Natural Disasters: Mass Movement & Geological Hazards

Lesson Outcomes

• Upon completion of the lesson, students should be able to identify different types of natural disasters.
• Students should demonstrate knowledge of safety measures and preparedness for natural disasters.
• Students should understand the impact of natural disasters on human lives and the environment.

Mass Movement

• Mass movement refers to the downward movement of surface materials due to gravity.
• Downslope movement and vertical movement are types of mass movement.
• Landslides are examples of downward movement, involving processes that result in the downward and outward movement of slope-forming materials.
• Ground subsidence is an example of vertical movement, characterized by the slow to rapid sinking of the land surface and is also known as sinkholes.

Geological Hazards

• Geological hazards can be grouped based on the onset time, as either sudden or slow phenomena.
• Sudden phenomena include earthquakes, landslides, mud flows, pyroclastic flows, tsunamis, flash floods, volcanic eruptions, tephra, and lahar.
• Slow phenomena include the formation of volcanoes, sinkholes, liquefaction, sand dunes, and shoreline and stream erosion.
• The Philippines is prone to geological hazards because of its location in the tropics and communities located at the bases of mountains and hills.

Geological Hazards in the Philippines

• In recent years, the Philippines has experienced a human-made disaster
• The Payatas Trash Slide happened when tons of trash, as high as 50 feet, slid down, resulting in 500 deaths.
• In 2011, Typhoon Mina caused incessant rains, which made flooded areas near the Irisan dumpsite collapse, causing a trash slide, burying five people alive in Smokey Mountain.

Rainfall-Induced Landslides

• Landslides, or downslope land movement, are a hazard in the Philippines.
• Notable rainfall-induced landslides in the Philippines:
  • November 1991: Ormoc, Leyte, leading to 6,000 deaths.
  • September 2003: Mining Village in Southern Philippines, resulting in 22 miner deaths.
  • December 2003: Panaoan Island in Southern Leyte, leading to at least 160 deaths.
  • February 2006: Guinsaugon, Leyte, causing 139 deaths and 973 missing.
• Even though the numbers may seem small compared to the other disasters that befell the country in recent years due to other causes (Typhoon Ondoy and Super Typhoon Yolanda), their toll on human life to properties and to the environment was insurmountable.
• Landslides are a natural process of the Earth's changing landscape and cannot be averted.
• Gravity acts on rocks and other materials on mountains.
• Downslope movement can be catastrophic when a large volume of materials are displaced by gravity.

Forces Determining Slope Stability

• Driving force: gravity acting on materials as a force that determines the ability of a slope to resist sliding.
• Resisting force: the force acting on the material and the friction holding it in place as a force that determines the ability of a slope to resist sliding.
• Slope steepness and the weight of material are key factors in slope failure.
• Slope steepness is directly proportional to the likelihood of slope failure.

Cherry Hills Landslide

• During the Cherry Hills Landslide, a slope averaging 20° was determined.
• A 20° slope is sleep enough for unstable materials to slide downward.
• Evidence and warning signs of a slope failure were already present months before the Cherry Hills Landslide incident.
• Hazard maps are being created by authorities to educate the population.

Agencies Mapping Hazard Risks in the Philippines

• PAGASA, MGB, MMDA, Manila Observatory, and PHIVOLCS have mapped the country's hazard risks.
• Project READY, Project AGOS, and Project NOAH are projects set up to aid the government and people to be prepared for disasters.

Color Convention used in Hazard Maps

• Red areas have high susceptibility to landslides with active landslides and tension cracks near communites
• Green areas have inactive landslides and tension cracks away from the community.
• Yellow areas are low to gentle slopes and lack tension cracks, also have a low landslide susceptibility rating.
• Violet areas have high susceptibility to flooding with over one-meter-high flood that remains flooded for several hours during heavy rains.
• Light pink areas mean low susceptibility to flooding. Flood height is usually below 1 meter.

Project Ready

• Project Ready aims to provide and conduct hazard mapping and assessment for effective community-based disaster risk management.
• Project Ready was implemented from 2006 to 2011 by the NDRRMC in collaboration with other government agencies (MGB, NAMRIA, PAGASA, and PHIVOLCS).
• Project Ready was funded by the United Nations Development Programme, AusAid, and Asian Development Bank.
• Project Ready addresses the problems encountered by local government units in handling disaster risk management.
• National and local involvement is required for disaster risk management.
• The national government will institutionalize standard measures and processes for disaster risk management.
• The project empowers vulnerable municipalities and cities to develop disaster risk management plans.
• The project targets the 27 high-risk provinces of the country.
• Project Ready's major components include multi-hazard and risk assessment, community-based disaster risk mitigation through early warning systems and education campaigns, and mainstreaming disaster risk reduction into local development.
• Project READY aims to improve the Philippines' safety and residents' awareness of hazard risks.

Rainfall-Induced Landslides and Soil Cohesion

• The amount of water added to sand should be just enough during the summer months.
• The right amount of water provides the proper surface tension between loose soil grains, also called cohesion.
• Cohesion results from static charges between small particles or surface tension of water between grains.

