h a l f b a k e r yAmbivalent? Are you sure?
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OK, so pyroclastic flows (PFs) are avalanches of rock, ash and hot gases that occur when an eruptive column collapses [link].
PFs are damned dangerous. Damned, damned dangerous. Just ask anyone in Pompei. Well, you can't, cause they're dead. The key factors affecting the flow of a PF are mass
density and velocity. if we can change one or both of these factors we could stop a flow before it reached a settlement and fried everybody.
There has been talk that a series of 30m high barriers on Vesuvius could stop a PF by redirecting the force upwards so the flow collapses, but i don't think that would work on its own. Most flows move at hundreds of kph, so they could just flow over the barrier (especially in the case of pyroclastic surges, which are more dilute PFs - they can top barriers with ease).
It seems to me that the key to stopping a flow is to slow it down ASAP. the slower it is, the less energy it has, the less capacity to entrain particles. Eventually the flow will collapse and deposit its mass, hopefully before it kills anyone.
So, assuming that the average flow has too much kinetic energy to stop just by placing a big-ass barrier in the way, I think the best way to slow it would be to introduce turbulence into the flow. Not being an engineer, I have no idea about the technical aspects of this, but assuming that we knew the most likely path of a PF, we could install large vats of compressed air facing against the flow, and fire them off as the PF passes. hopefully, this would cause the flow to collapse, or at least steal some energy and minimise its range.
This, combined with barriers, should reduce the risk to the inhabitants of volcanic regions. This problem is getting greater by the year as the populations of volcanic regions are increasing greatly (volcanic slopes are very fertile).
pyroclastic flow site
http://www.geology....work/Pyroflows.html
look at the size of that column [sambwiches, Oct 04 2004, last modified Oct 21 2004]
Santa Maria
http://volcano.und....at/santa_maria.html
Santa Maria and Santiaguito [sambwiches, Oct 04 2004, last modified Oct 21 2004]
The Cascades Range
http://vulcan.wr.us...uptions_4000yrs.gif
Lassen went pop in 1948; anyone remember St. Helens ? [8th of 7, Oct 04 2004, last modified Oct 05 2004]
Montserrat Volcano Observatory
http://www.geo.mtu....ies/soufriere/govt/
Busy place .... [8th of 7, Oct 04 2004, last modified Oct 21 2004]
Mt. St Helens Pyroclastic Flow
http://volcanoes.us.../What/PF/PFMSH.html
Some nice pics [sambwiches, Oct 04 2004, last modified Oct 21 2004]
more PFs
http://link.springe...01-0189-7ch002.html
a little on flow dynamics [sambwiches, Oct 04 2004, last modified Oct 21 2004]
Heimaey
http://en.wikipedia.org/wiki/Eldfell
Icelanders stop lava flow destroying their harbour [Cosh i Pi, May 30 2007]
[link]
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Croissant, but more for the spirit of the idea than the idea itself; volcanos have a nasty habit of reminding us just how puny humans actually are. |
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I would suggest float tanks of water with explosive charges underneath, which blast up into the leading edge of the flow, cooling it rapidly. |
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i thought about adding water, but i think explosive addition of it would add to the surging gas about the main flow and make it SUPER-BIG. but as i say, i'm no engineer. |
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A proportion of the energy released by the explosives is converted into the kinetic energy of the water, and will still be small compared to the energy of the flow front. I think the answer is to disrupt the saltation layer, cool the gas, and accrete some of the suspended solids with the water. It's a million to one chance but it might just work. |
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These structures are going to have to be pretty darned big to be effective ...... therefore, expensive. And they would as like as not have to be resistant to earthquakes, and big lumps of molten rock dropping on them from a great height ..... |
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They will have to be put in place whenever a crater is shaping up to blow. That probably means trenching with a 'dozer, laying the charges in the bottom, putting a water tank on top, then guarding the whole thing to make sure no-one steals the explosives. Then you wait for the pyroclastic flow.......... |
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The only thing with enough waft to get a volcano's attention is a Tac-Nuke and in that case the cure is worse than the disease. |
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Yeah, I wouldn't have much confidence in our ability to prevent the flow from occurring. All we can do is focus on mitigation. Given enough money, we could actually engineer the route of a flow. Obviously they follow the path of least resistance, so we could assume that any valleys leading from the volcano would be prime routes. |
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If we could afford it we could build in a series of barriers set into the sides of a valley to form a 'zig-zag' kinda shape, forcing the flow to slow. You'd still have the problem of the gas surge, but if you could stop the solid material you might save a town or two at the bottom of the valley. |
