We're only beginning to understand the historic nature of Helene's flooding | Hacker News Reader

We're only beginning to understand the historic nature of Helene's flooding

arstechnica.com

52 pointsrntn37 comments

I was supposed to be there Monday. I am flying out Friday instead. My family, obviously, doesn't have power and water yet. The best estimates they have are a month for water and power may come back in the next few days. I26 north I40 west are impassible and may be that way for months. Friends businesses are gone. It is a mess. I plan on doing the only thing that non-emergency people should do in things like this: drive my family out of there as soon as I arrive. Fewer people = fewer problems. As to what got us here? Bad things happen to well prepared places (Japan in 2011?) but having lived in truly well prepared places, I gotta argue that US infrastructure and planning isn't great compared to a lot of other countries (definitely better than some though). Infrastructure always pays for itself in the long term but we have so much short term, selfish, anti-science thinking that it is scary.

> I26 north I40 west are impassible and may be that way for months.

The latest I've heard is they're expected to be fixed by Sept 2025. So just shy of a year.

It was a bad disaster. I was scheduled to be there, but took a look at the NOAA rainfall map the night before and canceled my trip.

Calling it unthinkable is really overselling it though. Mountain towns and roads flood when it rains a lot. If you search past years google search results for Chimney Rock, it floods with 3" to 5" inches of rain. The town is just a few feet above water level - I've walked along the river.

So you're saying it's a region already especially vulnerable to flooding. How is that supposed to be better?

They're not saying it was better, they're saying it was foreseeable.

Doh! Thanks, that makes sense.

I think they mean more literally historical, in the sense that like this is when people will remember becoming aware that inland mountain ranges are catastrophically vulnerable to hurricanes now.

> Meteorologist Ben Noll said that the level of moisture transported to western North Carolina is more than 1.5 times greater than any event in the historical record for the region.

This is a little more than "mountains flood when it rains" it seems.

> Mountain towns and roads flood when it rains a lot.

> Chimney Rock, it floods with 3" to 5" inches of rain.

This was no ordinary flood - Chimney Rock was basically wiped off the map. This isn't a "dry it out and replace the sheetrock" situation, most of the town is gone.

That's not to say building several feet above a mountain river is a smart idea (or any river, for that matter), but this level of destruction hasn't been seen since the 1916 flood.

"mountain towns and roads flood when it rains a lot"

Hurricanes never hit Western NC

How is that even possible? The Southeast gets slammed by hurricanes constantly, and it's not like these mountains are a thousand miles inland.

A couple of factors keep most hurricanes away from the mountains. There's a strong gulf stream current that tends to lead hurricanes up along the east coast, sometimes as far north as NYC. And also the Gulf of Mexico can draw hurricanes away. Hurricanes dump so much rain on land that they have to be constantly replenished by water sources, otherwise they'll peter out pretty quickly. Land also exhibits something like drag on hurricanes and tends to slow them down.

I bet there have been some hurricanes whose edges have grazed central NC, but the most intense weather (heavy, sideways rain, storm surges, and 200mph+ gusts) are experienced at the edge of the eye. It's rare that hurricanes are so big and intense that land far away from water experiences the worst of it.

Source: grew up in south Florida.

From the article, this might be rare but it has happened before:

> Perhaps the only comparable event, in which a storm struck the Gulf of Mexico coast and dropped incredible amounts of rainfall hundreds of miles away, came in 1969 with Hurricane Camille. After making landfall in extreme southeastern Louisiana, Camille tracked over Mississippi, Tennessee, Kentucky, and West Virginia before combining with a frontal boundary over Virginia and producing epic rains of up to 25 inches.

Sure they do. Agnes and Hugo both tracked quite a bit west, for example.

Rains a lot is an understatement. It was unthinkable. Entire towns are gone. 3 day rain totals of 20"+ across the region.

https://climate.ncsu.edu/blog/2024/09/rapid-reaction-histori...

The article notes a comparable event in 1969. It’s not unthinkable, just rare, and might become less rare with climate change.

I don't see 1969 in the article but,

Perhaps the only local event in the same ballpark as this one for the southern Mountains is the flood of July 1916, when a remnant tropical storm caused rivers to swell and inundate Asheville and other mountain towns. For more than a century, that event has loomed large as the area’s flood of record.

