What’s the Story Behind the Flooding in South Carolina? #scflood

If you are following the news, you know that South Carolina’s been hit with epic flooding this weekend.  We are resisting the urge to go out looking for too much trouble, so I refer you to the wonders of other parts of the internet for the big dramatic photos.  Meanwhile, a tutorial on why we are inundated so dramatically, when there’s not even a real hurricane experience going on or anything.

We Don’t Get This Weather

Because I live on high ground, even as the state is descended into a true emergency, our street has fewer puddles than usual.  South Carolina gets storms — intense rain that drops in a giant bucket in a matter of minutes, or sometimes hours or days.  The stormwater systems are optimized for moving the odd deluge out to sea in one big gulp, except of course for the amusing puddle system that gives us something to talk about in the hot, 105% humidity mugginess that follows a heavy storm.

What we don’t get is the kind of steady wetness for weeks on end that’s killing all the drought-resistant cultivars and causing mushrooms to cover even the reputable lawns:

What we have is a stormwater system that essentially depends on the last storm being long gone before the next one shows up.  99% of the time, it’s a system that works well enough.  We get “flooding” but that just means “really big puddles.”  What you are seeing on the news is stuff we never, ever see.

Down on the coast, of course, you’ve got miles and miles of swamps and lowlands, so it’s not the world’s biggest surprise that rain = floods.  In the uplands, though, to understand why we’re getting catastrophic flooding, you need to understand the pond-dam system.

What does a working pond look like?

Here’s a video of drainage pond at work:

Run-off from everything is channeled into drainage ditches, which feed a pond.  The pond is a built-structure.  It’s something someone dug out on low(er) ground as a collection point for rainwater.  99.9% of the time, it provides a scenic stormwater-and-other-wildlife refuge.  If you own a pond you might stock it with fish, or paddle around it on a little boat, or out in the country use it as a watering hole for your livestock.

The maintaining of a pond is an art people argue about, and your neighbors are particularly interested in making sure the earthen dam on the downhill side of the pond holds together.

Ditch Drainage is Not Like Concrete or Plastic

The rain this weekend hasn’t been intense.  On high ground with a concrete sewer system, the roads are clear.  What has been happening, though, for weeks on end, is that the ground has been getting wetter and wetter.

An earthen drainage system, whether it’s a naturally-formed creek or a dug-ditch, operating in a part of the world where it’s never very wet for all that long, isn’t sized to channel all the water it receives.  A significant portion of the water is meant to soak into the ground, and in the process refill the many wells and springs that dot the landscape.

Because the ground has been slowly getting saturated over the past several months, relatively more water is being channeled through the ditches.  This is what a creek-and-ditch system looks like when it’s carrying more water than it’s engineered (why yes, these things are engineered) to handle:

So this is one cause of the flooding.  The other part, and this is behind some of the more dramatic failures, is the tremendous load that’s built up on the ponds.

A Pond is Not Just a Hole in the Ground

When we think of a pond or a lake, we tend to imagine a little cup in the earth where water settles because it has no place else to go.  There are ponds and lakes like this.  There are naturally-occurring dammed ponds (video tip: National Geographic’s Rocky Mountain Beaver Pond), but most of South Carolina’s lakes and ponds are earthen dams.

The way these work is that you stop up a naturally-occurring stream or river at the low end, so that water fills in the space behind it.  When you do this, you typically install a mechanism that allows you to release water in a controlled manner.  In normal weather, this is a great way to mitigate seasonal flooding and give you a scenic watering hole into the bargain.  Ponds fill up, you let off some water, they fill up again.

Because of the unusually wet weather we’ve had for the past couple months, ponds and lakes in the region came into the weekend’s storm already full.

Here’s a video of a typical pond, similar to the ones that collapsed in the City of Columbia this week:

If you look towards the horizon, there’s something you don’t see.  It’s the dam.  The reason you don’t see it is because even though the owner lowered the pond earlier this week, the ongoing rain, in saturated ground, filled the thing right back up again.  So now the pond is flowing over its edges — as are others like it around the state.

This particular pond drains into a marshy wetland, and this particular pond hasn’t actually had its walls burst on the dam-side.  So what you get is merely this:

A very wet, not quite impassible road, with water flowing over the property on either side.

Pavement Is Not a Sponge

The catastrophic flooding around the state is a combination of wet soil, plus over-full drainage systems (including controlled-release of full ponds and lakes), plus catastrophic failure of certain dams, unleashed on roads or bridges that don’t have the capacity to withstand a sudden massive increase in water flow.

In some areas, though not all, the flooding may be a case of victims-of-our-own success.  The South is terrible and backward, as I am quick to remind you at frequent intervals, but not enough people have accepted this message.  Thus, unlike the forward-thinking parts of the world, we’re growing.  We are getting new people (terrible and backwards ones, of course) and we are getting wealthier.

Even with proper stormwater improvements to go with, the reality is that infill development (a good thing) doesn’t hold water the way a stand of forest does.

As we rebuild post-flood, something we need to acknowledge is that although this is the kind of thing that never, ever happens here — a true “perfect storm” of rare weather conditions — South Carolina is also built up, at this writing, into a place unlike what lay here before.

File:Flood North Carolina.jpg

Photo: The southeast floods of 1916. The Spring Street approach to Smith’s Bridge in Asheville, North Carolina From: “The Floods of July, 1916”, published by Southern Railway Company, 1917.  Archival Photography by Steve Nicklas, NOS, NGS [Public domain or Public domain], via Wikimedia Commons


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