Causes of Landslides

• As people move around the surface of Earth, they are kept from flying into space by gravity.
• The same could be said of an airplane in flight—the balance of forces acting on it is what enables it to stay on air.
• The balance between the angle of a slope and its load is key to preventing landslides from happening.
• Factors disrupting balance can lead to landslides:
  • Constructing a structure over a slope that increases the load.
  • Road-widening projects can make the slope angle steeper.
  • Deforestation can make the soil absorb more water.
• Overloading in a slope can be seen in some mountainous parts of the country, particularly in Baguio City.
• Building houses on steep slopes often undercuts and adds additional load to the slope, making the ground more likely to slide.
• The moisture from built-in swimming pools increases soil moisture, which contributes to the likelihood of a landslide.
• Water saturates slopes, making them heavier and more loose.
• High water is particularly more likely to occur in the Philippines for two reasons: the frequency of typhoons and deforestation for housing projects or farmlands through "kaingin".
• The absence of a natural absorber of water in the soil makes the water saturated, effectively weakening the bond between soil particles and increasing the likelihood of slide.

Landslide Factors

• Factors are independent with catastrophic effects if simultaneous such as when Mount Pinatubo erupted.
• Rains from Typhoon Diding resulted in lahar flows that killed around 300 people.

Types of Landslides

• Landslides are classified by type of material, type of movement, and rate of movement.
• Debris: Material coarser than 2 mm.
• Soil: Material finer than 2 mm.
• Types of movement include falls from clips, lateral spreads, flows, and topples.
• Rate of movement depends on slope steepness, grain size, moisture, thickness of moving mass, clay mineral type, and clay content.
• Types of Landslides: rockfall, rockslide, debris slide, debris flow, earthflow, mudflow, and snow avalanche (in other countries).

Rockfalls

• Rockfalls happen in zones of weakness or in steep mountainous regions.
• Large masses of rocks become loose and separate from a steep slope or cliff, causing them to fall and break into smaller fragments and sweep downslope.
• Rockfalls can be triggered by earthquakes, slope undercutting, and blasting in mining works.
• The cliffs are at least 40°, which increases the likelihood of rockfalls; rocks can collect in a fan-shaped pile of rock fragments at a cliff base, called talus slopes.
• The kinetic energy can demolish residential houses and highways are often built following the base of precipitous, requiring measures for falling rocks.

Flow

• Materials become heavier when water saturates ground resulting in debris flow or mudflow.
• Flow: The movement of material down a slope in fluid form.
• A combination of rock and mud can lift objects along its path.
• The debris flow can cause flooding as it blocks streams of other water ways.
• Flow often leaves a upside-down funnel shaped deposit where the landslide material has stopped moving.

Debris Avalanches

• Downslope movements of rocks and soil with extremely rapid velocity are called debris avalanche.
• Dangerous avalanches can flow from 100 km/h to 300 km/h and may contain boulders as big as a house.
• The Yungay, Peru avalanche on May 31, 1970 traveled at 270 km/h.
• The Yungay avalanche contained 50 to 100 million m³ of granite, glacial debris, and ice, and fell at a distance of 400 to 900 meters.

Karl Von Terzaghi

• Karl Von Terzaghi developed an interest in astronomy and geography at an early age.
• In 1900, he entered Technical University in Graz where he took up mechanical engineering.
• He developed an interest in theoretical mechanics, geology, and highway and railway engineering.
• In 1924, he published Erdbaumechanik (Soil Mechanics) which was considered revolutionary in the field of soil mechanics.

Slides and Slumps

• Slide: a downslope movement of soil or rock mass on defined surfaces of rupture
• There is a zone of weakness separating the slide material from stable underlying materials
• Translational Slides move along a surface of rupture that is curved and concave-upward.
• Translational Slides are also referred to as slump.
• Occurs most frequently in homogenous, cohesive materials such as soil or artificial fill.
• Translational slides are characterized by movements on weaker surfaces that are less parallel to a slope.
• This makes translational slides faster and reach farther distances, more dangerous compared to rotational slides.

Creep

• Creep: a relatively less dangerous mass movement involving the slow downslope movement of surface soil and weak rocks.
• Creep causes tilting fences, power lines, and trees.
• The greater the tilt angle, the longer the object has been subjected to soil creeping.
• There is lesser movement as the depth increases because most of the movement operates or occurs on the surface.
• Accelerants include the expansion and shrinking processes- wetting and drying or freezing and thawing.
• Creep is an indication that the slope is unstable.
• Movement could accelerate and transform into a faster-moving and more dangerous slide.

Avalanches

• An Avalanche: an example of a deadly downslope movement.
• Avalanches are mostly associated with snowcapped mountains.
• A 12 inche deep avalanche can move tons of snow at high speed, enough to bury people and animals.
• An avalanche mostly depends on orientation, windspeed/direction, and vegetation.
• More prone to avalanching slopes are inclined 30 to 40 degrees, but will also form on gentler slopes if conditions are just right.
• Increased temperatures risks avalanches because melting water may fill in pore spaces between soil grains.