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I worked this out for whole thing out for a volcano in Guatemala and it seemed like it would work. |
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The trouble with permanent walls is that volcanos are dynamic. Some regularly throw off parasitic vents; one vent popping up in the wrong place and your elaborate defence system is useless. |
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The idea of a bomb-and-water-bag system that can be deployed in a few days seems to have more potential. Of course, the ground shock from the lifting charges is going to disrupt buildings in the vicinity so the PF deflector needs to be deployed far from the protected area and close to the source in my opinion. |
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I don't think it would do much for the "column collapse" type of PF as that's an "ongoing" event due to the huge mass of the collapsing column, but it might stop a "burst" event. |
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I agree, in the event of column collapse you're pretty much buggered, but we can do something about dome/vent collapse. |
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Looking at Santa Maria in Guatemala [link] it would be possible to predict the path of any PFs with a fair amount of confidence. If the volcano was effectively monitored, you could watch new vents and domes develop and work out the chances of them erupting. |
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// If the volcano was effectively monitored // |
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And therein lies the problem; The USA, Japan and Italy can probably afford the kit and the personnel, and "they have the technology". Third world volcanoes probably get monitored because sicentists form Western countries want to use them for data gathering, not for any altruistic reason. So unless a wealthy western country wants to field-test a mitigation system somewhere safely far away from its own citizens, third world countries (who probably have the biggest problem) are up sh1t creek. |
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Better work fast, [sambwiches]. Yellowstone lake is tilting faster every year ...... |
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That's how the world works, 8th. If everybody had money we wouldn't see thousands of people die in earthquakes and volcanoes in LEDCs. Its sad, but a fact of life. |
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I'm thinking that only volcanoes such as Vesuvius and St. Helens would be the subject s of mitigation methods. |
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As an aside, the number of volcanoes that are candidates for this is actually very low. As anyone who knows about volcanoes will know, there are different eruptive types. Hawaiian volcanoes tend to erupt very liquid lava, and we wouldn't expect PFs. Some volcanoes, such as St. Helens and Krakatau, are famous for explosive plinian and peleean eruptions, forming large eruptive columns and PFs. |
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As a result, we would only have to monitor a few volcanoes around the world (plinian type, settlements nearby). I'm sure the US gov't can afford this, and Vesuvius attracts a lot of research dollars because of its fame. Everyone else can go to fiery, white-hot hell. |
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Vesuvius would indeed be the chappie to go for, providing that the Phlegrean fields don't get Upwardly Mobile first. Pozzuoli is looking ropey too, and the whole bottom of the bay is bulging according to the echo soundings. |
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People died at St Helens because they were stupid and stayed put; well, that's the Mk.1 Human for you. If we're looking at the narrow criteria that you've defined then suddenly the problem becomes much more manageable. |
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Any suggestions for a test site ? I think there are some nice places in Japan you could try. I have all the equipment here for you; Item, party baloon full of water, Item, firecracker, item, single use camera, item, airline ticket (single). It goes without saying that you ticked the box marked Cremation rather than Internment......... |
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[LATER] Montserrat ? [link] |
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a few people died at st helens who were outside restricted areas, as i remember. as you said earlier, //volcanos have a nasty habit of reminding us just how puny humans actually are//. we can't even guess where the danger areas will be in some cases. |
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monserrat would have been a great test if we'd thought of it. |
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Agreed; sea water would be available in unlimited quantities to fill the "troughs". |
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Seriously, maybe you should email a link to this idea to the USGS and the BGS. |
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screw that. i've had it with the damn USGS. they don't reply to e-mails, even if you say please. |
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Cooling the leading edge of the flow enough could in itself create a wall. (+) for taking on the possibly insurmountable challenge of lava. |
