25 September, 2008

The Mathematics of Inevitability

Sometimes, it really sucks being trained as an engineer.

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Two weeks ago Tuesday afternoon, I was packing to go to a conference in Corpus Christi, Texas. Then we got the word that the conference was postponed because of in impending hurricane. By Wednesday evening, the projections said that Ike would be directly overhead of us here in San Antonio on Sunday morning, as a category 1 or 2 hurricane.

I sent an email to my congregation, warning them of the possibility of a hurricane (just in case someone wasn't listening to the news), and telling them to use their common sense on whether or not they should try to get here for worship on Sunday. I planned out a couple of alternate routes for myself for Sunday morning (the two most obvious ways to get from my house to the church campus have streets that flood).

We weren't alone--events got cancelled all over San Antonio. Kids' activities, high school football... and the Texas Longhorns rescheduled a football game in Austin. Now there's a sign that the world just might be coming to an end.

On Sunday morning, Ike was... over five hundred miles away. In Missouri, for crying out loud.

The very best minds we have, using the most sophisticated computer modeling we have, missed their guess by five hundred miles. Some things we still don't know how to predict, or else are inherently unpredictable.

And then there are other things, whose behavior we know very well how to predict. And that's why it sucks sometimes to be an engineer.

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Engineers, you see, are trained to understand the way the world works, and to make it a better place. I studied with Phil Bedient, and I know that hydrology is a fascinating and complicated discipline, but if you over-over-simplify, this is true:


V=Volume of water
R=rate of rainfall
I is a coefficient for the percentage of the impervious surface of the land, from 0 to 100%
T=time of rainfall
A=area on which the rain falls

Just for fun, I've used this basic formula (yes, I'm a nerd) to calculate the rate of rainfall, based on the amount of time it takes to fill up a trash can with the runoff from the roof of my house.

Applying to the impending hurricane:

Houston, my friends, is a great big place. It's flat as a pancake, with a huge portion of it paved over or developed. By late Thursday evening, our best guess had changed, and a storm five hundred miles wide was heading for the city, where it was about to rain very, very hard.

And the engineer part of my brain said: It's going to flood, and at least a few people are going to die. The only question is where, and how much.

I've also studied roadway design and traffic flow, and even if there's not such an overly simple equation to show you, I know that if you made every highway single-direction flow out of town, and somehow got the residents of the city to move with military precision, with no breakdowns or accidents, you still couldn't evacuate four million people in less than two days, even if you wanted to.

Which meant that when the mayor of Houston (or the disaster response people) said that they were "taking a calculated risk" when they only ordered the evacuation of certain portions of the city, that's true, but it's only partially true. The other side of it is that they know that they can't evacuate the city that fast, and people with pretty good models for runoff and floods (like the aforementioned professor) know where it's going to flood first, so they move those people first.

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Seven years ago this September, I remember having a similar moment about an impending disaster. I didn't see the first plane hit the world trade center, and I was hoping it was a particularly horrible accident. But when the second plane hit, I was sitting on my friend Brian's couch watching it on TV, and it was clear that this was deliberate.

I was stunned for a few minutes, wondering how on earth, and who... and then I started thinking about what was going on. Suddenly, the part of my brain that studied high-rise building design jumped over and overlaid itself on the part of my brain that had been an airport consultant for a few years, and I knew--I knew--that the towers were coming down.

I stared at the wall, and saw in my imagination the curves from the steel construction handbook that describe the strength of steel as a function of temperature. I saw, dancing before my eyes, the homework I had done in high-rise design and in structural stability class. And I turned to Brian and said, "Oh, God, they're gonna collapse."

It's just the mathematics of inevitability.

(by the way, it's not that I'm a particularly good or smart engineer. I'm certain that every one of my classmates came to the same conclusion, wherever they were scattered around the country, only they got there faster than I did)

I got up to call my only friend who worked in the World Trade Center, and got as far as picking up the phone, before realizing that he's pretty smart guy, and was (if he was even in the office that day) already on his way out of the building. I put the phone back down, and went and sat back down on the couch, and waited for the horrible scene I knew was coming.

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When a city floods, whose fault is it?

You can't blame the hurricane. It didn't decide to turn North, it just happened.

You can't really blame the city engineers, either. They designed a bayou system for Houston that will handle some tremendous storms. If I remember right, Braes Bayou is designed for the five-hundred year storm. (That means a storm of such intensity that it occurs, on average, once every five hundred years) But it was designed for a five-hundred-year storm in the city in which it was built... and Houston kept growing. Several years later, the runoff from all that extra pavement still flows downhill (such as that is in Houston), and it gets to the creeks and ditches and bayous as intended, but there's more of it than there used to be.

So who's to blame now? Should we tell Mrs. Martinez on the west side of the city that she is not allowed, after all, to realize her dream of owning a house in America? Should we forbid St. Martin's from constructing their enormous new worship space? Make the members of Second Baptist Church park on the grass rather than paving over a parking lot the size of Massachusetts?

Even if maybe we should say some of those kinds of things, we probably won't..because this is basically a free country, and people are going to do what they're going to do. There are laws in place in many inhabited areas that require new construction to be offset by the creation of retention ponds, which makes me feel a little better. But there are plenty of good people who find ways around those laws, or who ignore them because constructing the water retention areas are sometimes expensive.

It's old news by now, but I guess we have to keep saying it. We must recognize that our lives are interconnected. What I do matters.

We breathe the same air, we share the same water supply. When I cut down a tree, we all have a tiny bit less oxygen to breathe. And when I pave over the land, there are people (literally) downstream who are affected.

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