11 March 2006

Geophysics for the common man, pt.1

Possibly one in a one part series, but who knows?

Today it is the short lesson of the Mississippi delta and how it works. I could start 12,000 years ago, with glacial loess descending gently upon the plains and the sea level way below where it is today. Glacial run-off carried much silt and sediment and such and started the base of things today in the delta region. In fact, it was swift enough to carve an underwater canyon on the continental shelf. So not much building of the delta went on then, but as sea levels rose and the shelf disappeared underwater, the flow would also slow as the glaciers finally retreated out of the entire basin. My guess would be about 10,000 years ago, give or take a few centuries.

So the Mississippi drains a pretty large part of North America. And all the dirt, sand, waste water, run off, chemical effluent, sewage discharge and such that goes into its tributaries ends up in the Mississippi heading downstream. The grade along the largest portion of the river is not that steep, so it tends to have a steady current, and deposits and entrains sediment as it goes. Sand bars build up, meanders form, meanders get cut off, ox bow lakes are the result and, in general things keep on heading downstream. Once the Mississippi hits the delta region the river finds a few direct routes to somewhat lower levels and spreads out. In spreading out the volume of water in each part decreases, and so does its speed. The Mississippi starts to deposit the stuff it has been carrying before reaching the sea. As parts of the delta silt up, the river shifts to other channels and deposits its sediment load in those areas.

What causes the river to shift beyond that the once a few decades storm surges? The main and over-riding factor of the entire delta is subsidence. When particles get deposited on the bottom, or across broader regions during flood times, they tend to be uncompacted and contain much water between the particles. If it were only sand, then it would drain out. But all that fine glacial deposition along with clay particles and silt get washed into the mix. And many very fine particles will adhere to other particles or larger particles. This then becomes a sticky, gooey mess that acts as a viscous liquid. Think of it as something like molassas, but with lots of grit thrown in, just enough to clog things up a bit and slow the flow. I call it *muck*. And being a liquid it tends to move in the direction of the more swiftly flowing Mississippi, as it follows the same gradient as that river, and it also goes down. It sinks. The entire delta undergoes subsidence that is down and out. Thrown in some plants on top and you get the wonderful ecosystem of the delta.

Floods, being peak sedimentation carrying times, tend to spill over the banks and into the rest of the delta and deposit much muck onto the surface. This muck dries a bit, plants grow and you get a general swamp like atmosphere. Notice that the surface of this area is barely above water level and the entire viscous mass goes down a few hundred feet or more in some areas before you reach bedrock. Luckily this stuff does not flow quickly, this mucky land. About a quarter inch to a half-inch a decade before man got involved depending upon many factors. Some areas barely budge, others almost literally flow so you can see it. And a big bonus is that due to the glacial retreat some islands got built up of a bit firmer stuff and formed a barrier to protect the entire delta from the south and east against hurricanes and the Gulf basin currents.

Now, the mouth of the Mississippi is a wonderful place to put a port for trans-shipping goods from low draft barges to deep draft ocean going vessels. Thus the French founded the City of New Orleans on the highest ground they could find. For a number of years floods overwhelmed the city and then a few storm surges from hurricanes added to the fun. Napoleon wisely sold it as part of the Louisiana Purchase and was rid of a city built in a swamp that was, perforce, sinking. The Americans had to rebuild it a few times before a decent flood and storm control system was put in place. And so starts the wondrous history of the New Orleans (pronounced Naawlins by the locals, so I hear) geophysics experiment!

From here you can check the side-bar for my other pieces on New Orleans. But this gives you the basic overall schema of how things work. Ocean level rises, glacial meltwater goes away, lazy big river deposits muck at its mouth. Muck subsides but is refreshed by the river. Man builds city on muck. From here the fun story of clearing land, getting the water out, compaction, extension, flood control, dredging and, also, increasing the rate of sinking happens.

Such fun!

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