Sunday, 27 March 2011

Mass Movement 101

To all Lower sixth geologists and geographers of both years! 

You have recently studied mass movement as part of your course, which is of course the downslope movement of rock and soil under the influence of gravity. Here is an explanation of the key types of mass movement with some videos and after uch request, this post will be bullet pointed for ease of reading, though, personally I feel you need to be reading and making your own bullet points!!

Here is a good intro and a full episode of "When Nature Strikes Back" on Landslides:




Rock Falls
  • dry movements
  • very fast
  • typical on hard rocks
  • more common on rocks that are heavily fractured and jointed as these can be exploited 
  • key weathering process = freeze thaw 
  • Triggers = Earthquakes, heavy rain, eruptions, 
  • Anthropogenic Influences = undercutting, traffic vibrations
This video is an explanation of the hazard at Yosemite National Park, it is very clear on the risks, why Yosemite is prone to falls, and about how to assess impacts and detecting falls:





You may have seen this one in class, but here's how the Norwegians approach mass movement management:






Testing your rock throwing capability:




The Swiss approach - useful for management:




A very technical rockfall management barrier:




Landslides

This is the downslope movement of a large block of material that moves as a coherent mass, ie it retains its internal structure until hitting the base of the slope and fracturing into smaller pieces. It is more common over wet periods, and on steep slopes, and often observed on coastlines. 




  • Relatively dry movements, prompted by water
  • Relatively fast
  • typical on relatively hard rocks, often those that dip towards the base of the slope
  • more common on rocks that are heavily fractured, steeply dipping and easily laden with water
  • key weathering process = water layer weathering, hydration, freeze thaw
  • Triggers = Earthquakes, heavy rain, eruptions, undercutting by the sea
  • Anthropogenic Influences = undercutting, traffic vibrations, building on steep slopes, terminal groyne syndrome, climate change
This fist video is of a region that has been undercut to allow road access, which is clear at the end, and has prompted a massive landslide. It was triggered by heavy rains, but you can clearly see some of the trees remaining upright until the landslide hits the bottom of the slope:



This next video is of a huge landslide that occurred in a malaysian mine, it was predicted but you can clearly see how once it starts, the slope is destabilised and more and more material becomes entrained in the flow:


This Landslide in Italy last year, was spectacularly caught on camera, much of the material remains intact, and it looks like a flowing liquid, but, as with the malaysian video above, there is very little moisture in here, it is nearly a pure gravity driven movement:




Why we shouldn't deforest our rain forests reason number 1003:


Another swiss video, excellent for monitoring and management of landslides! Very informative and some good shots of testing barriers


And here is a barrier being tested:




So, in short, landslides can be managed through structural responses, ie building retaining fences, careful monitoring and avoiding potentially dangerous situations such as undercutting and building on steep slopes.

Slumps

These are very hard to catch on camera, they are similar to a landslide, but the slumped rock remans intact, just resting at a lower angle with the beds and trees tipping back towards the original cliff profile. The largest of the UK's slumps is at Lyme Regis, which you will see on the fieldtrip.


  • Fairly wet, and relatively fast movements
  • Material does not travel far from source
  • typical on soft rocks, often those that are unconsolidated or weakly consolidated
  • most common on muds and clays on the coast
  • key weathering process = hydration
  • Triggers = Earthquakes, heavy rain, eruptions, ash fall, saturation, snow melt
  • Anthropogenic Influences = terminal groyne syndrome, climate change, building on cliff tops.



Here is a photo of a rotational slump from Barton on Sea, note the surface is still vegetated, just at a lower position than  the rest of the cliff:


Unfortunately, i cant find any videos of a slump in progress, if you know if one, post a link below!

Mudflows/Debris Flows

This is a considerably wetter movement than any of the others, here water is the key, in that the excess water equates to extra weight, and the force of gravity acting on the slope causes the rock face to loose all internal structure and to mix with the water to form a dense fluid capable of transporting large boulders. 


  • Very wet movements, prompted by water saturating pore spaces
  • Relatively fast, but speed slows with change in slope and distance from source
  • typical on soft rocks, often those that are unconsolidated
  • most common on muds and clays
  • key weathering process = hydration
  • Triggers = Earthquakes, heavy rain, eruptions, ash fall, saturation, snow melt
  • Anthropogenic Influences = terminal groyne syndrome, climate change building on cliff tops, forest fires
The first video gives a good idea of the movement of a debris flow, this is a flow on the Andes and clearly shows that even at slow speeds, mud flows can move large boulders through traction:


This second video is from California, a region used to debris flows, this one is caused by a forest fire which exposes the soil and with heavy rain and no support, the soil is easily entrained in surface runoff. Note the tractor at the end,  I am not sure if he is meant to be there or not!



This shows an aggressive mud flow in Afghanistan prompted by snow melt, the region is clearly prepared/used to these events as the road is closed off and the gully well eroded:


In terms of management, debris flows can be identified before they start as the conditions that prompt them are fairly well understood. When this happens, barriers can be put in place as the following video explains:


They can be managed structurally, as this video of a barrier under construction shows:


Management

All Mass Movement can be managed through mitigation (lessening the hazard once it occurs) or management (trying to prevent it from happening). As every with geography, the basics for the region are:

  • Education
  • Hazard Mapping
  • Land use Planning
  • Evacuation Procedures
  • Prediction
  • Monitoring
Specifically for mass movement, the biggest enemy is water. Water can be removed through drainage, either surface or below. In places like Lyme Regis, where much of the cliffs are drained, this is done at great cost sub surface in order to preserve the attractions of the area. 

Surface runoff needs to be managed as well as movement through the rock, and no coastal cliff can be controlled without taking into account the coastal as well as terrestrial processes, which is why when considering coastal management, we need to consider coasts as a constantly changing, over different time scales and working as a system, with inputs from both the land and the sea. 

So, not quite bullet pointed all the way through, but a short video summary of different types of mass movement and their management. Post any questions below, remember for AS geography, all physical essays on the coast will need a mention of geology and the risks of different rock types. Mass movement for the A2 is a risk in most tectonically active regions and more of a risk in Developing countries. 

See you all on Monday

Millie


2 comments:

  1. Your blog is amazing so happy I found it! I am doing the OCR syllabus and wondered which one you are covering? Thanks, A2 geo student.

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