Ground Subsidence

• Ground subsidence is the gradual sinking or downward movement of the Earth’s surface.
• Subsidence is the slow lowering of the land surface.
• Subsidence can damage infrastructure like buildings, roads, pipelines, and other utilities.

Collapse

• Collapse is the fast opening of the land surface and movement of surface material into the underlying cavities.
• Ground collapse is a sudden and localized event where the surface falls or caves in, often forming a hole or depression.

Sinkhole

• A sinkhole is a type of collapse that happens when soluble bedrock, like limestone, is eroded by groundwater.
• A sink hole creates a void that causes ground above to collapse.
• Sinkholes vary in size-from ten to hundreds wide; they can "swallow" houses and roads, or drain streams, lakes, and forms of wetlands.

Common Types of Processes in Sinkhole Formation

• Dissolution
• Cover Subsidence
• Cover Collapse

Dissolution

• This occurs when the soil cover is thin and highly permeable.
• This is when water dissolves soluble bedrock (limestone, gypsum, or salt) over time, creating underground voids.
• The dissolving rock then causes the ground above to gradually subside.

Cover Subsidence

• It forms when sandy, permeable sediment sits on top of limestone bedrock.
• Sediments covering the bedrock move into underground voids over time forming depressions at the surface.
• The ground appears to sag rather than collapse suddenly.

Cover Collapse

• This can occur suddenly and without warning.
• Happens when a cavity in the bedrock grows large enough that the overlying material can no longer support itself.
• This results in the sudden, dramatic collapse of the surface, forming a deep hole.

Prevention and Mitigation

• Use the susceptibility maps provided by the MGB.
• Avoid building in steep slopes and mountainous areas or on slopes which exceed 30 to 45 degrees on a hillside.
• Avoid areas abundant of loose debris on a slope, slopes with fine-grained doil, or areas where rainfall can seep through the ground.
• Avoid locations of previous landslides.
• Shallower sides are more likely to develop on slopes with little vegetation and lack of trees.
• Relocation and evacuation should be made if there are structures identified as unsafe.

Engineering Solutions for Landslides

• Adding a buttress will help the slide resist movement.
• The resisting mass can also be increased by piling heavy boulders on the toe area.
• Slope angle can be lowered to reduce risk movement.
• Shotcrete (cement mixture) can restrict water and slow weathering.
• Rock nets, rock fences, and rock ditches can prevent rocks from reaching buildings or highways.
• Removing water out of the soil increases cohesion for which trees and shrubs are very helpful.
• To strengthens rocks with fractures, one can drill holes which are slightly upward into the slope, and insert perforated pipes for drainage or drill rock bolts and rock anchors.

Landslide Preparedness

• Recognizing signs of an imminent landslide is important to make safety considerations in building or plan for sudden split second decisions.
• Springs, seeps, or saturated grounds are a warning sign.
• New cracks or unusual bulges on street pavements, sidewalks & depressions in the road beds should be watch out.
• Bending telephone lines/poles and retaining walls/fences are also observable signs.
• Broken, underground utilities can also be indicators.
• Check doors and windows for frames out of plumb.
• Listen for unusual sounds like cracking wood from trees, or faint rock rolling sound that increases in intensity, indicating how far the landslide is.

What To Do Before a Landslide

• Check a ground assessment of the property before buying it or building anything on top of it.
• Look at the history of the location, since prior events greatly increase the chances of it happening again.
• Ask for landslide hazards from authorities like the local officials, the Mines & Geosciences Bureau, or the Department of Environment/Natural Resources.
• Seek sound advice on the area and get corrective measures that can be done.
• Use flexible pipe fittings for utilities.
• Plant ground cover on slopes and build retaining walls.
• Build deflection walls to divert debris flow but be careful to not damage other households.

What To Do During a Landslide

• Be wary of intense short bursts of rain (while staying on the loop with the latest updates for rainfall warnings.
• Evacuate immediately.
• Stay out of path.
• Moving debris will always generate unusual sounds due to the cracking trees/boulders.

Prevention steps during landslides

• Avoid small flows or trickles of mud.
• Be cautious of sudden change in the amount of water flow.
• A change in the color of the water might indicate a landslide activity upstream.
• Inform authorities/neighbors as needed for them to assess/evacuate.

What To Do After A Landslide

• Stay away as there is still a danger of additional slides.
• Listen to the emergency broadcasts.
• Watch out for incidence of flooding triggered by landslides/debris flows.
• If trained, provide help to those in need/inform the responders about survivors’ locations.
• Be aware of any elderly people/PWDs in the neighborhood who might need assistance.
• Be on the lookout for broken utility lines, roadways, etc, and immediately report to the utility companies.
• Note erosion caused by the loss of ground cover can lead to flashfloods and additional landslides in the future.