Friday, September 02, 2005

What's a levee, and why, why, why...

There were times today, as I stared at the grasses where my sampling frame had fallen, when my work seemed ridiculously unimportant. I know there are always tragedies unfolding somewhere in the world, and some huge tragedies pass without our notice. Still, that knowledge does nothing to ease my mind about the devastation in New Orleans.

I've been reading a lot about it on the Internet. At first, people were just sharing what they knew of the events, and how to help. Then there was a shift to dismay and anger as the situation got worse instead of better. Today, perhaps because the relief efforts seem to be taking hold, people have begun to tackle the questions of why. Why did so many people stay? Why were there no buses to move those who couldn't drive? Why is the relief taking longer than we expect? Why? Why?

Some have suggested, in comments ranging from subtly patronizing to blatantly dismissive, that all this was just the natural consequence of building a major city below sea level. That seemed much too simplistic for me. I had to ask:

How did New Orleans get to be below sea level in the first place?

Was there really a decision some decades ago to pump out a hole and put buildings in it?

According to Wikipedia, there was, but there were also other factors. When New Orleans was first established nearly 300 years ago, the site was chosen because it was high ground. It was also a very important location, near the mouth of the Mississippi River which allowed water transport deep into the interior of the continent. Because of these natural advantages, New Orleans grew rapidly. It stretched out along the natural levees (raised banks) that bordered the river and its former channels. Behind the levees was swampy ground that flooded often, limiting the growth of the city. Finally, early in the twentieth century, the city began using pumps and canals to drain low swampy land for new construction, allowing great expansion. Obviously, this lower land was vulnerable right from the start. However, some other slow and subtle changes have made the situation worse.

No, I'm not talking about climate change.

One change is a complication of draining the low land. Constantly pumping out water has allowed formerly saturated soil to shrink and settle. Gradually, over the last century, the ground under New Orleans has sunk.

Another change is a partly natural process. When a river floods, leaving the confines of its channel, its flow is spread over a much larger area and therefore slows down. In slower water, silt settles to the bottom. Much of this silt ends up on top of the river bank, where the water first slows down. Over time, with repeated flooding, a ridge builds up along the bank. Then the river floods less frequently, so more of the silt is dropped inside the river channel, instead of out on the floodplain. Eventually, the entire river can be raised above the surrounding land. In fact, the term "levee" comes from a French word meaning "lifted."

Of course, once there are buildings on a floodplain, people want to protect them, so they build the levees even higher, and the river floods even less frequently. More silt builds up in the riverbed, and less gets added to the floodplain. Slowly, slowly, the water level moves even higher compared to the land.

These changes, happening over most of the last century, and to some extent over the last three centuries, have put the city lower with respect to surrounding waters.

Then there are changes to the coastal marshlands. When a storm surge driven by a hurricane comes onto the Louisiana coast, it loses much of its height and force in a huge area of marshes. However, these marshes have been disappearing since the 1930s, when the levees of the Mississippi were raised for greater flood protection. Silt from floods used to build up the marshlands, but now that silt goes straight out into deeper water in the Gulf of Mexico. At the same time, the marshes have been eroding faster than before, because of numerous canals cut through them for shipping and oil exploration. Almost 2000 square miles of marshes have vanished since the 30s, and the loss continues at a rate of "roughly one acre every 33 minutes" (National Geographic, October 2004). If you start into the article I've linked, and the opening description of the hurricane's devastation seems slightly off, just notice when it was written.

Having read all this, I now realize that New Orleans wasn't always a disaster waiting to happen. Its vulnerability grew slowly over many decades. In the meantime it had already become a major port city, an important cultural centre, and a treasured place for hundreds of thousands, if not millions of people.

What happens now? How much of the city will be rebuilt? Can the artificial levees be built to withstand higher storm surges? What about the problems of subsidence, and silt deposition in the riverbed, and loss of coastal marshlands? What can be done to slow or reverse these changes? Or will they continue unabated? When the next major hurricane hits, will the devastation be even worse?

