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Wednesday
Jan032018

What is Coastal  Erosion?

Have you ever wondered why coastal cliffs always have warning signs such as “Danger- falling rocks” or “Don’t stand too close to the cliff.” This is because of coastal erosion. It may sound like a technical word, but is simply the wearing away and redistribution of material on cliffs from different forces such as waves, rainfall and high winds. Here we explore the mechanisms which propagate this detrimental process.

As bad as it is, coastal erosion is actually responsible for shaping the most amazing coastlines on all continents, from the Twelve Apostles in Australia to our famous White Cliffs of Dover. The type of coastline that erosion shapes will vary depending on the type of rock, the strength of waves and the type coastal management in the vicinity.

 Flickr | Tim Finley

Major causes

The main force acting on a cliff face is actually water; it is the most powerful agent on Earth.  Have you ever wondered how many times waves bash against the foot of the cliff in just one day? When you think about it, all that wave action is going to cause some damage sooner or later. After a while, pieces of rock will break off and in the worst case scenario, dramatic mass movements such as landslides will occur.

The British Geological Survey (BGS) have classified 2012-2013 as years with the most recorded landslides in the UK. In 2012 alone, there were 75 UK landslides compared to a typical annual average of 60. Extreme weather was to blame for this, especially heavy rainfall, which caused slope failures and reduction in soil strength. It is no surprise that it was the wettest year on record, leading to detrimental effects on coastlines. Landslides occurring after periods of high rainfall are the most dangerous due to the amount of water locked up inside the cliff, becoming heavier and slumping at any random point! This is why it is not safe to walk too close to a cliff…

 Flickr | Barney Moss

Believe it or not, the severity of the waves crashing onto the cliffs is not only dictated by wind and storm activity, but by the moon. If you don’t know already, the tidal cycle is governed by the phases of the moon. There are two types of tides: Neap and spring tides. The former occurs on the quarter and three-quarter moon phases and causes less forceful and calmer tides. Spring tides occur on full and half-moon stages and are powerful, sadly bad news for the cliffs! Research indicates that the frequency of coastal erosion is dramatic on the spring tidal cycle.

Which rock type is the most erosive?

There has been an abundance of research into which rock type is the most erosive. Believe it or not, chalk is considered the most susceptible to landsliding due to its capacity to become saturated. This saturation highlights that at a cliff setting, chalk absorbs moisture and becomes heavier at the toe of the cliff, which is a significant preparatory factor for landslide activity. A famous chalk cliff area is the UK Kent coastline, including the White Cliffs of Dover. There have been several landslides here, all owed to the buildup of groundwater within the cliff.

Clay is also up there on the list, its porous characteristics promote its ability to crack after being wetted and dried by wave action. Water simply gets into cracks and reduces the strength of the material.

 Flickr | Julian Jones

Types of erosional processes

Cliff erosion can occur in different forms depending on rock strength and rock type. As you can imagine, soft rock cliffs retreat more rapidly. According to coastal scientists, Brunsden and Prior in 1984, there are five key types of movements on a cliff: 1. Rock falls, where large pieces of rock collapse from the face of the cliff. 2. Translational slides, involving sliding of material down the cliff face. 3. Rotational slides, where material collapses from the middle part of the cliff leaving a curved face. 4. Mudslides, where silt or clay moves across the cliff surface irregularly. 5. Mudflows, where material moves down the cliff face in a ‘slurry’.

As you can see, some of these are falls, flows, and slides. It is assumed that falls are the most abrupt movements likely to occur on chalk cliffs, whilst slides are the most common and occur where there has been internal cracking such as on clay cliffs.

Flickr | Adam Pritz

Holbeck Hall Landslide

One of the worst coastal erosion disasters on record is that of Holbeck Hall, North Yorkshire. A four-star hotel was completely destroyed as a huge rotational landslide occurred in June 1993. The probable cause of the landslide is a combination of rainfall (140 mm in the two months before) and issues related to the drainage of the slope. It is suggested that 27,000m² of soil slumped into the sea, which is completely unimaginable! Intense rainfall is associated with this mass movement event, resulting in the muddy cliff turning into sludge and easily flowing downhill quite rapidly, taking the hotel with it!

Flickr | Alan Parkinson

A key point to consider with coastal erosion is that it is unpredictable, dictated by many forces in the coastal environment. It will intensity with rainfall and extreme weather, two things accompanied by the much dreaded topic of ‘Climate Change’. Could we see an increase in coastal erosion with Climate Change? It certainly should not be ruled out and coastal management should be strengthened at the same time.

 

By Sophia-Harri Nicholaou - Online Journalism Intern

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