Please read the scenario, then choose the poll option that seems closest to how you would feel, as a reader, if presented with it. Imagine a range of tall mountains. They are high enough to be covered in permanent snowpack, and it's a big mountain range, hundreds of miles across. As you might expect, this mountain range is the origin of a lot of rivers. About 100-200 miles away, along one of the rivers fed by this snowpack, is the capital city of the made-up kingdom that is the setting of my current fantasy WIP. One of the mountains is a volcano, and it erupts. The eruption is arbitrarily powerful--imagine it as a 6 or 7 on the VEI, if you like. This melts a good deal of the snow, which then floods down the flanks of the mountain range. It's enough runoff, coming quickly enough, that the river running through my capital city overflows its banks even hundreds of miles from the mountains, and does so with enough force to wash away buildings, etc. This is a fantasy setting, but I don't want to use magic or any supernatural explanation for this part, because of reasons. Does anyone, off the top of their head, see any problems? I don't want a lecture on hydrology/volcanology, I'm just trying to find out if the average reader would or would not be able to suspend disbelief for this part.
I have a concern that the volcano is going to melt enough snow all at once to flow into the river all at once, to cause an abrupt flood rather than gradual increase-- but the volcano won't be hot enough to just vaporize the water. You have to provide a lot of calories to melt snowpack, but if you provide them all at once, you might just vaporize the top layers and leave the rest still as snow and ice. And volcanoes aren't really known for their moderation.
Iceland floods are unique, @Steerpike, and they recur with regularity. I've driven across the flood plains there, they don't build a lot on them. What happens is the water accumulates into large lakes of melted snow and ice then like a glacial flood when the ice wall breaks a massive flood follows as the lake drains. Volcanoes commonly cause floods called lahars.
Aaand I would know. I saw the Mt St Helens flood first hand. Eruption-Triggered Avalanche, Flood, and Lahar at Mount St. Helens—Effects of Winter Snowpack
This is my problem with the scenario. If the river were a huge closed pipe, I think it would work, but it's not. I think the extra water would overflow near the mountain, and mostly be used up well before the hundreds of miles. I did once study fluid dynamics, if that gives me any credentials. But only one semester, and I haven't touched the subject since. Hmm. What if you went with a large, deep body of water instead of a river?
Sorry but this is wrong. See my post above. Volcanoes are of two main types, shield volcanoes like those in Hawaii that low gas less viscous lava pours out of, and strato or composite volcanoes where the lava is viscous and full of gas. The latter are explosive like those along the Pacific Rim. They become the tall cone shaped volcanoes that erupt with huge explosions and pyroclastic flows of hot gasses and ash. A pyroclastic flow
But lahars do flow long ways. Some of the ancient mud flows from Mt Rainier reach all the way to Puget sound. If the snow pack is deep a pyroclastic flow can melt it all at once, literally.
Skip the vulcanologists! I think the major problem is the geography around your city. Where I live, the River Severn floods on quite a regular basis. The problem for Ironbridge is that it's a narrow, rocky gorge just after an area of low-lying farmlands, through which the river meanders slowly, and is then constricted by the gorge; what is a five-foot flood over the water meadows is suddenly narrowed, and becomes a ten- or twenty-foot flood through the town. If you exacerbate that by having a massive bridge with massive piers crossing the river at that point, and you've built in a choke point that will trap trees and other debris (never underestimate the amount of debris that a river in flood will pick up) and pile up the water even more. Once you've got the basic geography in place, it doesn't take much of a cataclysmic event to cause catastrophic flooding.
