Saturday, August 17, 2013

When The Mountain Falls

The Ruines de Séchilienne is (are?) not classified as a real landslide, geologically speaking, from what I understand of my hasty translation of several sites on the subject, including French Wikipedia. Instead, it's an "internal progressive rupture caused by fracturing." While this has been going on for about 10,000 years it's been a growing problem for the last 30 or so, and the government is rightfully worried that another big rockslide could dam the Romanche river, which runs along the curve of the mountain below the pile of debris. That would likely bury two villages and drown several others (including the one I'm currently living in), creating a lake that would displace about 7,000 people. Worse, the dam would then eventually break and a cascade of water would rush down the narrow valley to Grenoble and wash it away, along with the large chemical plants in the region that make chlorine and hydrogen peroxide and many other nastier compounds. There's about 50 miles of river before it reaches Mont Sec ("Dry Mountain") at the end of the Belledonne range, and the massive ice fields in the Massif des Écrins where the river starts send a lot of water down the valley; it's not a wide river, but it's fast.

Three million cubic meters of rock tumbled down in November of 2006, according to a documentary I found on line, and over a hundred million are immediately unstable. The whole of Mont Sec is full of fractures and, as you might infer from the name, there isn't much in the way of underground water sources, so apparently - and geologist Mom will either support or refute this statement - when the water from snow and rain freezes and drains each year, the whole edifices loses more and more coherence. The documentary shows the alternating layers of harder and softer rock that are sliding down, and I imagine that if a giant fist whacked the top of the mountain it would all fall apart in sections like a chocolate orange.

There used to be a town where the cone of rubble sits, but fortunately everyone was moved out in the 1980s when it became obvious that they were in danger. The road was moved over away from the bottom edge of the mountain, on the other side of the Romanche, and there's an ongoing project to move it even further away and route it through a tunnel/bridge configuration on the opposite side of the valley. Another project is to try to move the river itself by providing it with an underground passage, but that's probably not feasible; they've constructed a sort of alternate channel on the surface already, but it wouldn't make a difference if a significant landslide occurred.

Of course, since we're talking geologic time here, it's not surprising that such an event has already occurred. Séchilienne was already washed away once (at least) by floodwaters from former lakes. At the end of the 12th century a rockfall blocked the Romanche farther up the valley, where Bourg d'Oisans is now, creating a lake 11 miles long. In 1219 the rock dam broke and the lake drained and scoured the valley all the way down to Grenoble.

There are half a dozen monitors tracking the movement of the earth at the most active part of the fracturing, the Ruines de Séchilienne, which are just around the other side of the slope I see when I look out the window to the south. According to a study published in 2010, the rate of movement "has increased from 0.5 m/yr in 1996 to 1.4 m/yr in 2008" in this zone. The goat dairy isn't menaced by the rock, but it's not high enough to avoid the water should the river be dammed. And that would be a damn shame, so let's hope that the giant fist of doom does not fall any time soon.

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