Lecture 4: Natural hazards and disasters Flashcards
(11 cards)
Landslide
A movement of rock, earth, or debris down a slope.
Types include:
* Falls, topples, slides (rotational and translational), flows, and spreads.
Can range in depth:
* Shallow landslides: 2–5 meters.
*Deep-seated landslides: >5 meters.
Natural hazard
A natural process that can occur with or without warning and that may affect the natural and/or built environment.
Disaster
Serious disruption of the functioning of a community or a society at any scale due to a hazardous events.
Components of disaster
Components of disaster
1. Susceptibility: where the potential disaster can occur: preconditioning & predatory factors (e.g. steep slopes, soft soils, deforestation).
- Hazard: the probability/frequency: susceptibility + triggering factors (e.g. heavy rainfall, seismic activity).
- Exposure: the presence of people, assets, or infrastructure in hazard-prone areas.
- Vulnerability: impacts of those who are exposed (depends on socio-economic factors, infrastructure, early warning systems, …)
- Resilience: How can people cope with, and recover from the impacts of disasters.
What causes landslides
Driving mechanisms: What causes landslides?
When shear stress exceeds the shear strength of slope materials.
* Factor of Safety (FS) = shear strength / shear stress.
* FS < 1 indicates slope failure.
FS depends on various factors:
* Preconditions: static characteristics such as slope steepness, soil type, geology.
* Preparatory Factors: slowly changing variables like tectonics, weathering, and land use (e.g., deforestation).
* Triggers: sudden events such as intense rainfall, earthquakes, reservoir filling.
Role of rainfall:
* Dominant landslide trigger globally.
* Rainfall increases pore pressure in soils, reducing shear strength and triggering slope failures.
* Soil moisture content!!
Deforestation
* Reduces root cohesion and evapotranspiration.
* Effects can be delayed (2–3 years) and persist up to 15 years.
Human-induced triggers
* Construction and road development.
* Artisanal mining and excavation.
* Reservoir impoundment and irrigation mismanagement.
Hazard & exposure assessment
Hazard & exposure assessment
Susceptibility Mapping
* Preconditions & preparatory factors
* The past is the key to the future
Hazard Mapping
* Preconditions, preparatory and triggering factors
* More difficult: needs a lot of data
Exposure Mapping
* Combines hazard zones with population density.
* High exposure in East African Rift (dense population + steep slopes).
Interaction with societal drivers
Land pressure = increased use of marginal lands.
Forest decline since 1958 driven by:
* Population growth. Conflict and displacement.
Results:
* Higher exposure. More susceptibility. More frequent landslides.
Land pressure also caused by mining, roads, and agriculture.
Role of climate change
Role of climate change
* Increases intensity and variability of rainfall.
* May raise soil moisture = key landslide factor.
* Effects on cyclone frequency are unclear.
* Interacts with land use.
* Slope stability may self-limit (not infinite increase).
INFORM index
Vulnerability and resilience
INFORM Index:
* Vulnerability = poverty, education, inequality.
* Coping capacity = governance, infrastructure.
* Sub-Saharan Africa scores high on both (i.e., high disaster risk).
Long-term Indirect effects
Apart from claiming casualties and destroying houses & infrastructure, landslides can also directly destroy crops and hence reduce farmer incomes
Yet, this potential income reduction also has indirect effects, even before a landslide affects the farmer.
Why? Because substince farmers need to minimize their risk rather than maximize their profit/income
One way to cope with this risk is to diversify: sell (part of the) unsafe plots and buy safe plots instead
However, ‘safe’ land will be more expensive then ‘risky’ land
-> Farmers under landslide risk are ‘forced’ to buy an expensive (thus smaller) additional plot
-> Farmers having a safe plot can invest in more ‘risky’ land as their income is already more guaranteed
Over time, this means that a farmer starting with a landslide-prone plot, will own less land than a farmer starting with a safe plot
Towards disaster risk reduction
Sendai Framework for Disaster Risk Reduction:
* UN framework focused on proactive disaster reduction.
* All disaster components considered: susceptibility, hazard, exposure, vulnerability, resilience.
* 7 global targets and 4 priorities.
* First priority: “Understanding disaster risk.”
Better understanding landslides
* Early warning systems!
