Coastal Systems: How Mass Movement Affects the Coastline

There are several types of mass movement that occur along a coastline and involve significant quantities of material being released and falling under gravity, usually onto the shore.

Rockfall

Rockfall occurs when rocks are broken down by freeze-thaw weathering, this loosened material is vulnerable to the elements. If the rock is at the top of a steep cliff face, it can fall directly to the shore. This can occur more so when a wave cut notch is created at the foot of the cliff by wave action, causing the overhanging rock to be unsupported and more likely to fall.

Soil creep

Soil creep is a process operating at the granular scale. It happens on gentle slopes and is noticeable from the wavy surface it produces. Damp soil moves very slowly down the slope as the weight of water pushes it forwards. Rain splash may release soil grains that fall further downslope.

Landslide

Landslides arise when rocks and unconsolidated material on the cliff face are saturated with water (rain or wave-splash). Eventually the material slips down the slope. Landslides occur more often on soft rock coastlines where there are natural joints in the rock, into which water can get. They occur on steep gradient slopes and are very similar to slumps, although slumps occur on shallower, concave slopes.

Rotational slumping

With rotational slumping, heavy rain is absorbed by unconsolidated material making up the cliff (often glacial till, or boulder clay). The cliff face becomes heavier and eventually it separates from the material behind at a rain-lubricated slip plane. Slumping occurs on a concave cliff face and contributes to this shape as successive slumps accumulate. Material at the slump foot (toe) has to be removed by wave action before more slumping can replace it.

Mudflows

Mudflows occur on very steep slopes along the coastline. Where there is limited vegetation to bind the soil together and the ground is very saturated heavy rain can produce sheet flow over the upper cliff surface. The soil continues to be lubricated and it eventually flows over the cliff face and down onto the shore at a fast speed.