9 April 2017
Papamoa: a brilliant video illustrating the mobility of landslides
One of the hardest things to explain about landslides is the ways in which changes in the structure of soils can cause dramatic reductions of strength, and thus increases in mobility. A brilliant video has been posted to Facebook by Emma Kapua that illustrates this point as never before, I think. This landslide occurred in the Papamoa Hills of New Zealand, one of many landslides triggered by the remnants of Tropical Cyclone Debbie as it passed over the country last week:
The force of nature! Papamoa Hills Regional Park 6th April 2017 after heavy rain from ex. TC Debbie.
Posted by Emma Kapua on Thursday, 6 April 2017
You will see that this starts as a classic, small rotational failure. It is clearly formed from intact soil at this point:-
As the landslide goes around the circular shear surface it fragments rapidly, and changes phase:-
Thereafter the landslide behaves as a fluidised mass, and thus accelerates rapidly down the steep part of the slope:-
Understanding the processes that allow some soils to respond in this way has been a key aspect of landslide research, and of course it is essential to appreciate these mechanisms if the likely impact of future landslides is to be determined. It is the failure to understand that landslides can show this behaviour that can lead to an under-appreciation of the hazard that some landslides can pose. Clearly once a slide is moving in this way then the destructive potential is very high.