At the few river gauges in the region that observed both Helene and the 1916 storm, the crests since Helene have broken those long-standing records. The French Broad River and Swannanoa River – which collided at high speeds and high volumes in 1916 to overtake Biltmore Village – both saw new record crests during and after Helene.

The French Broad River in Asheville rose 1.5 feet above its previous highest crest, and downstream at Blantyre, the river surpassed its 1916 crest and was still rising when the gauge stopped reporting on Friday afternoon.

The Swannanoa River at Biltmore crested at 26.1 feet, more than five feet above its 1916 maximum and slightly above the apparent 26-foot crest in April 1791, making this effectively the worst flood along the river since North Carolina became a state.

And I agree that it will become more common with climate change.

Not sure what is going on, because I don’t see your quote. What I see is:

Perhaps the only comparable event, in which a storm struck the Gulf of Mexico coast and dropped incredible amounts of rainfall hundreds of miles away, came in 1969 with Hurricane Camille. After making landfall in extreme southeastern Louisiana, Camille tracked over Mississippi, Tennessee, Kentucky, and West Virginia before combining with a frontal boundary over Virginia and producing epic rains of up to 25 inches.

Did a previous version of the headline or article use the word "unthinkable"?

Accumulated Cyclone Energy (ACE) https://tropical.atmos.colostate.edu/Realtime/

The figure in parentheses is ACE, averaged over 30 years up to and including September 30th: 77.8 (94.1)

There's more detail here, including a helpful chart: https://tropical.atmos.colostate.edu/Realtime/index.php?loc=...

EDIT: Interesting that Beryl had more ACE than Helene. I wonder if that figure will change as the effects from Helene are investigated further?

No, Helene's ACE will remain unchanged until the post-season analysis of both storms. It's a measure of the storm's duration and intensity; Helene was a rather short-lived storm with the intense period only occuring for a short time compared to Beryl which is why it has such a lower value than Beryl.

https://en.wikipedia.org/wiki/Accumulated_cyclone_energy

For more information there's a Wikipedia entry:

<https://en.wikipedia.org/wiki/Accumulated_cyclone_energy>

I'm unfamiliar with the term, or what it means, though the concept seems interesting.

What it apparently isn't, is a measure of the total available or potential storm energy within a basin, which seems to me an interesting concept. I'd presume that would mostly be dependent on sea surface temperature, and SST might well be the best measure of that which exists.

If ACE is the total energy released via cyclonic storms, then the ACE:TPSE ratio might indicate future risk within a storm season for a basin.

Storm incidence is predicated on other factors, so far as I understand most especially high-altitude wind shear and total humidity. High wind shear and low humidity can both depress storm formation and development, and may have been a factor in the August lull for the 2024 North Atlantic hurricane season.

If you want to get a feel for what some of the rain people are talking about is like, see if you can find a model of rainfall intensity for your area. A good place to look is on the websites of your state government departments that have to deal with drainage and runoff.

For example here in Washington that would be the Department of Transportation. They need to be estimate how often a given area will get X inches of rain over Y minutes for a range of X and Y in order to figure out things like how big a culvert they need to put under a road. That information is in chapter 2 of their hydraulics manual which the public can download [1].

When someone mentions some ungodly amount of rain in a flood area, such as the 8" in 6 hours mentioned in another comment, you can take that rate (1.33"/hr) and then see how often you expect to see that rate where you are and for how long.

Doing that for my location, I expect to see 1.33"/hr every 2 years--but only for 8.5 minutes at a time. Every 5 years I can expect to see it for nearly 14 minutes. 18 minutes every 10 years. 25 minutes every 25 years. 32 minutes every 50 years. 38 minutes every 100 years.

Basically, then, if I think back over the last couple of years or so and remember the worst 8 minutes of rain I had, and then imagine it going on for 6 hours instead of just being an 8 minute pulse I'll have some idea of how much freaking rain that was.

Alternatively, I can see that I expect an interval of 6 hours with 0.19" of rain every 2 years, 0.24" every 5 years, 0.28" every 10 years, 0.33" every 25 years, 0.37" every 50 years, and 0.40" every 100 years.

So people who got 8" in 6 hours got 20 times as much rain as people where I live expect to see in 6 hours if they live here their entire life.

