Coastal Mass Movement

Sub-aerial processes Sub-aerial processes are those processes which operate at the coast but do not involve direct contact with the sea. Material is loosened and made more vulnerable by sub-aerial weathering and mass movement.

Weathering Salt weathering - sea spray enters cracks. Later the water evaporates to leave crystals of salt. Further evaporation enlarges the crystals. The growing crystal exerts force on the rock. The rate of salt weathering is most rapid in well-jointed rocks. Frost shattering - rain-water or sea-water enters cracks. Later the water freezes to ice and expands. This exerts extra pressure on the rocks and makes cracks become larger. Thawing of the ice allows the water to trickle into the new cracks. The rate of frost shattering is most rapid in well-jointed rocks. Frost shattering is slower than inland because sea-water freezes at a lower temperature than freshwater. Furthermore, frost is less likely at the coast than inland. Wetting and drying - water enters sediments and causes expansion. The sediment contracts when it dries out. Repeated wetting and drying causes stress fractures in some rocks, such as clay and shale. Other processes of weathering - hydration, hydrolysis, carbonation and biological weathering may also occur at the coast.

Mass movement - Mass movement occurs when weathered material moves downhill under the force of gravity (often lubricated by water). Mass movement is particularly active at the coast because undercutting of rocks by the sea makes them unstable. There are two basic types: Rockfalls occur when the waves undercut the cliffs and weathering loosens pieces of rocks on the cliff face. Rockfalls are most common on cliff coastlines with resistant rocks such as chalk or limestone Landslips occur when rocks become saturated with water. The slip is triggered by undercutting by the waves or by loading by heavy rain. The saturated material flows out from the base of the cliff to form a tongue of mud.

Mass movement This is where land movement at a range of speeds results in destruction of property and/or loss of life. It is often triggered by human activity. In its broadest sense it is the movement down slope of any weathered material (regolith) under the influence of gravity. The type of movement is influenced by: Angle of slope (steeper is faster) Nature of regolith Amount and type of vegetation Water Type and structure of rock Human activity Climate

Slow movements Unlikely to be hazardous. Soil Creep: Speed is below 1cm per year. Common in humid climates, and can be nearly continuous. Solifluction: Very slow, and of limited importance. Occurs in highland of Scotland at a speed of 5 to 10cm per year often due to a thawed top layer moving over a frozen lower layer. Fast movements Involve both mud and earth flows: type is dependent on amount of water involved. Earth flow: Occurs on slopes between 5 and 15 degrees, often after the regolith has become saturated, and flow then results. Vegetation can be destroyed and speeds range from 1 to 15km per year. Mudflow:The best-known example is that of Aberfan. A small mining community was virtually destroyed as coal spoil heaps were placed on slopes over 25 degrees 200 metres above the village. The heaps became unstable as they were saturated from springs. A massive mudflow resulted on Oct 21st 1966 in which 147 people were killed, 116 being children. Rapid movements Slides: A movement of material 'en-masse' which remains together until impacting the bottom of a slope. Rockfalls: Slopes must be extremely steep. They are a result of a variety of causes, for example, extreme weathering such as freeze-thaw action may loosen material. Slumps: Usually found on weaker rocks (Clay), that becomes saturated and heavy. Undercutting of cliffs by wave action can also be a cause, as can human activity increasing pressure on rocks, as shown in Scarborough in 1993, where the Holbeck Hall Hotel slumped into the sea.

A classification of mass movement

4 star hotel in Scarborough, North Yorkshire (1993).

Black Ven, Dorset Coast

Osmington Coastal Path, Dorset Coast

How Landslips work (1) Rain water sinks through the porous rocks but once it reaches the underlying clays it can sink no further. The water builds up along the junction between the rock layers and seeps out of the cliffs as a series of springs. Rain water sinks through the porous rocks but once it reaches the underlying clays it can sink no further. The water builds up along the junction between the rock layers and seeps out of the cliffs as a series of springs. After periods of prolonged rainfall, the build up of water increases the weight of the cliff top. Increased poor pressure reduces the friction and allows large sections of the cliff top to break away. After periods of prolonged rainfall, the build up of water increases the weight of the cliff top. Increased poor pressure reduces the friction and allows large sections of the cliff top to break away.

How Landslips work (2) As the cliff top block subsides, it rotates along the slip plane within the cliff, resulting in the flat surface tipping back towards the cliff. As the cliff top block subsides, it rotates along the slip plane within the cliff, resulting in the flat surface tipping back towards the cliff. The main type of landslide that occurs at the coast is called slumping. The main type of landslide that occurs at the coast is called slumping.

Slumping (also landslips) Slumping can occur after heavy rainfall or earthquakes Back tilted slopes Large blocks break away Sliding surface is concave Soft boulder clay cliffs can be undercut by the sea and slumps are common. Slumps occur in weaker rocks and involve some rotational movement.

Landslides in Dorset The Dorset and East Devon coast contains a wide range of landslides, both large and small. The Dorset and East Devon coast contains a wide range of landslides, both large and small. The coast is unstable because there are numerous situations where porous strata (eg: chalk and sandstone) lie over impermeable clays. The coast is unstable because there are numerous situations where porous strata (eg: chalk and sandstone) lie over impermeable clays.

Old Harry Rocks

Another view Sliding surface is concave Clay Sandstone

Impermeable Clay Permeable Sandstone Rotational Slumping Sheer Face

Your turn Make notes on sub aerial weathering: Make notes on sub aerial weathering: - Weathering - Types of mass movement