Mass Movement

Coastal Mass Movement

Mass movement is a critical process along the coast that influences the formation of coastal landforms. Coastal mass movement influences the rate of coastal retreat and contributes to coastal erosion. When a coastal cliff or slope is continuously eroded and weathered at its base, it becomes weak. In its weakened state, the base cannot support all the material above it, thereby making the cliff or slope susceptible to the force of gravity and mass movement. This is aided by rainwater, which lubricates the cliff/slope material. When mass movement causes the material to move downhill, the result is that the cliff or slope moves backwards.

Effects of mass movement

The material removed by mass movement is transported by the ocean and deposited along other parts of the coastline. Therefore, mass movement plays a major role in the development of both coastal erosion landforms and coastal depositional landforms.

In addition to altering the coast, coastal mass movement can have negative physical, social and economic impacts. Some of these are:

1. Damage to or loss of land
2. Damage to or loss of property
3. Damage to or loss of infrastructure
4. Injury to people
5. Unstable ground surfaces

Types of Mass Movement

The types of mass movement that you need to know are:

1. Slumping
2. Sliding – Landslides
3. Rockfalls

Slumping Mass Movement

In slumping, the soil or rock debris moves downhill along a concave or curved plane in a rotational manner. The upper part of the slump moves vertically downward, but the lower part moves outward, forming what is known as a toe at the bottom of the slump area.

Slumping occurs where permeable rocks overlay impermeable rocks, such as clay. Rainfall penetrates the permeable rocks, causing them to become saturated and heavy. The rain also causes the lubrication of the top of the impermeable layer. The extra weight of the permeable layer causes a curved plane to be formed. The rock debris and soil then slump along this plane.

Mass Movement Sliding

A slide occurs when a portion of the soil or rock along a steep slope suddenly gives away and moves downhill, usually aided by heavy rainfall. Sliding mass movement occurs on slopes weakened by weathering such as landslides, rockslides, and mudslides. We will look specifically at landslides.

Landslides

As stated above, a landslide is one type of sliding mass movement. A landslide is a sudden, rapid, downslope movement of large blocks of rock debris and soil along a definite or slide plane.

Landslides occur when the rock and soil are saturated with water from either rainfall or repeated wetting by waves on slopes at the coast. The rock becomes heavier due to the addition of water and begins to fall along the plane. Landslides occur on slopes with a steep gradient and are common in areas:

1. Of high relief
2. Where rocks are being undercut, e.g., at the coast
3. Where water gets into joints in the rocks

Rockfalls

A rockfall is the fastest moving type of mass movement where rocks break away from the cliff face and fall, bounce or roll downslope. Rock falls are typical in areas that experience freeze-thaw mechanical weathering, which causes chunks of rock to lose contact with the cliff face and fall. The fallen rocks form a scree slope at the base of the cliff.

Examples of Mass Movement

Now that we know the different types of mass movement, it's time for us to examine some examples of where a mass movement has occurred.

Slumping

Hornsea is a coastal town located in East Riding of Yorkshire. The area to the south of the sea defences is prone to slumping because the beach area is not expansive enough to protect the cliffs from the sea. Major slumping occurred here in 2008, where 8.75 m of land slumped. In 2019, about 50 m of the cliff slumped. In this latest slumping event, the scar left by the slump was initially 95 cm deep but grew to 300 cm within a week. Slumping in this area has resulted in coastal cliff retreat. As a result of the constant slumping, some people have had to relocate their holiday homes which are located along the cliff, as there is the fear of them being lost due to the slumping mass movement. The image below shows the extent of the slumping.

Evidence of slumping near houses along the Hornsea coastline - Ian S, CC BY-SA-2.0 Wikimedia Commons

Landslides

Sidestrand is a village which is located along the Norfolk coast. The coastline cliffs in the village are prone to landslides, which may also be accompanied by mudflows. Heavy rains in June 2019 resulted in a landslide which caused a large portion of the cliff to collapse onto the narrow beach and ocean below. After the landslide, The Sidestrand Hall School, which is located on the land above the cliff, had to have a survey completed to determine whether its perimeter fence would need to be moved.

Example of a landslide along the coast at Sidestrand, Norfolk- Evelyn Simak, CC-BY- SA-2.0, Wikimedia Commons

Rockfalls

In April 2021, the UK experienced its biggest rockfall in 60 years, when 300 m of the cliff along the Jurassic Coast in Dorset collapsed onto the beach below. It was estimated that approximately 4,000 tonnes of material fell as debris. At the time of the rockfall, persons were warned to stay away from the area as further cliff failure was expected. The impact of rockfalls like this is that they help to change and shape the coastline.