slope processes and development

Cards (14)

  • slopes fail if:
    • there is a reduction in the strength of a slope
    • there is an increase in the stress a slope is experiencing
    • combination of both
  • factors that reduce shear strength:
    • removal of basal support e.g. erosion by river/sea, "cut and fill to make roads
    • removal of underlying support (layer under surface is weakened) e.g. waste from a quarry is piled on a spring, which saturates base
    • organic effects e.g. tree roots or burrowing animals reduce cohesiveness of slope
  • factors that increase shear stress:
    • pressure e.g. pore pressure, freeze thaw, pressure release
    • transient stress e.g. earthquakes, vibrations from roads
    • weight e.g. new buildings, added water from heavy rain or burst pipes
  • rainsplash erosion
    when raindrops fall, material is thrown at equal distances but due to the slope, material falls further downhill, causing a migration of material down the slope over time
  • sheet wash - uniform sheet of water flows down hill, picking up loose material and depositing it, often after snowmelt
  • rills - water flows through fine channel networks, as water favours cracks over flat surface, then the water erodes the cracks further, and moves more material
  • gully - water is concentrated in channels, leading to gully erosion, which expands the gully
  • creep - gravity and rainsplash erosion cause soil to slowly move downhill, so poles lean, walls break, and trees have a curving growth pattern in the trunks
  • heave - When water in cracks in the rocks freeze, it expands as ice, increasing the cracks size, before melting again leaving the larger cracks. This process repeats, often daily, causing the soil to rise up, and on a slope, the soil gradually moves downhill.
  • Rock falls occur on slopes over 70 degrees, where pieces of material fall off the rock. Rock falls reduce horizontal pressure on a cliff face, allowing the growth of vertical cracks, which leads to more rock falls. Happens often after glaciation has occurred. Glaciers excavate deep valleys with steep sides, but when they melt, the sides are less supported and liable to rock falls. Rock falls produce scree which accumulates as fans at the base of the slope, which can join together to form a continuous slope.
  • Rotational slumps
    • During the summer, clay dries and forms cracks. In autumn, rainwater fills these cracks and saturates a layer of clay. This forms a small mudflow out of the slope, which causes the layer of clay above it to slump forward. For sand above clay, the sand is permeable, saturating the top layer of clay. Slopes with no basal support are more likely to slump as the shear strength is reduced.
  • Profile change:
    • top of slope is steeper
    • bottom of slope is gentler
    • material has been removed from the top, and deposited at bottom
    • (for rotational slump, there is also a stepped nature)
  • mud/earth flows:
    1. increased pore pressure from water breaks the cohesion of materials in the slope, reducing shear strength
    2. increased weight of the slope from water increases shear stress
    3. water acts as a lubricant in any lines of weakness which reduces shear strength
  • slumps and slides:
    1. increased weight of slope from water increases shear stress
    2. water acts as a lubricant in any lines of weakness which reduces shear strength