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Mass movement & Ground Water

Mass movement

  • Movement of weathered material down the slope due to gravitational force
  • Movement may be gradual or sudden depending on the gradient of the slope, weight of the weathered debris & presence of a lubricating agent such as water

 

Soil Creep

  • Slow & gradual but more or less continuous movement of soil down the hill slopes
  • Movement is not very noticeable, especially when slope is fairly gentle or when soil is well covered with grass or other vegetation
  • Most common in damp soils where water act as a lubricant so that individual soil particles move over each other & over the underlying rock
  • Though the movement is slow, the gradual movement tilts trees, fences, posts & so on which are rooted in the soil
  • Soil is also seen to accumulate at the foot of the slope or behind obstacles such as walls, which may burst by weight of the soil accumulated

Soil creeping

Soil Flow / Mud Flow (Solifluction)

  • When the soil is completely saturated with water, soil particles easily move over each other & over the underlying rock
  • Soil act as a liquid mixture & soil flow or mud flow occur
  • In Ireland such flows are known as Bog-Burst

Mud flow

 

Landslide (Slumping or Sliding)

  • Very rapid movements resulting in large mass of soil & rock falling suddenly
  • Landslide usually occurs on steep slopes & by earthquakes & volcanic activities
  • Landslides are often caused by the lubricating action of rain water
  • Slumping is usually common where permeable debris or rock layer overlie impermeable strata such as clay
  • Water sinking through the permeable layer is halted by the clay
  • Damp clay provides a smooth slippery surface over which the upper layers slides easily
  • Man often enhances the possibility of landslide by clearing natural vegetation for agriculture & housing which allows more water to penetrate through soil & rocks
Landslide
Slumping

Groundwater

  • When rain falls on earth it is distributed in various ways.
  • Some is immediately evaporated & thus returned to atmosphere as water vapour.
  • Some is absorbed by plants & gradually returned to atmosphere by transpiration from the leaves of the plant.
  • Much of it flow into rivers & streams eventually reaching seas & oceans as run off.
  • A considerable amount of water received from rain or snow, however, percolates downward into the soil & rocks known as groundwater.
  • Groundwater plays an important role in mass movement & weathering and is also important as a mean of natural water storage.
  • It re-enters the hydrological cycle by way of springs.
  • A spring is simply an outlet of stored groundwater, released at a point where water table reaches the surface (a man-made outlet for groundwater is known as well)
  • The amount of water available to form groundwater depends to some extent on climate, nature of the rocks (absorbing power) & seasons of the year.
  • Absorbing power of the rock is determined mainly by its porosity, permeability & its structure.
  • For ex. Sandstone is both porous & permeable, Clay is highly porous but impermeable, Granite is crystalline but pervious
Ground Water
Groundwater Aquifer

Water table

  • Water which seeps through the ground moves downward until it reaches an impermeable layer of rock through which it can not pass.
  • If there is no ready outlet for the groundwater in form of spring, water accumulates above the impermeable layer & saturate the rock.
  • The permeable rock in which the water is stored is known as aquifer & surface of saturated area is called water table.
  • Depth of water table varies with seasons, relief & type of rocks, as it is far below in hilltops but is close in flat surface areas.

Water Table

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