How a Dam Bursts


The stream was dammed for the old mill well before 1976. That’s the only certain date I can give, but by that time the old mill was gone and you had to be a digger to find its foundations (I was.) There were a couple of mature hickories growing in the ruins. Let’s say the dam had stood for a century, give or take a human generation.

It was a small earthen dam on a lowland stream, holding back a hundred acres of pond water that drained from a few thousand acres of forests and fields. It rose perhaps 15 feet from its base to the outlet, perhaps a bit less. It was simply but well constructed with a hard clay interior and plenty of earth above high water, for weight. Twin four-foot pipes at one end drained away beneath the adjacent road.

The road came after the dam, I think. It ran on a fill laid across the steep stream bottom, below the dam and intersecting at one end. The little pocket between the dam and the fill had its own culvert, which it needed because the dam seeped a little. I never saw water on the face, but the ground adjacent was muddy even in dry weather. That would have been the pond’s water column pressing on groundwater, that percolated up on the other side.

Most of the time the little system of pond, dam, and spillway was quiescent. When it rained the pond rose and the outlet pipes ran. When it was dry the water stayed below the outlet and stagnated. When the local beaver clan got too ambitious some responsible person would usually tear out their work. If they repeated the offense, well honestly, trapping those guys is a lot of bother. Back when it was my job, I let Mr. Savage handle it after dark.

Let’s stop and talk physics. Percolation is how we model the way fluid seeps through a porous barrier and similar physical situations. The math is nice and abstract; it’s based on the idea of a network of particles each in contact with its neighbors. Whether any gap lets fluid through is up to the nature of the fluid and the particles, and a bit of chance. Percolation of water through earth is driven by pressure — it takes force to push water through those gaps.

Let’s stop and talk engineering. An earthen dam can allow water to percolate within limits and still go on standing forever. As long as the percolating flow doesn’t go too fast with too much volume, the particles of earth won’t budge. Raise the pressure and more water will percolate, maybe starting to move a grain here and there.

When enough grains have moved to open a connected channel, the system has entered a new realm where percolation no longer matters. The phenomenon of interest now is erosion. As long as the water flows the channel will widen.

Let’s go back and talk physics. Pressure is a force, as is weight. The pressure at the base of a dam is the weight of the water above it. Deeper water, more pressure, more percolation. Now isn’t that so simple a caveman could get it?

Cue the ominous music. Some fool, it’s always some fool, decided to raise the water to improve the fishing. The damn pond already had great fishing, panfish, and predators — crappie, a range of bream, largemouth, and pike. Maybe the would-be Fish Lord wanted his picture in Field & Idiot or something.

A few dozen tons of riprap and rubble will stop a spillway in no time at all. Did you know that? He raised the outlet by three feet.

Nature didn’t get around to filling the additional impoundment for over a year, I’m told. I imagine the stagnant water became offensive in the summer. But Atlantic hurricanes can drop a lot of water a hundred miles inland. I forget which storm it was, might have started with ‘A.’ The dam had seen bigger storms every third year since it was built, but the extra depth and pressure this time were the end of it.

Witnesses told me that when the dam went, it went fast. The wall of water topped the road and ran long enough to take that too. The state repaired the road. The dam was private. The fool with the rocks moved away.

Most of the dam is intact today, only the middle quarter failed — the deepest part, of course. It was undercut by pressure driven seepage that evolved into erosion from beneath. The undisturbed core is plain to see in the remaining segments; it really was a fine dam. You can look at it today, the outlet pipes, the rockpile, the missing segment, and the swamp behind it all. The simple physical truth stares you in the face and says something completely unprintable.

There’s probably a metaphor here for a constitutional republic, one that delivered well on its original promises and good intent and could have gone on indefinitely. Then a few comfortable and privileged fools wanted more. Being privileged, they took that which properly belonged to the community into their own hands and remade it. Being fools, they broke it.

I’ll skip it, that’s too easy. Besides, our fools are still fishing in high water. As more and more outlets are blocked, however, I wonder what it will look like when the regime changes and the real erosion kicks in.

There are 9 comments.

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  1. Arahant Member

    Barfly: As more and more outlets are blocked, however, I wonder what it will look like when the regime changes and the real erosion kicks in.


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  2. RightAngles Member

    Well put.

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  3. Gary McVey Contributor
    Gary McVey

    Well, this one’s a surprise gut punch, isn’t it? Good job, Barfly. It not only works as a sobering political metaphor; it actually works as a straightforward explanation of timeless knowledge about dams, water pressure, and erosion. 

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  4. Randy Webster Member
    Randy Webster

    I once had a long conversation with an engineer about static head.

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  5. Jon1979 Inactive

    Here in the West Texas oil patch it turns out at the dead center of the area where billions of barrels of oil and trillions of cubic feet of recoverable natural gas have been unlocked by fracking and horizontal over the past decade they built an irrigation dam back in the 1930s, for a 300,000 acre/foot lake. Only problem was the same geological forces that put all that oil and gas there also put a ton of fresh water-soluble karst in the formation under the dam, and the presence of the added water’s been trying to erode the dam since then.

    They almost lost it back in the early 1970s when a series of sinkholes formed, and had to do emergency grouting to save the west side of the earthen dam. And earlier this decade, the Texas Commission on Environmental Quality ordered the dam spillway’s gates locked open and the capacity of the lake halved because new voids were found under the dam (which turned out to be a smart move — drought had dropped the lake’s level to under 30,000 acre/feet at the time, but 2014 flooding pushed it to almost 200,000 acre/feet at one point, with the spillway’s gate’s fully open. People were going whitewater rafting down the thing after the rains ended). They’ve been trying to re-grout the new voids but have found so many that the engineering company has recommended a toe-and-blanket system be installed right now, which is designed to keep any internal erosion under the dam trapped in the dam, so that subsurface channels aren’t created to speed the flow of the water and increase the chances for erosion and dam failure in that area (and the oil companies are also banned from drilling within 3,000 feet of the dam as an extra safety precaution).

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  6. David Foster Member
    David Foster

    The sense of security more frequently springs from habit than from conviction, and for this reason it often subsists after such a change in the conditions as might have been expected to suggest alarm. The lapse of time during which a given event has not happened is, in this logic of habit, constantly alleged as a reason why the event should never happen, even when the lapse of time is precisely the added condition which makes the event imminent.

    –George Eliot, in Silas Marner

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  7. Jason Rudert Coolidge
    Jason Rudert

    Good to see you back here. 

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  8. David Foster Member
    David Foster

    Somewhat related:  There’s an interesting novel in which the protagonist works as a dam designer:  The Testing of Luther Albright, by MacKenzie Bezos (presently Mrs Jeff Bezos.)  The protagonist is faced with three simultaneous stressors:  a dam that he designed has been found to have potential flaws, the house that he built has developed defects, and his relationships with his wife and son (especially the son) are troubled.

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  9. Gary McVey Contributor
    Gary McVey

    It’s funny; there are former C-minus students in the “study of how scientific papers are written” who call themselves scientists. There are people who never studied engineering, writers with personality problems and bad ideas who call themselves social engineers.

    But on our side of the culture, here’s someone with a huge vocabulary and a rare gift for explaining how the world works, delivering a free, captivating four minute course in civil engineering, and he calls himself Barfly. 

    Have we got a monopoly on irony and wit, or what? 

    • #9
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