And why do I spend my time monitoring the regrowth of grass on old oil well sites?

3 comments:

Mary Ann said...

Nice to find some objective information about New Orleans instead of the endless ranting I've been reading.

Why are you studying the regrowth of grass on old oil well sites? I'm looking for more information to help people understand why oil drilling in ANWAR is a bad thing. Any ideas?

Laura said...

Why do I study grass regrowth on old oil well sites? Well, I don't really study; I monitor. After the site's productive years are finished, the equipment is removed, the well is sealed off and buried, contaminated soil is cleaned up, the site is recontoured, topsoil is replaced, and the site is reseeded. I come and check once a year to see what's growing, and how well. The work is requested and paid for by an oil company. They need a decent, documented cleanup job so that they don't suddenly face unexpected costs when a landowner notices an uncorrected problem some time later.

As for helping people understand why oil drilling in the Arctic National Wildlife Refuge is a bad thing, I do have some ideas. First, I think it's important to understand why people think it is a good thing. They may have seen figures or even a map showing that only a small area will be affected. They may have heard that caribou have thrived in the vicinity of the Prudhoe Bay oil facilities; or that the people living closest to the proposed development are in favour of it. They may have seen pictures of a flat, featureless plain covered with brown grass or (more likely) snow, and heard stories of swarms of mosquitoes.

Unfortunately they will not be very receptive to information that doesn't fit their understanding. I occasionally comment on a conservative blog, small dead animals, and I sometimes draw scorn, but sometimes I seem to get a serious hearing from at least some readers. I think this happens when I acknowledge valid points made by others, make just one or maybe two clear factual points of my own, and don't do ANY blaming or labelling of persons or groups.

Having said all that, here are some bits of information that I have found that might be helpful.

Parsing the President on ANWR
Note that some of the comments to this post are very informative.

The Class Menagerie
An inside look at a university class trip to the proposed development area, with background on what's at stake for different groups.

And for perspective on how views get polarized around misinformation, perhaps even innocently produced:
CNN: Lying with pictures

When the above post was quoted on small dead animals, I commented: "Well, careful there. The Brooks Range is not at the southern edge of the ANWR, but fills most of the centre of the ANWR, and comes up close to the southern edge of the 1002 area (the area set aside in 1980 for potential exploration and development). (See maps here.)
Mick gives some details about distance to the nearest peaks from the 'drill site,' but as far as I can tell, he is taking his 'drill site' from this map showing a 'Proposed Development Area (to scale) - 2000 Acres.'
I have about seven years of work experience in the oil industry, and let me tell you, the 2000 acres of development won't be in one neat 2000-acre square. Kate, you grew up in oil country, you know that. The technology has improved, so the pads can be smaller and draw from larger areas, but the large oil reserves being discussed in ANWR won't be accessed from one compact mega-pad. Nor will the proponents be stating any exact drill sites at this stage. Mick mentions 'the equivocation on which proposal is being discussed' - you bet there's equivocation. Has been for years. Just try googling 'ANWR 2000 acre'."

So, Mary Ann, I hope you may have found a few new ideas in all of that. Good luck. I wish I still had links for all the sites I looked at back in March. There were beautiful scenes of the coastal plain in summer. There was a pro-development page, posted by a senator, I think, with a video titled "Fly over ANWR!" that showed totally snow-covered land so flat that you could hardly see the scene changing (it almost looked like a blank white screen), with helicopter noise, for several minutes, nothing more. If I find any more links I'll pass them on to you.

Mary Ann said...

Your point, about understanding why people think it's a good thing, is well taken.

I have a Republican friend whom I sometimes use as a test case. He thinks it would be okay to drill for 10 or 20 years then return the area to refuge status. I have a knee jerk reaction - but not enough data to respond to him.

Have you read Where Mountains Are Nameless?

I'm off to check out your links. Thanks.