Natural Hazards: Flooding and Landslide Risk Management

Risk of Flooding: Natural Phenomena and Impacts

Floods are natural phenomena, occurring almost periodically in coastal or continental areas due to increased water levels and waterlogging potential. Flood-prone lands are typically near water channels that converge within a vast watershed.

Causes of Flooding

Natural Causes

• Climate and Weather: Cyclones, rapid melting of ice and snow, or climates with marked periods of drought followed by torrential rain fronts.
• Obstruction of Natural River Channels: Blockages that impede water flow.

Human Causes

• Direct:
  • Works in the riverbed: dykes, dams, and pipelines.
  • Dam breaks and sudden water releases from reservoirs.
  • Mining works.
• Indirect:
  • Deforestation and loss of vegetation.
  • Poor cultivation practices and erroneous land use.
  • Waterproofing due to increased urbanized areas.
  • Soil erosion phenomena that favor torrential runoff.

Damage and Consequences of Flooding

Floods cause the erosion of large areas of land and the deposition of uprooted and dragged materials in other locations. These flood-prone terrains are often very fertile, which unfortunately leads to the misguided construction and habitation of settlements by many people. However, these remain areas of significant flooding risk, liable to be inundated again in historical time. Floods also cause changes in channel geometry, thereby altering the course of water for future events.

Flood Prediction Strategies

The prediction of floods should be based on:

1. Meteorological forecasts.
2. The cyclicity of an event (return period).
3. Risk maps.

Flood Prevention Measures

Structural Prevention

This involves the preventive creation of physical structures, such as:

• Swamps, dams, dikes, and levees.
• Installation of rain gauges along channels for early alerts.
• Increasing the capacity of existing channels.
• Creating new channels.
• Reforestation of mountainous areas.

Non-Structural Prevention

This includes prevention methods that do not require building structures, for example:

• Regulating the use of flood plains and prohibiting construction within them.
• Developing comprehensive risk mapping.
• Implementing civil protection plans.
• Utilizing flood simulation models.

It is important to note that alpine streams typically pose fewer problems due to their large slopes and confined channels. However, small, flat wadis near the sea, where the channel widens, present a higher risk.

Understanding Landslide Risks

Landslides are movements of materials down slopes that occur on hillsides and slopes of the land surface due to the force of gravity. The main types of movements include: landslides, flows, and avalanches.

Factors Controlling Landslides

The factors influencing landslides can be categorized as internal and external.

Internal Factors

These may be intrinsic or extrinsic to the slope material itself.

• Intrinsic:
  • Lithological: Rock type, degree of consolidation, material thickness of mulch and soil.
  • Structural: Presence of failures, diaclases (joints), and bedding planes.
• Extrinsic:
  • Environmental: Climate cycles (e.g., freeze-thaw), changes in vegetation type, and fluctuations in the water table.
  • Morphological: Angles of slope and steepness.

External Factors

These are external triggers that can initiate or accelerate landslides:

• Increased water content.
• Vibrations (e.g., from earthquakes or human activity).
• Human activities such as logging and fires.