23 December 2013

A Christmas recipe for perfect landslides

Posted by Dave Petley

Context for the recipe

As Christmas approaches the newspapers in the UK are full of recipes for the perfect mince pie / christmas pudding / turkey.  That set me thinking about the recipe for the perfect landslide.  This is what I came up with:

Raw ingredients

You will need a pile of weak rocks.  If these aren’t available, and shops do tend to sell out of useful things this close to Christmas, a pile of strong rocks with many joints will be a satisfactory alternative, and indeed may give some more spectacular large failures.  Avoid very strong rocks with very few joints – this will give you a Scandinavian landscape, which is not completely without landslides but will tend to disappoint in terms of the frequency of events:




Volcanic soils are a perfect introduction to landslide generation for those looking for a quick win.

Initial preparation

Subject your rocks to a few million years of rapid uplift (I find that something over 5 mm per year will be ideal).  This should create an over-steepened landscape, but to try to avoid making your landscape too steep – in this case you’ll tend to get rockfalls rather than landslides.  Your landscape does need to have a decent slope gradient though.  To create the perfect conditions you will need a landscape that has steep slopes right down to the rivers, but with occasional flatter areas (perhaps old river terraces) – you will see why below.

You will now need to marinade your landscape in a warm humid climate to generate really deep weathering – ideally to 20 metres depth or more.  Daily temperatures of over 30 C and a millimetres of rainfall every day will be ideal.  If you can get your soil to look like this then you will be doing well:




Shortly before you want to cook your landslides, I suggest that you completely deforest your landscape, ideally with aggressive clear-cutting to leave completely exposed soil, like this:




Creating the correct oven environment

To get your landslides going properly you will need two key aspects to your oven environment.  First, you will need very heavy rainfall.  Ideally this should be occasional (every few years), very large rainfall events, either as individual storms (a strong tropical cyclone is a good example) or more prolonged rainfall with occasional cloud burst events (such as the SW monsoon or the heavy rainfall in some parts of the world associated with El Nino events).  Second, you should subject your landscape to occasional strong earthquakes.  Generally speaking, rare very large earthquakes will give better landslides than frequent smaller events.  Very satisfactory results can be achieved by a combination of a strong earthquake followed by a large rainstorm.

Advanced landslide cooking

For those looking for a truly exceptional set of landslides, you can pep up the recipe with the actions of people.  Ideally, your population should be poor, living on the isolated flatter areas you created in the landscape above, and rapidly developing.  In doing so, they should throw away their traditional agricultural techniques to grow inappropriate cash crops on steep land that is being cleared.  To get these crops to market the local authorities should build new roads without any assessment of the stability of the route corridor and with no engineering measures to provide drainage or to stabilise the soil.  The international aid agencies will probably provide funds to support this type of approach.  This example from Nepal will provide a good template:

13_12 recipe 4


The impact of the landslides will be increased significantly if you encourage the local people to move from their safe terraces to live beside the road.

Serving suggestion

A light sprinkling of global warming, and in particular increased storminess and higher rainfall intensities, should keep your landslides going well into the future.