Well your work is fantasy so you can really just get away with 'because fantasy lol' but if you want a more realistic explanation, I think it's certainly plausible. Googling it quick, I can't find an instance of a volcano with snow on its peak erupting and causing mass melting but I DID find volcanoes that do have snowy peaks, so they do exist. I think Shadowfax brought up an invaluably great point that the water from the melting peaks doesn't really have to be the cause of the flood itself, a sudden river swell full of debris is far, far more than enough to cause a chokepoint (especially a bridge) to clog and flood huge areas. A couple years ago I worked as a news correspondent for a crappy little local news paper and the biggest story I ever covered involved a guy having his home destroyed in a flash flood. At first I was completely confused by the story when my editor asked me to go report on it. I kept thinking "Flood? What flood? I didn't hear shit about a flood, how did a flood happen here?". Even when I found the guy's house after following the address I was given, I still thought "this can't be the right place, no flood happened here". At least until I saw his house, which was clearly and thoroughly destroyed by flood water. What happened was exactly what Shadowfax just described. This guy lived right next to a small piss-creek river that you'd never thought would be able to flood, right next to a road overpass that bridged the river. But after a storm knocked out another smaller bridge a short ways up the river, that bridge basically clogged the overpass bridge (lol) and this caused a minor flash flood that pretty much flooded his entire yard with like five feet of water. A really, really, really shitty thing to happen to a pretty good seeming guy, but hey. Your best bet would maybe be pair the two disasters up into the ultimate shitty combo. Maybe the initial eruption caused river-side homes to crumble or roofs to slip off or whatever. They fell into the river, swollen and rapid with melted peak water, and then all that junk clogged up a massive bridge that fucked everyone up. That'd be my suggestion.
Thank you all for the good ideas. You've certainly shown me a lot of angles that I didn't think of before, especially the point about debris creating short-lived reservoirs along the way. It sounds like this could go either way, but I'm thinking there would be enough doubts that I might go with my backup location for the flood, which would be a different town in the foothills--closer to the volcano--and in a narrow valley, which seems much more flood-able. Maybe like the Armero case @GingerCoffee mentioned. I'll already have a couple other MCs who weren't going to be affected around there, so if anything I could play it for even more drama. Thanks again everyone, really.
You didn't look very hard. These are two different Iceland floods, different volcanoes, different years. And a volcano in Chile: Thousands of nearby residents have been evacuated, partly because of the risk of snow-melt from the eruption causing floods.
As others have mentioned, the only issue seems to be the distance the city is from the volcano. That said, when I first read the title, I instantly thought of volcanic tsunamis... There have been issues on volcanic islands with the huge amount of lava suddenly hitting the ocean and creating a high pressure bubble that turns into a tsunami when it hits a nearby coast. So it might work if there was a bay or other large body of water between the volcano and the city. I'm still not sure that it would work several hundred miles out, but under the right conditions, I believe tsunamis actually grow in magnitude as they travel.
Not exactly. You may be confusing what happens when the wave travels up a narrowing bay or fjord. In that case the water piles up making the wave higher and higher. Otherwise a tsunami wave is going to spread out like any wave and as such it loses strength. Consider the physics, just as the wave piles up in the narrow bay, it weakens as it spreads out wider and wider, resulting in the the fixed amount of energy being spread out over a wider area. In addition, friction slows the wave as it moves forward acting like a slow break. But you are right about landslide tsunamis being potentially larger than earthquake generated tsunamis depending on the amount of material that slides. The massive landslide that occurred when the flank of Mt St Helens gave way would have generated a mega tsunami had that slid directly into the ocean. It's hard to see the scale on this video but that peak was 5,000 feet above its base and lost 1,300 feet of height in the eruption.
Indeed, thanks for catching that! As I admittedly know very little about tsunamis, I quickly scanned over an article and clearly misunderstood what they were saying. Even in ideal conditions, the power of the tsunami will never grow. That said, I believe it's entirely possible that the height of the waves hitting the shore could be greater at a location further from the source than at a location nearer -- depending on the subterranean topography. Consider that many locations hit by a tsunami aren't line-of-sight with the source; in these cases, the tsunami is bouncing off of underwater land masses. If two of these "reflections," so-to-speak, should interfere constructively at a given point along the shore, the height of the waves could nearly double; conversely, other locations might see almost no fluctuation in water level at all... One of the main reasons the damage from a tsunami can be so uneven throughout a region.
Yes, topography is a critical element. A tsunami wave has a top and a bottom. When the bottom hits the ocean floor it drags causing the top of the wave to pile higher. It's the mechanism which causes a regular wave to break. But unlike in the movies showing a giant breaking wave for a tsunami, the actual wave is much wider perpendicular to the shore. In both tsunami and hurricane swells, the leading edge comes as a swell rather than a breaking wave. Now that we have some incredible videos of the Banda Ache and the Japanese tsunamis, movie directors are going to have to decide if they want to go with myth or fact with the next big tsunami movie.
