Hazards Resulting from Mass Movements

Mass Movement

Mass movement is the downslope movement of material (rock and soil) under the force of gravity. It is the umbrella term for a wide range of specific movements including landslide, rotational slumping and blockfall. It occurs when the downslope gravitational force exceeds the resisting forces of friction and internal rock cohesion.

​The type of mass movement depends upon lithology:

unconsolidated material (like boulder clay) - slumping

consolidated rock (like carboniferous limestone and granite) - sliding

Blockfall (or rockfall) 

This occurs on slopes >40', where a rock fragment breaks away and either drops vertically (so it isn't in contact with the cliff) or bounces downslope. 

It's initiated by mechanical weathering

• freeze-thaw

• salt crystal growth

• which break the cohesive bonds in the rock

• By marine erosion

• Hydraulic action

• Abrasion

• Undercutting cliff by creating a wave-cut notch

• Notch removed supporting material that supplied the resistive force holding up the rock

Cliffs prone to blockfall have:

• a geological structure with many joints, faults or bedding planes

• steep, near vertical dip of strata

• they're often also in an earthquake-prone area

Blockfall is very rapid, taking only a few seconds to occur. 

They may involve the detachment of single fragments or of a whole section of cliff that breaks up as it descends (which occurs by the undercutting of a wave-cut notch). 

In April 2013, there was a large blockfall in St Oswald's Bay on the South Dorset Coast where an 80 m section of chalk cliff was detached overnight. 

​Rotational Slumping

Rotational slumping involves rock failure and movement along a curved rock plane.  The slumping material usually moves intact as a single mass, without any internal deformation of material. It's slower than blockfall, often occurring in 'slow motion', and may take minutes, hours, days, or even years (for huge masses) to occur. 

Rotational slumping occurs in:

• ​weak rocks, e.g. clays and shales

• unconsolidated material, e.g. boulder clay, sands, gravels

• rocks with complex geology, e.g. where permeable rock strata overlie impermeable beds 

Slumping is facilitated by the presence of water, which adds weight (increasing the gravitational force) as well as lubricating it, reducing friction. 

An example of rotational slumping is at Christchurch Bay, in Barton-on-Sea, near Lymington in Hampshire, where unconsolidated sands overlie clay. The bedding plane between sand and clay dips seawards. 

In dry weather, soil above sand cracks, funnelling water into permeable sand. Increased pore water pressure along lines of percolation form lines of weakness in the sand. Water accumulates in the lower sand as it is unable to percolate into the impermeable clay. Pore water pressure lubricating the bedding plane encourages the movement of sand. The weight of the water adds to the downslope gravitational force, while wave erosion created a notch at the cliff foot, removing support. Eventually, slumping occurs.  

​Landslides

A landslide is the downslope movement of discrete blocks of rock down a flat/linear slip plane, maintaining contact with the cliff surface throughout.  The discrete blocks are released by mechanical weathering of well-jointed rocks, (e.g. carboniferous limestone). Gravity then pulls the loosened block down the relatively flat slip plane of the joint or bedding plane, to the cliff foot.  Landslides can also be caused by marine erosion of a cliff foot undercutting blocks weakened by jointing. The removal of support allows gravity to release the block, resulting in sliding. Rainstorm events can encourage a landslide, lubricating the slip plane, reducing the resistance. Landslides occur in consolidated rocks with joints or bedding planes sloping seawards. 

Flows

Flows occur when unconsolidated fine grained sediment, e.g. silts and clays, mix with large volumes of water. They're common in weak rocks such as clay or unconsolidated sands. They become saturated, lose their cohesion, and flow downslope. Heavy rainfall, combined with high waves and tides can contribute to saturation. Earthflows are more viscous than mudflows, and contain larger sediment. In cold environments, earthflows known as solifluction occur in the unfrozen layer between the permafrost and the tundra vegetation turf. 

Types of mass movement may be classified by:

• the speed of movement

• the water content 

• the type of sediment