SpaceMika

The target audience of my thesis is me, back when I first started this degree. It is the scientist with only limited geoscience exposure, and who is trying to figure out what’s important and what isn’t in landslide papers and is more than slightly confused by it all. Although the style will be technical and I will be using appropriate vocabulary (“entrain” really can’t be replaced with “scooped up and mixed in” even if it has more accurate denotations), what I choose to include and how I lay it out will be guided by this philosophy.

I spent a recent bus journey meditating on how to express this balance of elegant simplicity while trying to find yet variation of “the debris went down the slope, across the valley, then spread,” I realized something phenomenal: in science, being repetitive isn’t dull, it’s consistent!

I have all my landslides in a database. My lovely CodeMonkey has written a little program to suck the data out and slip it into a standard, consistent paragraph. I am going to hand-edit, but the structure will be written by machine.

Awesome.

The volume of this landslide is a bit strange to work with. The published volume does not match the figures in the same paper — even at the most conservative usage of the figures paired with bulking the volume, the volume differs by at least a factor of two. I know, x2 is pretty much negligible in most physics (it’s within an order of magnitude!), but it’s pretty important when calibrating a numerical model.

My current theory is that two different volumes are being used. This event had a large amount of material slump just a short distance, and a smaller volume run out a longer distance. In the figures, the two sets of material are differentiated. I think that if I treat the large slump as part of the path (exclude it from the volume) and just run the smaller volume on top of it, I should get the correct volume.

But this is exactly why papers should list exactly how it is they’re getting the volumes they’re using! Is it from field investigations? Air photos? Measuring what, exactly? It’s important!

The material for this event is unusual: it’s a loess slide. If I use the volumes from the figures and calibrate to the correct debris distribution (including the large pile-up at the base of the main scarp), it seems like a slightly runnier material than rock avalanches. Tentatively, I’m placing it as a Voellmy fluid with the parameters 0.15, 500.

DAN-W in action

This is how I spend my nights: clicking little dots onto other little dots, adjusting with more dots to make the spline even around tight turns, and hoping all the external data I’m using is cross-consistent. I’ve been trying to get the 1987 Val Pola, Italy profile correct, but so far no matter what I use for input (figures from papers, slices taken from DAN-3D terrain, or profiles extracted from googlemaps) my volume is ending up at 60-70 million cubic meters instead of the 40 million cubic meters I should get. Sigh.

On September 12th, 1717, a huge landslide came down off Mont Blanc in the Alps and ran across Triolet Glacier. Boulders, water, and ice were entrained in the flow. The landslide killed 7 men, 120 oxen or cows, and destroyed a whole lot of cheese (Grove 2004). The travel path came down the slope, splashed up the valley side, then continued down the valley about 5 km (Grove 2004 citing De Tillier 1968 & Porter and Orombelli 1980).

Triolet Glacier on Mont Blanc: location of the profile. Image credit to Google & HeyWhatsThat.com.

From Grove, “The debris of the giant rockfall from the Aiguille de L’Eboulement which swept the Triolet glacier in 1717 is estimated to have a volume of 16-20 million cubic meters and have descended 1860 m over a distance of 7 km in a few minutes.” However, this debris distribution has been revised quite a bit by other authors — Deline and Kirkbridge (2008) vastly reduced the scale of the event to 7.3-9.8 million cubic meters with the deposit covering 2.9 square kilometers to a depth of 2.5-3.4 meters.

Profile of Triolet Glacier: this is the cross-section I’m using for my DAN-W model. Image credit to HeyWhatsThat.com.

So far, modeling it hasn’t worked for me. I suspect this is because glaciers are slippery.