22 February 2018

River of rock: a fantastic video of a granular flow from New Zealand

Posted by Dave Petley

River of rock: a fantastic video of a granular flow from New Zealand

From New Zealand comes a new video of a “river of rock“, a somewhat beautiful granular flow.  This was triggered by Ex Cyclone Gita, which made landfall across the central part of New Zealand, bringing heavy rainfall and, of course, landslides.  The “river of rock” was captured on video by Donna Field on the Rakaia River:


This is a magnificent example of a granular flow.  Whilst they appear to be very exotic, granular flows are quite common and have been well-described in the literature.  In essence the pebbles behave as particles, allowing behaviour that is akin to that of a fluid.  Of course the density of the fluid is somewhat higher than that of a more familiar liquid, such as water, and the particles are many times larger, but the river of rock is still able to generate the sorts of flow structures that we see in water:-

river of rock

The river of rock granular flow on the Rakaia River in New Zealand. Video captured by Donna Field in New Zealand in the aftermath of Ex Cyclone Gita.


I suspect that this one comprises particles that are of a comparatively even size – Simon Cox from GNS suggested to me that the material is probably greywacke.  It may be that this uniformity lies behind the unusual characteristics of this flow.

Stuff.co.nz has an article about the granular flow. They note that:

“A river of shingle rock flowing down Terrible Gully has left eight farms cut off from town in Rakaia, near Canterbury’s Mt Hutt.  Incredible video of the “rocky river” was captured by Donna Field of Cleardale Station as she surveyed the damage from former Cyclone Gita on Wednesday.  The flow of careering shingles closed Double Hill Run Rd which services the farms in the South Island region. It’s an event that has become a regular occurrence for the farmers in the area.

I guess it may not be too hard to understand why this location was named “Terrible Gully”.

Many thanks to the several people who pointed this one out to me.  Much appreciated.