GEOLOGICAL HAZARD : CAUSES AND SIGNS

• Geological Hazard


  An extreme natural event in the crust of the earth that pose a threat to life and property
• Related Geological Hazards


  • Rainfall-induced Landslide
  • Sinkholes
• Rainfall-induced Landslide


  Landslides caused by heavy rain due to storms and southwest monsoon (habagat) is the sliding down of soil, rocks, or mud from an elevated place like a mountain or cliff resulting in houses or structures, properties and even people being buried
• Rainfall-induced Landslide


  • Rain-induced landslide, Panaon Island, Southern Leyte, December 2003
  • Rain-induced debris flow, Brgy. Mayana, Barili, Cebu, December 2014 – TS Seniang
• Types of Rainfall-induced Landslide

  • Creep
  • Slump
  • Rock/Debris Slide
  • Rockfall
  • Debris Flow
  • Mudflow
• Creep


  Imperceptibly (Unnoticeable) slow, steady, downward movement of slope forming soil or rock
• Slump


  Involve both soil and rock, travel short distances and vary from slow to very rapid
• Rock/Debris Slide


  When debris from rockfalls and slumps mix with water, they can transform into debris slides that can travel great distances
• Rockfall


  Rockfall and topples involve rocks, are very to extremely rapid and can travel great distances
• Debris Flow


  The initial debris slide can pick up speed and develop into a debris avalanche or debris flow. If the mass erodes or follows a channel in the topography, it will generally be called a debris flow
• Mudflow


  Mudslides generally involve saturated soils moving at very to extremely rapid velocities and travel great distances
• Two Forces Affecting Landslide

  • Driving Force (DF) - cause the slope to move
  • Resisting Forces (RF) - stabilize the slope and prevent movement
• Factors Increasing Driving Forces

  • Steep slopes
  • Lack of plants and trees in mountains
  • Rocks weakened due to weathering
  • Breaking of rocks
  • Slopes that are too heavy
  • Natural triggers
  • Intense rainfall
  • Ground vibrations created during earthquakes
  • Volcanic activity
  • Man-made triggers
• Factors Increasing Resisting Forces

  • Removing excess water from slopes
  • Adding buttress material at base
• Signs of Landslide


  • Cracks on the ground, highways, or concrete floors
  • Utility posts, trees, gates, and walls of buildings tilt
  • Bulging (mass movement of rock material) ground appears
  • Utility lines underground breaks
  • Ground water seeps to the surface
  • Water in creeks or rivers becomes murky
  • Land movements and debris downslope
• Sinkhole


  An area of ground that has no natural external surface drainage- when it rains, all the water stays inside the sinkhole and typically drains into the subsurface. Sinkholes can vary from a few feet to hundreds of acres and from less than 1 to more than 100 feet deep
• Sinkholes


  • A sinkhole appears in barangay Banhigan, Badian town after the storm
  • Sinkhole appeared in Purok Jalandoni, Barangay Oringao in Kabankalan City, Negros Occidental
• Types of Sinkholes


  • Dissolution sinkholes
  • Cover-subsidence sinkholes
  • Cover-collapse sinkholes
• Dissolution Sinkholes


  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 preexisting 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
• Cover-subsidence Sinkholes

  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, are smaller, and may go undetected for long periods
• Cover-collapse Sinkholes


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

  • Decline of water levels - drought, groundwater pumping (wells, quarries, mines)
  • Disturbance of the soil – digging through soil layers, soil removal, drilling
  • Point-source of water – leaking water/sewer pipes, injection of water
  • Concentration of water flow - stormwater drains, swales, etc.
  • Water impoundments - basins, ponds, dams
  • Heavy loads on the surface - structures, equipment
  • Sinkhole left open to take more water may continue to grow and can contribute to the appearance of more sinkholes nearby
• Signs of a Sinkhole


  • Trees or fence posts that tilt or fall foundations that slant
  • New small ponds that appear after rain
  • Cracks in the ground
  • Sudden drainage of a pond
  • Rapid appearance of a hole in the ground
  • Dips, depressions, slopes that appear in a yard
  • Dead patches of grass or plants
  • Sinkholes in the neighborhood, etc.