Why Helene flooding in NC was so bad;Atlantic notes; Coastal GA River Update

Quick recap on current storm situation, Kirk and Leslie are both fish storms. Although NHC boosted the Gulf blob to a 40% chance of becoming some kind of tropical disturbance, it isn’t likely to develop in to anything too threatening since it is encountering a frontal boundary that should both limit development and push the thing in to the Florida Peninsula (oops – it developed first and is likely to become a major hurricane before being pushed in to Florida). TD 11-E remains along the far southern Mexican coast pumping unneeded rain into the area.

Elsewhere, Typhoon Krathon made landfall on Taiwan, killing at least two and probably causing on the order of $2 Billion in economic impacts according to my TAOS(tm) model, and there is a decaying storm in the South Indian Ocean far from land (SH012025, Cyclone Ancha – note that the 2025 Tropical Cyclone Season in the southern hemisphere started September 1st so their ATCF ID is already 2025).

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The rivers in Coastal Georgia are seeing the impacts of Helene as water dumped upland is making its way to the ocean. Note that neither river basin encompasses the areas of worst impact in North Carolina, so for those who have asked about possible debris and contamination, that is not a factor for either the Savannah (which does drain from a very small area in far southwest North Carolina) or the Ogeechee (which does not get water from that area at all). Nevertheless, both rivers are at or above flood stage, and some localized flooding is expected over the next few days. Here are the latest forecast hydrographs and flood stages for the Savannah …

and Ogeechee rivers.

Again, these are “normal” floods (not to be dismissive about that, bad for those who are in the vulnerable areas, but not likely to intrude in to areas that rarely see flooding).

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So why were the floods in North Carolina so catastrophic? There are a number of factors. In general the rain that fell was roughly the same as we got from Hurricane Debby …

But that’s not the whole story. In mountainous areas there is a phenomena known as orographic precipitation (or orographic enhancement). This is because moist air flowing over elevated topography (orography – the technical term geologists use for mountains) causes higher rain rates on the upslope side as that air is forced upwards. Here is a diagram from a detailed study of an island in Korea during a passing Typhoon:

So, once all that water hits the ground it has to go somewhere. Some will infiltrate into the ground, but that is a slow process and once rain rates exceed a certain level depending on soil and depth to bedrock (and impervious surfaces like buildings, roads, and rocks), it just runs off. In coastal areas like the Frogmore Metroplex (Richmond Hill/Savannah/Statesboro/HHI/Beaufort), being so flat, the water spreads out and moves slowly. That’s not great if you are in one of the low (well, lower) lying areas in the way, but at least the water can’t easily concentrate and start flowing at high speeds like it can in the mountains in the narrow valleys.

Why does that matter? Here we get to use Math AND Physics (with the caveat these are not detailed calculations, just some grossly simplified numbers to give the average person a feel for the order of magnitude of these forces). The force that water (or wind) exerts on an object is calculated as the density times area (A) times the velocity squared (v*v), or F=ρ∗A∗v².

Let’s take a place that is inundated with a meter (three feet) of water, and that water pushing against the wall of your home. Around here the water is probably not moving much faster than a mile per hour (.45 meters per second, m/s). The metric system unit of force is the Newton (N), which is the amount of force required to move a mass of one kilogram (2.2 lbs) one meter. The typical human adult can push on something like a wall with a force of around 400N. That meter of water moving at .45 m/s is exerting a force of 250 Newtons (N), which is a bit more than half of the force a typical person would exert pushing as hard as they can on that wall. So it will make a mess, drop a lot of mud, cause mold, destroy your electronics and carpet – but it won’t knock your house over.

In Chimney Rock NC the water was moving over 30mph (14m/s), which for 1 meter deep (three feet deep) water is a force of 196,000N. That’s about twice the force a bulldozer would exert pushing into that wall! And the water was over 10 feet deep in many places. So you can easily see why many of these communities along the rivers are simply leveled. They were literally bulldozed by the water. If the foundation fails, entire houses can be lifted up and moved for miles, with debris creating an almost solid wall traveling downstream, taking all in its path.

As a reminder, storm surge and waves often move at high speeds (and anyone knocked down by a wave at the beach knows how powerful water can be). So always keep in mind that is why we evacuate the islands from storm surge – moving water is a truly powerful and dangerous phenomena.

By comparison, the 72mph wind gust that knocked over our Magnolia tree probably hit it with around 1/10th of the force of that mythical three foot deep water moving at 30mph, which is about the same as being hit by a Honda Accord at 30mph. Enough to snap it at a weak point.

So you can easily see why the damage in the North Carolina mountains is truly catastrophic, and the number of deaths may well top 1000. I’m trying hard not to rant about how utterly distorted our priorities are as a country because much of this could have been prevented by good infrastructure planning and better priorities (especially given our imperial adventures that dominate the news cycles). I have little patience for those who exaggerate the phenomena – bad as it is – calling this storm unique or similar terminology. This was predictable and within the expected limits we should plan for and be able to deal with. Calling it otherwise has the effect of excusing the impacts of disasters like this as an “act of God” or nature or climate change (which is likely be a factor) or whatever. This is on us as a society.

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Here is a note from the North Carolina State Climatologist, Dr. Kathie Dello:

The destruction from Hurricane Helene in western North Carolina has been distressing to watch — much more so for those who are waiting to hear from family and friends in the region. As a public service center for the state, our office wants to share not only the story of the storm, but also how to assist in the recovery.

While the desire to lend a hand in person can be compelling, note that officials are turning away individuals who show up to help. The best way to support cleanup and recovery efforts is through the North Carolina Disaster Relief Fund, which is accepting donations that go directly to nonprofits working in impacted communities.