Landslide mitigation

The best way to avoid damage, However, in the case of areas such injury or loss of property is to avoid as , it would be Living with a known danger areas altogether. impractical to try to prevent Some landslips have been triggered landslides in the long term. Limited by human activity such as mining, intervention can work sometimes to changing coast gravel extraction or even just help stabilise areas where nearby digging fossils out of a slope face. roads and houses are at risk. The removal of vegetation can also destabilise a vulnerable slope. Coastal landslips and rock falls:

Why do they occur?

Rock falls The National Trust has adopted 10 principles defining how we will manage the coastlines in our care. Those As with landslips, rock falls can strength of the rock. The overhang that particularly apply to the issues surrounding coastal occur without any warning but now breaks away and falls to the beach erosion are: we are considering material that below. The hollowed out stretches falls almost vertically rather than of cliff along a beach are often simply sliding. As before, it is the referred to as the ‘undercliff’. • The Trust accepts that the coast is dynamic and weather conditions, the specific changing and will work with the natural processes of geology and erosional forces that As with landslides, this is an coastal erosion and accumulation wherever possible act together to create cracks and ongoing natural process. However, fissures along fault lines in the rock. with rising seas and more violent Often, these fault lines travel deep storms, we must expect to see • The Trust will plan in the context of projected sea level into the cliffs and so when they accelerated rates of cliff erosion rise and will favour coastal realignment wherever this finally fail, enormous volumes of and consequently, more frequent material become detached. rock fall events. The general public can reasonably be accommodated are often unaware of these Chalk and Sandstone sea cliffs are dangers, especially when using cliff • The Trust will only support interference with natural particularly susceptible because paths that run close to the cliff coastal processes where it believes there is an wave action can rapidly erode the edge. toe thus creating a heavy overhang. overriding benefit to society in social, economic or Eventually, the weight of the So look out for warning signs! environmental terms overhang overcomes the sheer

For further information please contact our West How will Climate change affect landslides? Office. Telephone: 01297 561900. Climate studies suggest that we are As sea water warms, thermal E-mail: [email protected] likely to experience hotter drier expansion will cause sea levels summers and wetter winters around the world to rise. Climate coupled with more severe storm scientists are increasingly able to If you require this information in events. These severe storms will make measureable predictions. alternative formats, please call generate very powerful waves that will readily scour beaches and the 01985 843579, or email toes of vulnerable cliffs. [email protected] Ltd. Group schematic: Halcrow except Spittles/ Black Ven Flux National Trust charity no. 205846 All images courtesy of the Tony 2008, registered © The National Trust The Upper Greensands of Black Ven turn a golden What does ‘Slumping’ mean? yellow when exposed to the air because they oxidise. Often, the first signs that a movement is about to take place is when longitudinal cracks appear in the land surface. Then ‘slumping’ occurs (see opposite). A marked ridge or step is formed and it defines the boundary between the material that is ‘on the move’ and that which is still intact. Usually the ‘slump’ remains horizontal (although it may turn upwards slightly) and the overall effect is to create a series of earth steps or ledges.

1.8 metres per year… the average rate The destructive power of the sea We may not like it but natural hazards are all around of cliff top loss in the past 60 years Waves crashing against a soft cliff will us and some can be pretty devastating... Landslip areas may be spectacular cause material from the bottom (or but they are very dangerous. Ground toe) of a cliff to be washed away. The In densely populated areas, a major The Geology is complex and in fact, movements will occur without undercutting of a soft sea cliff causes landslip can cause loss of life and there is not one but a series of landslip warning, easily trapping the unwary material further up the cliff to become property on a massive scale. systems that are acting semi- walker. unstable. As the angle of the inverted Fortunately we do not have such sites independently of each other. The main slope steepens, so it becomes more in but we do have sites that focus of activity in recent years has The mud, shale and ‘debris toe’ of a recent landslip likely that the material above will slip. are both intriguing and dangerous been at the Spittles (lower, western Just where the land frequently moves. end) of the site. In storm conditions, sand and sea such a region exists near Lyme So why does land slip? spray combine to ‘sandblast’ the cliff Before Regis National and it is known as ‘Black Ven The area forms part of the face adding to the erosional force. and the Spittles’. The word ‘Ven’ is an Trust’s Estate. It also Short-term weather patterns include Landslips occur when the stability of a old English word for ‘bog’. It is forms part of the highly acclaimed storms, heavy rains, frost, ice and sloping land mass fails. They can be The problem has been made worse After East and Dorset World drought conditions. They act together triggered by many things including reputedly the largest mudslide in in some coastal locations where hard Heritage site. to affect the stability of the top soil. earthquakes, wave action (and quite Europe and has been studied by sea defences have interfered with the When these stresses occur alternately, commonly by sudden heavy rainfall) Geologists for generations. natural movement of sediments ‘Special Area of water can penetrate cracks and travel but local geology really determines Black Ven is also a along the coast. This can resulted in Conservation’ (SAC) deep below the surface. whether the land is going to fail or not. The youngest rocks here are the . These are beaches becoming depleted. Where golden-coloured sandstones at the strictly protected sites designated the beaches once protected the Water will penetrate easily if the rocks It is important to realise that this is summit of the cliffs. They are under the EC Habitats Directive and in neighbouring cliffs, they are now are porous (such as chalks and an entirely natural process and the approximately 100 million years old this instance it is because the unable do so because they have sandstones). If a geological ‘layer’ is beauty of our coastline is sustained by and are very soft and very permeable vegetated sea cliffs are an important been starved of vital replenishing impervious (such as Clay), then rain allowing the process to continue to water. Below these golden (and quite rare) type of habitat. sediment. sandstones there is a thin layer of water becomes trapped and creates a largely unhindered. 185 million years… ‘slip layer’. impermeable stiff blue clay known as The middle picture (right) shows a the age of the oldest rocks at Gault Clay. recent rock fall at Burton Beach in Black Ven Dorset. The lower picture shows a dangerous section of cliff that has clear vertical faults in it and could collapse at any time. What are the characteristics of a landslip that are Even with close visual monitoring, it easily seen? is not possible to accurately predict when the next fall is going to happen. The most obvious feature are the At beach level, the debris will spread stepped terraces. These are out in a fan-shaped arc and much of it vegetated but separated by steep, will be washed away by waves and bare slopes. Land will slip along lines storms. The muds and clays will A falling rock can reach speeds of up to 100 metres per second… of weakness known as ‘shear planes’. disappear first, often leaving a ‘fan’ of that is about 224 miles per hour!! Each falling rock can weigh Where a slip has occurred recently, harder boulders along the waters’ several tons the terrace below will be covered in edge. Playing on the ledges seems like good fun but it is very dangerous! loose debris and mud.