Why Appalachia flooded so from Helene's remnants | Hacker News Reader

Why Appalachia flooded so from Helene's remnants

scientificamerican.com

31 pointsanigbrowl20 comments

This is utterly terrifying:

> ... three-day totals that were well above 20 inches at multiple stations. For context, a three-day-long precipitation event in Asheville, N.C., the largest city in the most-affected region, is considered to be a once-in-1,000-year occurrence if it produces 8.4 inches of rain. (A once-in-1,000-year flood is one that has a 0.1 chance of happening in any given year.) The longest period that the National Oceanic and Atmospheric Administration calculates that out to is 60 days, for which a rainfall event in Asheville is considered to be a once-in-1,000-year occurrence if it produces 19.3 inches.

I've long known that we'll bee seeing "hundred year" and "thousand year" events much more frequently than their name suggests, but I didn't really fathom storms this far off the charts.

I'm not sure how you even begin preparing for or shoring up infrastructure against these sorts of extreme events.

The traditional way to harden infrastructure against flooding was to locate 'most everything expensive or important on higher ground. It was always the poorest part of town that got built on the flood plain, or next to the swamp, or whatever. And when a major bridge or something had to be built at a low elevation - its construction was often "heavy stone, and lots of it"-durable.

The seemingly high occurence of rare events is an artifact of how much we're measuring. If you have a thousand independent weather stations, every year you'll see a "thousand year" event at one of them. We just didn't notice the other 999 that didn't.

Same deal as how data centers manage equipment failures. If the drives have a million-hour MTBF, but you have a million drives, you're replacing one every hour. Thousand-year events happen all the time when you have a thousand trials.

This one may have been something wilder like 1 in 20k years, and among 1k endpoints you'll see that every 20 years, and this could be comparable to Katrina which was now 20 years ago.

How do you shore up infrastructure: either you spend a ton of money, or you don't do it and deal with the cleanup afterwards. Each additional 9 of reliability costs 10x. At some point the cleanup for one point is less than shoring up all of them.

These are not purely random events. Every weather station doesn't rolls the dice on their own.

If I put 1000 weather stations in my back yard, it doesn't suddenly flood every year.

> The seemingly high occurence of rare events is an artifact of how much we're measuring. If you have a thousand weather stations, every year you'll see a "thousand year" event at one of them. We just didn't notice the other 999 that didn't.

That would only be true if all those weather stations are fully independent. They're definitively not.

Drives should be independent, but all drives in a datacenter would similarly share their environment. Run the datacenter too hot and your MTBF will be out of spec.

Some of Benoit Mandelbrot’s seminal work focused on Operational Hydrology. This work informed subsequent developments in extreme value theory. Frequency and level of tail risk events may not be characterizable by moments.

https://courses.physics.ucsd.edu/2016/Spring/physics235/Mand...

Based on current costs and the increasing rate of events of this type, we are spending about 0.3% of global GDP on rebuilding from this type of event, and will cross 1% by 2035.

That may not sound like much, but 5%, which at current rates of increase we could see by 2060, is enough to put economic systems into a death spiral.

> Then there’s the terrain. In terms of response, mountains mean there are fewer roads to any given town, hampering both evacuation and response efforts, Camp says. Water will always flow downhill, no matter what, but mountainous terrain constrains where it can go. That means water cascading down slopes will more quickly accumulate in lower-elevation areas, worsening effects—and it will pick up speed as it travels, potentially making the flood even more dangerous.

I gained a deep respect for rains and flooding in the 2005 Alstead NH flood, where 13 inches of rain pooled above the town in ~20 hours, unable to drain effectively through the culvert, and burst through as a massive downhill wall of water. https://www.wmur.com/article/remembering-deadly-flood-that-r...

Mountainous terrain giving water speed is truly terrifying. Often too there are channels and valleys which experience massive amounts of water. It's nightmarish to imagine how much damage this could be, how hard it will be to support the people and places impacted so deeply.

Reminds me of hurricane Irene in Vermont. They got less rain but some areas got 7" in a short timespan and b/c of the mountains and narrow valleys it decimated some towns. I went to help cleanup and in one place a tiny 50' wide river rose probably 20' and was over 1/4 mile wide covering the entire valley floor. It was really sobering to see the power and damage that can be caused so quickly. It also really illustrated how bad severe rain in mountainous terrain can be

Water rushing downhill can wipe away whole towns. The World War 2 "Dambusters" raid against 2 large dams caused a huge amount of damage mostly by water from the reservoirs.

Is it not the case that engineers, meteorologists, and emergency managers in Asia, where typhoons hit mountains closer to ocean and much more frequently, have more knowledge and experience to provide with these sorts of things? Mudslides, floods, and evacuations are the norm over there. The Appalachians are pretty far inland, but maybe there's something to gain from looking towards Asian experts and storm preparation examples.

Title typo: Why Appalachia Flooded So Severely from Helene’s Remnants

Feels like automated title moderation.

HN posts have limits on title length. In such cases, it’s not unusual to post the original title as a comment.

HN seems to remove certain adjectives and, I guess, adverbs to minimize the sensationalizing of the titles (I assume). I was surprised by it recently when a title I submitted got "Massive" auto-removed even though length maximum was respected. Made the title more humdrum, but I can understand the rationale.

It makes sense in a poetic English sort of way.

Helene's effects makes me wonder how a similar storm will act in 10, 25, and 50 years time.

Question: can we quantify how much more energy storms are getting from the water and air nowadays vs 10/25/50 years ago? We have Accumulated Cyclone Energy per year, but I can't find historical charts comparing the curves each year. There is this which does have ACE per-year over time, and you can chart it with the drop-down at the top: https://tropical.atmos.colostate.edu/Realtime/index.php?arch...

> Helene's effects makes me wonder how a similar storm will act in 10, 25, and 50 years time.

...similarly?

i.e. Will the cyclone dump more water or less? Why? Will future cyclones move faster? etc etc.

Right. In which case it would no longer be a similar storm.

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