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 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

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

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

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.