Is SC Really Having a 1000-Year Flood? I don’t think so. #scflood

Statesmen have a right to poetry in their oratory, and I give Governor Haley every allowance for referring to the flooding in South Carolina as something that happens “once in a thousand years.”  The point she wishes to make is that the present events were utterly unexpected, and I agree with her.  There are countless cases where you can glance at the land and the forecast and be utterly unsurprised by a bit of flooding; the recent disaster in the Midlands is not one of those events.

All the same, I’m going to hazard that if we look at some snatches of recent history — and by “recent” I mean less than a century — the picture is a bit different.

How much water does this place get?

Seasonal fluctuations in water levels are normal, and so are variations from year to year. Sometimes summers are wet and winters are dry, sometimes it runs the other way around.  We are used to the rivers and creeks doing their thing, going up and down.  It entertains.  In recent memory, however, there has been nothing like the overwhelmingly massive flooding we are seeing now.  The evidence, unfortunately, is that this is because we are forgetful.

The Lake Murray Dam was completed in 1930, and one of its purposes was flood control.  If you look at river flow records on the Congaree, you’ll notice some very high-water years prior to the building of the dam.  But note: This year’s record high during the floods at 155,000 cfs was exceeded by a long shot in 1936 at 231,000 cfs.  Take a look at the data — there were yet other big-flood years since the dam was built.

For data from this year’s flood, check out the feed at the Congaree Riverkeepers Facebook page.  Side note: The Gervais Street Bridge was completed in 1928, which means it’s already survived 300,000+ cfs.  Don’t fail us now, little bridge!

The geography of the city underscores this point.  In the past twenty years there has a been a flurry of development along the river, as if someone suddenly noticed there was a major scenic area right there in the middle of everything.  But if you had shown up in 1990, here’s what you would have found on the riverfront: A chicken factory (still there), a couple of water treatment plants (ditto), and a prison.  The scenic Vista tourist area just up the hill from the river (around the recently-built Columbia Convention center) was all industrial property — factories and industrial-supply houses.

(In 1990 there were two pioneering apartment complexes — but nothing at all like what you see today.)

Part of this was due to the river’s history as a transportation pipeline and as a power source for mills.  Likewise, there’s obvious logic to laying out the state capital farther up on the more buildable flat lands at the top of the hill.  But if you look at the siting of the mill villages on either side of the river, you’ll notice that all the historic development leaves plenty of elbow room for flooding.  Not only is the obvious floodplain kept clear, but there’s a bit of extra cushion before you reach the historic structures.

We who are living in 2015 are surprised by so much rain and what it does to a river or creek.  Someone building in 1915?  Either they knew to expect massive floodwaters, or else the evidence of their ignorance has since washed away.

Just how old is this place, anyway?

The City of Columbia has been around for ages, but what we overlook in thinking about the present floods is just how new most of the residential areas are.  Forest Acres, on the eastern side of the city where the most disastrous flooding is occurring, was only developed into neighborhoods from the 1950’s forward.

What did 1950’s southern suburbia look like? My great aunt, who bought a brand new home on the other side of the river in the avenues of Cayce (West Columbia), explained how Knox Abbot drive — one of the major arteries feeding the city — was then a dirt road.  Urbanization as we know it is new.  Very new.

We think of sixty-five years as being a long time ago, but in terms of weather cycles and flooding probabilities, it simply isn’t.  Unless you mean your city to be disposable, it has to be built for the weather you can reasonably expect during the lifespan of the homes and neighborhoods you hope to keep.

What place for dam-protected neighborhoods?

The pond-dam system of flood control is what makes large portions of creekside land in central South Carolina arable and habitable.  Without the dams in Forest Acres, the neighborhoods as we know them could not function — so they will almost certainly be rebuilt.  Alongside a wild creek is not a good place to put a tame neighborhood.

Something to understand before asking petulant questions about why the dams in Forest Acres and farther afield weren’t made to handle more water is that there is much more room for error in a rural, undeveloped setting — which is where most dams were situated, for lack of urbanization, until only very recently in South Carolina history.

Dirt roads are prone to erosion, which is why asphalt is so popular.  But when you have a dirt road, you can see the dirt wash away.  In contrast, the terrifying sudden collapse of paved roads happens because the dirt below has eroded, and the pavement gives way only when it is no longer supported.  In other words: Until recently, the roads that got washed out by the odd dam-failure didn’t necessarily do so in a way that took anyone by surprise.

Likewise, in a rural area there’s more room for absorbing floodwaters.  Build your home on higher ground, and if the dam goes you’ll lose some crops, but it isn’t, if you do it right, your entire life’s everything.  The Forest Acres floods have been so catastrophic because pavement serves as a channel and funnel for water, not a sponge.

How do you build a disaster? Put your history on backwards.

We have a disastrous paradox going on in the Midlands:

  • Portions of our pond-and-dam systems are built as if water levels we’ve experienced historically aren’t going to come around again.
  • Our infrastructure, which radically affects how floodwaters impact our lives, is the thing that has radically changed. A mostly-paved city experiences water runoff in a fundamentally different way than farm and forestland does.

Essentially we’ve pretended to control the thing we can’t change, and have failed to account for the thing we can and have changed.

It’s an error, and hopefully it’s a once-in-a-thousand-years error.  But if we do not correct ourselves, we will see this again.

File:Gervais Street Bridge, Gervais Street spanning Congaree River, Columbia (Richland County, South Carolina).jpg

Photo: Jack Boucher [Public domain], via Wikimedia Commons


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