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:
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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:-

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.
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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.
Some types of landslides seem to require activation by extreme weather events such as hurricanes and summer cloudbursts. In the Terrible Valley event, I notice a lot of angular rock pieces and brown clash or silt.. “Just Add Water”, LOTS OF WATER!
In western Oregon, there are muddy canyon deposits of gravel, rock, wood and mud that have been filmed 1996, moving like the deposits in this video. In the Pliocene geologic record of the Willamette Valley (Troutdale Formation) , similar buried deposits “set up” like cement and are described that way.
There is a similar mechanism in the Franz Josef and Fox glacier Rivers flows in Westland, on the other side of the Southern Alps. You observe the base of the respective glaciers as the ice face tumbles into the milky glacial rivers. The ice breaks up rapidly. The noise of the bouncing ice is deafening – a sight to hear and behold.
What does the “granular flow” you used here mean ? Why not grain flow or debris flow ?