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// bomb-and-water-bag system //
Would the bomb be necessary? If the water were in a plastic container, could the disruption be accomplished by the PF itself? |
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Taking on lava flows is a different story, shz. PFs only contain solid rock, ash and gas. |
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Saying that , I can't see any problem with building moats around volcanoes and filling them with seawater. this might be effective in stopping, or at least slowing, many lava flows. |
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Lurch, I'm not confident about the water-bag method. I don't think we could do it on a scale large enough to cool the entire flow. |
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The original idea is to just destabilise the flow by forcing compressed air into it against the flow. While this wouldn't stop the flow in its tracks, it might reduce its energy enough to allow the effective use of barriers further down the slope. |
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// cost to build and maintain? // |
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// not scarring the landscape? // |
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Like, more than a pyroclastic flow does, [AO] ? <snigger> |
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// Would the bomb be necessary? // |
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I think you need to blast the water up into the flow, and do it fast. I don't think "disruption" would be enough, otherwise you could do it just with a strip of explosives. I'll do the math tomorrow. |
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If anyone has any knowledge of the flow dynamics of pyroclastic flows, I would welcome any input. I'm just a geographer, so I'm only trained in the causes and effects of these things. you seem to have a good amount of knowledge, 8th. any new ideas? |
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I'm just inferring flow behaviour based on my knowledge of blast mitigation. We need a proper vulcanologist. Do you think we can use a net, or should we dig a pit and bait it with some seismographs ? |
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Not just a cheeky ploy to bump the idea back up and encourage further voting/annos. Oh no. |
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You've got to cool the rock, and that means a lot of water. Silicate rocks have an average density of 2-3gm/ml, which means they're 2-3 times as dense as water. Molten lava has a temperature of between 750C and 1250C; to solidify it you need to cool it to below 700C. Let's say the average temperature is 1000C, which means you need to cool it 300C to stop the flow. |
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Water at STP (20C) can be raised to 100C before it turns to steam, which means a kilogram of water can usefully absorb 30 x 1000 = 3kCal of energy. Cooling n equivalent volume of rock 300C requires up to 300 x 3 x 1000 = 900 kCal of energy. So every cubic foot of lava will require, on average, between 200 and 300 cubic feet of water to cool it to the point where it solidifies. |
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(My math may be suspect- I've got a terrible cold right now- but I'll guess I'm right within an order of magnitude ;-) |
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All this assumes you have a relatively slow lava flow. I've seen films of pyroelastic flows that filled valleys in a few seconds. Hard to find pumps big enough to deal with that. |
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Mje, you'v forgotten heat of vaporization - 2.26 MJ kg. for water. |
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Not sure that you would need to cool much rock, really. These pyroclastic flows are bits of rock floating on a bed of hot gases, and it is the fact that the gases are choking and really hot that kills you. That just means you are dead before it buries you, though, so that might be a good thing! |
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However, I reckon that the best trick might be to dig a gravel bed then part fill with water. The flow would come down the hill, and fall through the gravel, slowing tremendously. The water probably wouldn't do much, but since it would get water in it when it rained anyway, why not try it? |
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It would be a little like stopping a herd of rampant sheep with a cattle grid. A big enough flock will walk over the bodies of the previous sheep, but the smaller groups will stumble and fall. |
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Agree with the intention. Dubious about the method. |
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However, the puniness of human activities alongside the fury of volcanoes shouldn't put us off: see Heimaey link. |
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Personally, I suspect the best techniques would be a forest of strong reinforced concrete pillars in the predicted path(s) of any flows. As folks have observed, pyroclastic flows will top barriers easily. But a forest of pillars will slow the flow and hopefully eventually take enough energy (kinetic and thermal) out of it for it to collapse. |
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