity.

·         Mass Movement-The movement of surface material caused by gravity, including landslides.

·         Slope Failure-The collapse or movement of a slope due to loss of stability.

·         Urban Flooding-Flooding in city areas due to poor drainage and impermeable surfaces.

·         Desertification-The process of fertile land turning into desert due to drought or mismanagement.

·         Hydrological Drought-A drought that affects water bodies like rivers, lakes, and aquifers.

·         Tertiary Effects-Long-term and indirect effects of a disaster, such as economic slowdown.

 

 

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