Module 11C: Geological Hazards - Mass Wasting

• Mass Wasting - The downslope movement of rock and regolith which can be natural or artificial in origin and triggers: rainfall, earthquake, human activity
• Erosion - transport of weathered material by moving agents
• Mass Wasting - primarily caused by slope failure (instability) due to gravity
• Slope stability - highly dependent on applied forces and gravity on a mass at an angled slope and properties of the material
• What are the four factors that affect slope stability?

  Slope angle

  type of material

  water

  Geology
• Gentle slope - when tangential forces are less than perpendicular forces
• Horizontal slope - kept in place
• Steep slope - When tangential forces are greater than the perpendicular forces
• Angle of repose - Steepest angle at which sediment deposit or soil is stable and will not move downslope

  dependent on the size of sediments
• Moist sand has a higher angle of repose than dry sand
• Water saturated sand will lead to slope failure
• Geologic discontinuities - (planar structures like beds & fractures) serve as “weak planes” where sliding may occur.
• Daylighting - When planar discontinuities (beds, fractures) are tilted in the same direction as the slope, increases likelihood of a sliding slope
• Rock Fall - Abrupt, downward movements of rock or earth, or both, that detach from steep slopes or cliffs (free-fall)
• What are the two kinds of slide movement of rock?

  Translational

  Rotational
• Translational slide - where the movement of rock or debris is along a planar surface
• Rotational slide (slump) - where the movement is along a curved/arcuate/concave up surface
• Flow - rapidly moving mass-wasting events wherein loose material is mixed with abundant water where there are long runouts at the slope base.

  Can be separated into debris flow (coarse material) and earthflow (fine material)
• Runout - how far a landslide travels
• Avalanche - rapid to very rapid landslides and is related to the collapse of slopes or volcanic edifices
• Soil Creep - extremely slow downslope movement of unconsolidated material and it takes years to have noticeable effect on the slope

  due to expansion and contraction of surface sediment and pull of gravity
• Soft mitigating measures - are those that do not involve putting up engineered structures
• Hard Engineering Measures - These are other engineering measures to prevent landslides
• What are the four main reasons why mass wasting occurs in the Philippines?

  Climate

  Tectonic Setting

  Topography

  Human Activities
• 2006 Guinsaugon Landslide - Guinsaugon village, as well as the other villages located along the footslopes of Mount Can-abag where more than 1000 people died
• 2012 New Bataan Debris Flow - specifically Barangay Andap (the most affected area), is located at the mouth of a mountain drainage network (alluvial fan) and this incident was triggered by category 5 supertyphoon Bopha (Pablo)
• What are the four recommendations on improving disaster risk response?

  Forestation

  Proper Land Management

  Deflection Walls

  Spreading Awareness