External Geodynamic Processes: Landslide and Flood Risks
Landslide Risks from External Geodynamic Processes
Flood Risk: Causes of Natural Inundations
Causes of Climatic Origin
- Coastal cyclones
- Extreme weather events
- Rapid ice and snow melt
- Temperate climates with marked periods of drought followed by torrential rainfall
Obstruction of Natural River Channels
- Direct anthropogenic causes: Construction in riverbeds, dam breaks, waterworks, sudden water release, mining and tailings
- Indirect anthropogenic causes: Deforestation, loss of vegetation cover, poor farming practices on sloping land, erosion
Factors that Influence Torrential Phenomena
- Climatic factors: Rainfall and evaporation
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Geological factors:
- Lithological permeability: Impervious materials increase surface runoff
- Structural arrangement: Strata and fractures influence water flow
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Hydrographic factors: Basin shape and size
- Elongated basins tend to have more diffuse responses to storms compared to compact basins.
Risks of Landslides
Materials on the Earth’s surface are subject to gravity. When materials are located on slopes, gravity can cause sliding. This occurs when the shear stress due to gravity exceeds the material’s shear strength.
Factors Influencing Slope Stability
- Intrinsic factors: Lithology and structure
- Extrinsic factors: Slope morphology, environmental conditions, water content
Types of Slope Movement
Landslides
Gravitational mass movements of rocky downhill material due to the appearance of a rupture surface. There are two main types:
- Translational: The rupture surface is more or less parallel to the slope surface.
- Rotational: Occur along a curved failure surface, common in clay materials.
Rockfalls
Free fall of blocks of different sizes from rock outcrops.
Flows and Landslides
Movements of loose materials, such as unconsolidated regolith. In the presence of clay, these can form mudflows. Mudflows on the slopes of volcanoes are called lahars.
Avalanches
Similar to flows but much faster.
Creep
A slow, continuous flow of the soil mantle. Solifluction is a specific type of creep associated with freeze-thaw cycles. Other types involve a combination of mudflows and slow, superficial soil movement.
Subsidence
The sinking of the surface relative to a stable reference level.
Causes
Lithology, tectonics, mining, fluid removal, surface water drainage, anthropogenic fill, seismic tremors.
Expansive Soils
Soils made of materials such as loam, silt, and clay can swell when hydrated and shrink when dried, leading to non-recoverable deformation and loss of stability.
Factors Influencing Expansiveness
Drainage conditions, thickness of expansive soil, climate, clay mineral type, vegetation.
Effects
Deformation of pavements and sidewalks, landslides, and damage to drainage pipes.
Prevention
Soil stabilization with lime, earthworks and fill with non-expansive materials, and proper planning.
Dune Movements
Accumulations of sand in deserts or coastal areas shaped by wind action.
Types of Dunes
- Barchan dunes: Individual, crescent-shaped dunes formed when there is a predominant wind direction.
- Seif dunes: Form in regions where the wind blows in two directions, developing a dominant arm.
- Pyramid dunes (Rhourds): Large-scale dunes.
- Coastal dunes: Dunes that progress inland from the beach. They can be classified as moving, fixed, or fossil dunes based on their degree of activity and evolution.