DRR - Q3 L1 - Geological Hazards

• Geological hazards are natural phenomena that cause major problems around the world
• The expansion and development of cities has led to an increase in impact and damage due to geological hazards
• Most of the geological hazards are related to natural conditions, although some may be due to human activities
• Landslide


  A ground movement on a sloping terrain

  
• Landslides do not happen on flat ground because of the angle on the ground, gravity induces the land to move downward

  
• Landslides are aggravated by rain because water is a natural agent for erosion

  
• Structures built on steep-slope mountains have a high vulnerability to landslide hazards especially during heavy rains

  
• Areas with steep slope, dense population and denuded terrain are distinguished by a high susceptibility to rainfall-induced landslide hazards

  
• Long or regular rain may saturate the topsoil and the bedrock, weakening the soil base of buildings or structures
• Without plants and trees whose roots can absorb water and hold the soil together, subsequent rain water can continue to loosen up the soil that anchors the buildings
• A heavy downpour of rain can quickly destroy these buildings and communities, giving way to landslides, mudslides, or mudflows
• Soil Creep Landslide


  A very slow downslope movement of particles that occurs in every slope covered with loose, weathered material

  
• Slumping Landslide


  A downward movement of rock debris, usually the consequence of removal of buttressing earth at the foot of a slope of unconsolidated material

  
• Debris Flow Landslide


  Happens when the slope becomes saturated with water, this then triggers a landslide of water-soaked mass of rock and soil that slides down the slope

  
• Rock Fall landslides

  Sudden slides caused by heavy rain the rock on the slope loosens and then slides down the slope

  
• Sinkhole


  A topographic depression created when groundwater dissolves the underlying limestone bedrock
• Sinkholes occur in areas where the soil foundation is made of soft minerals and rocks such as limestone, salt beds, or any acidic rocks
• The depth of sinkholes ranges from a couple of meters to several mile deep
• How sinkholes occur


  1. Water from the rainfall seeps underneath the soil through the cracks and fissures

  2. As water passes through these cracks and fissures, it erodes the soil and forms a conduit system

  3. These underground water systems increase in size as the soil is carried by the water through internal erosion

  4. This can either form a void filled with air with an underground drainage

  5. If the void is clogged with clay, then it forms a depression which then accumulates water and forms a pond

  6. Otherwise it forms a hole once the cover collapses into the void which can be either filled with air or water
• Cover Collapse Sinkhole

  Develops suddenly (over an hour period) thus, causing catastrophic damage
• Cover Subsidence Sinkhole


  Gradually grows where the sediment covers are permeable and contain sand
• Dissolution Sinkhole


  Occurs in areas where calcareous is exposed on the ground or where thin layers of soil and permeable sand are also covered. Limestone or dolomite dissolution is most intense when the water first reaches the rock surface
• Artificial Sinkhole


  Such types of sinks may be caused by various human activities, including groundwater pumping and building
• Landslides can be caused by natural or human factors

  
• Natural causes of landslides


  • Climate
  • Earthquakes
  • Weathering
  • Flooding
  • Volcanic Activity
  • Forest Fires
  • Gravity

  
• Human causes of landslides


  • Mining
  • Clear cutting

  
• Rainfall-induced landslides are the most common and widespread damaging landslides in the Philippines, with an average of 20 typhoons each year due to prolonged or heavy rainfall

  
• Rainfall can be a cause for landslides, but soil conditions are also very critical
• The features of precipitation occurrences, including the accumulated volume of rainfall, length and severity, and the slope of the terrain show the greatest effect on the stability of the slope and the rate of occurrence of landslides and debris

  
• The precipitation-induced landslide depends on the type of land covered and the type of soil

  
• Sinkholes occur when sedimentary rocks are dissolved
• Pumping water underneath the ground causes drainage to alter their flow which causes the soil to be eroded by water
• Broken pipes underneath the ground can also cause internal erosion making a void underneath the ground
• Altering drainage systems cause water to branch into other routes where there is soluble rock can cause erosion through the flow of water
• Sinkholes


  Cause internal erosion making a void underneath the ground. Altering drainage systems cause water to branch into other routes where there is soluble rock can cause erosion through the flow of water.
• Types of Sinkholes


  • Cover Collapse Sinkhole
  • Cover Subsidence Sinkhole
  • Dissolution Sinkhole
  • Artificial Sinkhole
• Cover Collapse Sinkhole


  • Develop abruptly (over a period of hours) and thus cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. Over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression.
• Cover Subsidence Sinkhole


  • Develop gradually where the covering sediments are permeable and contain sand. In areas where cover material is thicker or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon and therefore may not be seen frequently. They are smaller and thus may go undetected for long periods.
• Dissolution Sinkhole


  • Occur in areas where limestone is exposed at land surface or also is covered by thin layers of soil and permeable sand. Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface. Aggressive dissolution also occurs where flow is focused in pre-existing openings in the rock, such as along joints, fractures, and bedding planes, and in the zone of water-table fluctuation where groundwater is in contact with the atmosphere. Solution sinkholes are generally small in size and also slow to develop.
• Artificial Sinkhole


  • These types of sinkholes can be caused by various human activities, including groundwater pumping and construction activities. However, the most common activities that result in artificial sinkholes are mining, drilling, considerable changes in weight, as well as a tremendous increase in water flow, such as a construction of an artificial pond.