Landslide Hazards

GEOLOGIC HAZARDS

• The probability of occurrence, within a specific period of time in a given area, of a potentially damaging natural phenomenon;
• Events in which natural phenomena such earthquakes, river floods, mass movements, volcanic eruptions, etc. cause the loss of lives and damage to property;
• Mainly occur in areas where the natural factors (ie. climate & geology) are unfavorable and where the natural equilibrium has been disturbed by man

IMPORTANCE OF GEOHAZARD STUDIES

•  Land use planning
•  Urban development
•  Disaster preparedness & management
•  To minimize loss of life
•  To minimize economic and social disruption

TYPES OF GEOLOGIC HAZARDS

•  Mass movements or landslide hazards  (pagtiris o pagrasay kan daga o bato)
•  Flood hazards (pagbabaha)
•  Earthquake hazards (linog)
•  Volcanic hazards (pagtuga kan bulkan)
•  Accelerated erosion (erosyon)
•  Coastal hazards

MASS MOVEMENT

Mass movement is the downslope movement of soil, debris or rock when the shear stress exceeds the shear strength of the material. It is also the denudational process where soil or rock is displaced along the slope mainly by gravitational forces.

Fall consists of the detachment of soil or rock from a steep slope followed by a more or less free and extremely rapid descent of of the material. The movement is largely through the air, alternated with the bouncing or rolling on the slope.

Topple is the end-over-end motion of rock down a slope. It involves forward rotation out of the slope of a mass of soil or rock, a point below the center of gravity of displaced mass. Topple mostly occurs in combination with fall.

Slide is the moving slab of soil or bedrock that moves downslope as a a whole, remaining in contact with the underlying material. The movement is parallel to planes of weakness and occasionally parallel to slope.

Flow is the viscous to fluid-like motion of debris. It is sporadic and sudden channelized discharge of water and debris, and usually involves great internal deformation. There are three types of flow: Earth-flow, Mud-flow and Debris flow.

Creep is the slow movement of soil or bedrock down the slope. It occurs where the stresses on the slope material are too small to create a rapid failure.

MORPHOLOGICAL ASPECTS SUGGESTIVE OF LANDSLIDES:

 

• Steep backscarps with convex accumulation in the lower slope.
• Cracks and depressions
• Morphological steps
• Failure planes and slicken sides
• Backtilting with deranged drainage
• Irregular and hummock relief

What Causes Landslide?

Geological Causes

• Weak and sensitive material
• Weathered material
• Sheared, jointed or fissured material
• Adversely oriented fractures
• Contrast permeability
• Contrast stiffness

Morphological Causes

• Tectonic/Volcanic uplift
• Erosion of slope toe
• Vegetation removal
• Subterranean erosion (piping, solution)
• Deposition of load on slope or crest

Physical Causes

• Intense rainfall/prolonged exceptional rainfall
• Earthquake/volcanic eruption

Human Causes

• Excavation of the slope or its toe
• Deposition of load on the slope or crest
• Drawdown of reservoirs
• Deforestation
• Irrigation
• Mining
• Artificial Vibration
• Water leakage from Utilities

MITIGATION AND REMEDIAL MEASURES (STRUCTURAL)

• Rock reinforcement such as pattern of rock bolt with wire mesh and shotcrete
• Use of rock sheds
• Use of nets and rock fences to catch falling and sliding rock
• Provide adequate and appropriate slope drainage
• Flatten the slope
• Soil improvement
• Cut or fill solutions
• Rock soil anchors
• Drainage
• Geofabric
• Grouting
• Use of retaining walls

MITIGATION MEASURES (NON-STRUCTURAL)

Direct (legal) regulations

• Land use zoning
• Subdivision regulations
• Building codes
• Land registration
• Open space controls
• Public works and engineering

Financial Incentives

• Taxation (negative or positive incentives)
• Insurance and mortgage policies

Public Land Development

 LEVELS OF GEOHAZARD MAPPING

Read More about this by Clicking Here: Landslide Hazards

Conversion of Mangrove Areas into Fishponds and Other Uses

Aquaculture provides a ready alternative source of protein and income that are otherwise sourced from wild fish catch. It is also one of the dollar earners for the country. In the case of the Caramoan Peninsula, aquaculture is very much practiced in the mangrove areas that have been cleared, excavated and diked for the purpose.

As a natural barrier, mangroves and related coastal forests serve a dual purpose: they protect the upland from the eroding action of seawater and at the same time, protect the marine ecosystem by trapping and filtering sediments from the uplands. Mangrove areas are specialized ecosystems serving as habitat for organisms that tolerate alternating fresh and saltwater periods or brackish environment.

Learn more about Mangroves here.

When mangrove forest are cleared to give way to fishponds and other developments, coastal areas lose their natural protection from the eroding action of moving water exposing the coastal communities to natural hazards. Toxicity also results when the soil is exposed to the air making the environment unsuitable for other species.

Probably with the exemption of small scale mining, these activities have cumulatively impacted on what has remained of the forests resulting to further shrinkage of the forest size and forest fragmentation.

A picture showing fragmentation of the forests in the uplands of Caramoan.

As forests lose their cover, the ability of the topsoil to resist erosion is lessened in high precipitation events. Sediments deposited on the river floor and sea beds hamper biological processes that could result to lower yields. Other disturbances from human activities like road construction also contribute to the process.

Picture showing siltation along the Caramoan Coast.

 

The Earth is Warming

And that is the obvious truth.

Below is a modelling study that concluded that by the time atmospheric concentrations of CO₂ reach 560 ppm “all coral reefs will cease to grow and start to dissolve” (Silverman et al. 2009).

Soaring CO₂ emissions will amplify the risk of conflict, hunger, floods and migration this century (UN 2014).

In 1997, Kyoto Protocol was agreed on different countries but still, the CO₂ concentration continues to rise until this day.

Let’s further know why the Earth is warming.

Radiative Forcing (RF) refers to an imbalance between incoming solar radiation and outgoing infrared radiation that causes the Earth’s radiative balance to stray away from its normal state.

A positive forcing (more incoming energy) warms the system (greenhouse effect), while negative forcing (more outgoing energy) cools it.

Total RF of the Earth System, relative to 1750, is positive and the most significant driver is the increase in the CO₂ atmospheric concentration.

New Normal Updated

Warming is likely to exceed 2°C by 2100 and could even exceed 4°C.

See Sen. Loren Legarda – BUHOS, A Climate Change Documentary here to understand more about Global Warming.