I'm curious what rates they get in those storms over shorter periods, such as 5 minutes. My tipping rain gauge takes about 100 ms to complete a tip, and it tips every 0.11", so if there was a spurt with intensity above 396"/hr it would not be able to keep up. Can those storms hit that over short intervals?

[1] https://wsdot.wa.gov/engineering-standards/all-manuals-and-s...

Insurance companies are undoubtedly moving out from servicing the areas. Voters continue to vote as if carbon dioxide and methane are not greenhouses gases that they emit. Such across-the-board denial of reality can have only one consequence: elimination.

Insurance companies probably won't be hit as hard as people think they will be. How many people in that area had flood insurance? Very few Americans have flood insurance, and they're generally concentrated in areas where there's storm surge, i.e. the coasts.

Unfortunately, a number of flooded properties were located outside the FEMA designated flood zones - even the 100 year+ zones. And they were located higher than the flood lines by something around 10-12 feet (in Ashville, NC).

I really hope a lot of changes are made coming out of this disaster.

FEMA will definitely redraw some flood zones after this.

We had flooding in Vermont on the same day in July as last summer’s flooding.

In a different deluge, one town had 8” of rain in six hours.

It would appear this is the new normal. Towns will have to be redesigned (some were built around rivers for power and transportation a long time ago).

Either i am having the biggest case of deja vu ever, or this post has time travelled.

I am certain i read this exact post and comments 2 or 3 days ago.

when a post enters the second chance pool, the timestamps of the post and comments get rewritten. if you hover over a timestamp you can see its original value.

What did people expect when they built in floodplains with no flood insurance? My question is why taxpayers have to keep subsidizing the cleanup and rebuilding in these areas.

think of it as your taxes and their taxes both paying into a national disaster insurance scheme run by the government.

I'd agree, but if it's a floodplain, I think it's fair to be annoyed when you pay taxes into this fund AND you pay into your own insurance, yet a bunch of folks choose not to buy insurance when they should and expect to get paid out of this fund instead. It should be a last resort, not an "I just decided not to buy my own insurance" backup plan.

there is very little living space in the US that is not subject to some sort of natural disaster. where i live in california, e.g., fire insurance is getting increasingly expensive and many insurance companies are opting out of offering it altogether. what you should be annoyed about is that you have to pay for private natural disaster insurance on a per-disaster-type basis rather than the government explicitly providing a national pool by default.

edit: also it's worth considering that the worse the land in terms of risk, the poorer the people who will be forced to live there because they were priced out of everything else. with climate change accelerating this will increasingly be a factor, and it is unconscionable to expect those people to bear the burden of insurance themselves. this is what a civilisation is for.

We should also fix the part where some dunce with a letter put a town where physics (at least occasionally) put a river.

What we call a civilization should have education at least equivalent to a seven year old sand castle architect. Not hubris so large we go broke attempting to occupy the waterspouts of the world just because there’s pipe and cable there.

> We should also fix the part where some dunce with a letter put a town where physics (at least occasionally) put a river.

How do you fix that, short of moving the entire town (which is going to cost almost as much as rebuilding it)? The dunce that established the town is long dead, and his relatives probably moved away to better pastures.

If I'm understanding you right, I totally agree with your idea of a national government-provided disaster insurance akin to universal health insurance. Throw it on the stack of things that should be like that and not run by for-profit companies. Unfortunately that will likely never happen. Except maybe Florida right now because it's almost happening by default.

First of all, this is perhaps the most cruel, hurtful response to 200 dead people and thousands of their loved ones and friends who lost entire cities overnight. Based on your generally-rightwing post history, though, I'm not surprised.

Second, natural disasters affect everyone, especially when we've decided to go full-steam ahead on altering the climate. Going from a category 1 to a category 4 in, what, 24-48 hours, is NOT normal. Except now it is, and it's millions of people who are vulnerable. It's only a matter of time before New York gets another hurricane.

You live in a society, and unless you want to conjure up some Mad Max hellscape world where everybody's just shooting at each other (based on today's rightwing zeitgeist, though, that wouldn't surprise me), I suggest you get used to the idea of the richest government in the world actually helping its citizens. Otherwise, what is the goddamn point?

Grow up.

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